Given in Loving Memory of Raymond Braislin Montgomery Scientist, R/V Atlantis maiden voyage 2 July - 26 August 1931 Woods Hole Oceanographic Institution Physical Oceanographer 1940-1949 Non-Resident Staff 1950-1960 Visiting Committee 1962-1963 Corporation Member 1970-1980 Faculty, New York University 1940-1944 Faculty, Brown University 1949-1954 Faculty, Johns Hopkins University 1954- 1961 Professor of Oceanography, Johns Hopkins University 1961-1975 m£ RttM. JtiLh^SU*-. cr o LT) t-n o a o m o ■ I '/ # '-- THE SEAS THE SEAS y ?7 OUR KNOWLEDGE OF LIFE IN THE SEA Iff- & AND HOW IT IS GAINED J f£\ BY F. S. RUSSELL, D.S.C., B.A. (Cantab) Assistant Naturalist to the Marine Biological Association, Plymouth ; Formerly Assistant Director of Fisheries Research to the Egyptian Government AND C. M. YONGE, D.Sc, Ph.D. Leader of the Great Barrier Reef Expedition, 1928-9 ; Formerly Assistant Naturalist to the Marine Biological Association, Plymouth MARINE j BIOLOGICAfcviTH 384 ILLUSTRATIONS . ftD;V -J67-,-OFvWHJCH ARE IN FULL COLOUR I I R P £ P Y L ! D r\nn 1 * A WOODS HOLE, MASS. ] g , Ji^, \ W. H. 0. I. |2 if tk I » L«g N gjp N . ^ § FREDERICK W^^^O^. LTD. AND NEW $&kK (All rights reserved) IfUU Copyright Frederick Warne & Co. Ltd. London 1928 PRINTED IN GREAT BRITAIN To E. J. ALLEN PREFACE ERRATA. Preface, p. X, line 20. Text figs. 36 & 37, are from " Science of the Sea," and insert Plates 63 and 64, from Charles Darwin's " Coral Reefs." Page 25, line 5. " Blue Algae" — should read "Blue- green Algae." ,, 145. Text fig. 32 — the words "male" and •' female " should be transposed. „ 205, line 9. " vulnerable " should read " invul- nerable." „ 208, line 4 from bottom. Delete — " with its huge mouth wide open." „ 239 line 21, for " Pacific Ocean " read " Southern Ocean." , 261, plate 95, in legend "x ca 20 " should read X Ca 20- V have been written by Mr. Russell, and Chapters I (second half), II, III, VI, VII, VIII, IX, XIV and sections of XVI by Dr. Yonge. We should like to take this opportunity of expressing our thanks to Dr. E. J. Allen, F.R.S., Director of the Plymouth Laboratory, and to the members of the staff of vii viii PREFACE the Marine Biological Association for the unfailing interest they have shown in this work, and for the many helpful criticisms and suggestions that they have made ; and especially to Mr. H. W. Harvey for his great assistance with Chapter X, concerning which the opinions of a modern hydrographer are invaluable. We are also especially indebted to the Council of the Marine Biological Association of the United Kingdom for their kindness in allowing the publishers to have photo- graphs specially taken at the Plymouth laboratory, and for putting the laboratory and library at the disposal of Mr. W. J. Stokoe. We are grateful to our wives for the great assistance they rendered in the preparation of the manuscript, and to the father of one of us, Mr. W. Russell, for his care in reading through the MS. It has been our aim to illustrate this book with as many original drawings and photographs as we could obtain, supplemented by many beautiful illustrations buried away in the mass of scientific literature and so unfortunately hidden from the eyes of most people. This end could not have been achieved without the cordial co-operation and assistance received from all sides — personal friends, societies, and publishers — and to all and sundry we wish to express our gratitude. We are especially indebted to Mr. Douglas P. Wilson for so lavishly placing at our disposal his many beautiful photographs of shore life from which we chose the illustrations for the following plates : 8, 9, 10, ii, 12, 14, 15, 16 bottom, 18, 20, 22, 45 top, 46 bottom, 87, 109 top, 127 bottom ; to Dr. Marie V. Lebour for the original paintings for Plates 40, 43, 44, 88 ; to Mr. W. Russell for the original paintings for Plates 31, 50 ; to Dr. T. A. Stephenson for Plate 16 top, and to Mr. H. O. Bull for Plate 29 ; and to Mr. Harry Vandervell for per- PREFACE ix mission to reproduce the original painting by Mr. Norman Wilkinson, Plate 102. For original photographs our thanks are due to Mr. C. F. Hickling for Plate 99 ; Dr. E. J. Allen, F.R.S., for an endpiece ; Mr. F. M. Davis for Plates 95, 118 left ; Mr. E. Ford for Plate 119 ; Mr. N. J. Berrill for Plate 7 bottom ; Professor R. Dohrn for Plate 5 ; Mr. Hammond for Plate 11 ; Mr. K. Hartley for Plate 96 right ; Institution of Civil Engineers for Plate 57. Permission has been kindly granted by the authors of many original works to reproduce illustrations, amongst whom we wish to thank Mr. C. Tate Regan, F.R.S., for text-figure 17 ; Dr. Henry B. Bigelow for Plates 42 top, 46 top ; Mr. Meade- Waldo for text-figure 22 ; Professor A. C. Haddon F.R.S., for text-figure 16 ; Professor Johs. Schmidt for Plate 35 and text-figure 13 ; Dr. Werner Klinkhardt for Plate 30 ; Dr. B. W. Evermann for Plate 60 ; Dr. W. T. Caiman, F.R.S., for Plates 80 top, 89 top ; Dr. J. Travis Jenkins for text-figure 49 ; to Professor S. J. Hickson for text-figure 65 from his " Introduction to the Study of Corals " ; to Dr. E. W. Cudger for plate 32 ; to Mr. J. T. Nichols for Plate 42. Our thanks are also due to the following societies and scientific institutes for allowing us to reproduce illustra- tions from their publications and in many cases for the loan of blocks ; the Royal Society, London, for Plate 35 and text-figure 17 ; to the Zoological Society, London, for Plate 123 bottom and text-figure 22 ; to the Natural History Museum, South Kensington, for text-figures 20 and 21 ; to the Marine BiologicaP Association of the United Kingdom for Plates 7 top, 49, 118 right, and text- figures 43, 45; to the Norfolk and Norwich Naturalists' Society for text-figure 18 ; to the Director of the Stazione Zoologica, Naples, for Plates 5 top, 27, 39, 41, 56 ; to the Institute of Oceanography, Monaco, for Plate 28 ; to the x PREFACE American Museum of Natural History for Plates 32 bottom, 42 bottom, 103, 104, 123 top ; to the Bureau of Fisheries Washington, for Plates 42 top, 46 top, 67, 69 ; to the Smithsonian Institution, Washington, for Plate 35 ; to the Ray Society, London, for Plates 77, 81 ; to the National Research Council, Washington, for Plate 54 ; to the Carnegie Institution of Washington for Plates 73 left, 93 ; to Harvard University, U.S.A., for Plate 89 bottom ; to the Discovery Committee, London, for Plate 103 top. To the following publishers and others we wish to express our thanks for their kind permission to reproduce many illustrations : The Controller of H.M. Stationery Office, London, for Plates 4, 7, and text-figures 2, 61, from the Report of the Scientific Results of the Exploring voyage of H.M.S. Challenger, and text-figure 56 ; to the Ministry of Agriculture and Fisheries for Plates 95, 111, 118 left, and text-figure 56 ; to Messrs. Bartholomew for Plate 3 from The Times Atlas; to Mr. John Murray for text-figure 3 from the Challenger Society's handbook, Science of the Sea ; and text-figures 36, 37, from Charles Darwin's Coral Reefs : to Messrs. Arnold & Co. for Plates 5 bottom, 45 bottom, 112 top, from Sir William Herdman's Founders of Oceanography ; to the Government Printing Office, Washington, for text-figure 13 ; to Messrs. Benn Bros, for text-figures 27, 28, 29, 30, from Discovery ; to Messrs. Frederick Warne & Co., Ltd., for text-figure 4 from Shell Life, by Edward Step, and Plate 112 bottom from Fishes of the British Isles, by J. Travis Jenkins ; to Messrs. Macmillan & Co., Ltd., for Plates 21, 25 left, 33, 38, 71 top, 72, 76 top, and text-figures 8, 10, from The Depths of the Ocean, by Sir John Murray and Professor Johan Hjort, for Plate 25 right from Sir C. Wyville Thomson's The Depths of the Sea, and for Plate 80 and text-figure 19 from the Cambridge Natural History ; to Herr Gustav PREFACE XI Fischer, Jena, for Plates 34, 36, 71 bottom, 73 left, 74, from the Reports of the Valdivia Expedition ; to the Cambridge University Press for Plate 61 from The Desert and Water Gardens of the Red Sea, by C. Crossland ; to Messrs. L. Reeve & Co., Ltd., for text-figure 34 from Corals and Atolls, by F. Wood-Jones ; to the Clarendon Press, Oxford, for Plates 66, 68, 82, from the Quarterly Journal of Micro- scopical Science ; to Herr B. G. Teubner, Leipzig, for Plate 70 from Professor A. Steuer's Planktonkunde ; to Messrs. Methuen & Co., Ltd., for Plates 80, 89, from The Life of Crustacea, by W. T. Caiman ; to Messrs. Duckworth & Co., Ltd., for Plate 84 bottom from The Great White South, by H. G. Ponting ; to Messrs. Putnam's Sons, Ltd., for Plate 94 from The Arcturus Adventure, by William Beebe ; to the Chemical Catalog Co., New York, for Plates 103 bottom, 104, 123 top, from Marine Products of Commerce, by D. K. Tressler ; to the Director of the Govern- ment Press, Cairo, for Plate 126 from the Report of the Egyptian Fisheries, 1922, by G. W. Paget ; to Messrs. J. M. Dent & Sons, Ltd., for Plate 124 from Reptiles and Batra- chians by Boulenger ; to Messrs. W. R. Deighton & Sons, Ltd., for permission to reproduce the original oil painting by Mr. Harold Webb as Plate 1 . Of the remaining illustrations the photographs for Plates 106, 107, 108 and drawings for text-figures 7, 35, 38, 39, 40, 41, 42, 44, 46, 60, 64, were supplied by Dr. Yonge, and the original paintings, drawings and photographs for Plates 17, 37, 48, 51, 91, 101, 115, 117, 125, and drawings for text-figures 1, 14, 15, 23, 24, 25, 26, 51, 52, 54, and 55, by Mr. Russell. Finally our greatest indebtedness is due to Mr. W. J. Stokoe for the preparation of the many coloured plates, the half-tone plates and text-figures. We wish to record our gratitude to him for the great skill xii PREFACE and endless pains he has taken to produce the coloured illustrations from original black and white photographs supplied either by Mr. D. P. Wilson or taken specially in the Plymouth laboratory ; for his care in producing the wash and line drawings under our directions ; and for his generous assistance and advice in all matters pertaining to the arrangement of the illustrations. F. S. RUSSELL. C. M. YONGE. CONTENTS CHAPTER PAGE Preface .... v ii L. General Introduction i LI. The Sea Shore - 26 [II. The Sea Bottom ... ^ IV. Swimming Animals 76 V. Drifting Life - - 1 10 VI. Boring Life .... - - - 135 VII. Coral Reefs 156 VIII. Colour and Phosphorescence - - - - 179 IX. Feeding of Marine Animals - - 196 X. Sea Water 216 XL Ocean Seasons 243 XII. Methods of Oceanographical Research - - 254 XIII. The Sea Fisheries 270 XIV. The Shellfish Industry 293 XV. Fishery Research - - - -> - - - - 319 XVI. Products from the Sea 341 Bibliography 361 Index 364 xiii THE SEAS CHAPTER I General Introduction The Oceans " Oceanus," son of Heaven and Earth, was the name given by the Greeks in days gone by to an ever-flowing river that they supposed to flow around the earth they thought was flat. Later the name became applied to those waters that were far outside the range of land, being first used to signify the Atlantic Ocean which lay beyond the Pillars of Hercules. To this day the name has the same significance and differentiates the great open water masses from the seas, gulfs, and straits, that lie around their borders. The Atlantic, the Pacific, and the Indian, are the three great oceans of the world. The first, the grave of the mythical Atlantis ; the second, named " El Mar Pacinco," by Magellan, so calm were his first weeks therein ; the third called after the great country that bounds it on the north. In addition, the waters that surround the North Pole and those that lie along the coasts of the Antarctic continent, are sometimes termed the Arctic and Great Southern oceans respectively. Around the borders of these oceans lie the enclosed seas cut off by narrow straits, such as the Mediterranean, the Baltic Sea, and the Persian Gulf ; the fringing or partially 2 THE SEAS enclosed seas, separated from the oceans by islands or peninsulas, such as the Behring Sea and the English Channel ; gulfs and bays, such as the Gulf of Aden, Gulf of Maine and Bay of Bengal ; and straits such as the Straits of Gibraltar and of Dover. Over two-thirds of the earth's surface is covered thus by the oceans and their adjacent waters, the actual pro- portion of the water to the land masses being according to latest computations about 2.4 to 1. These water masses are not distributed evenly over the surface of the earth, only 43 per cent, lying in the northern hemisphere as opposed to 57 per cent, in the southern. The ratio of water to land also varies in the different hemispheres and it is possible to divide the earth into a water hemisphere whose centre lies a little south-east of New Zealand and a land hemisphere with a centre near the mouth of the Loire in France. Even in the land hemisphere the water area exceeds that of the land by a small amount, while in the water hemisphere only one-tenth is dry land. Along the coasts of the great continents the water is comparatively shallow and a shelf is formed, either by erosion of the land through the ceaseless battering of the waves against its shores, or by the seaward extension of deposits of mud and silt brought down from the interior of the continents by great rivers, or by the gradual sub- mergence of the land itself. Thus there is a plateau, the Continental Shelf (Fig. 1), from which the dry land emerges above the water level. This area of shallow water, extend- ing down to a depth of about one hundred fathoms, varies considerably in width. It is widest in those regions where there has been a gradual submergence of land, such as in the North Sea into which also are carried mud and silt from the many rivers on the surrounding land. It is narrowest where there has probably been an upheaval of GENERAL INTRODUCTION 3 land and where there is an absence of rivers to extend, in a seaward direction, with their deposits such shallows as have been formed by erosion through wave action. Such regions are on the western coast of North Africa, and the Californian coasts of America. The effect of rivers in widening this shelf can be very clearly seen on the north coast of Egypt where it stretches out for over forty miles in the neighbourhood of the Nile mouths, while 100 miles west of Alexandria the 100 fathom line is reached within five miles. TIDAL CONTINENTAL EDGE SEA SURFACE OVER 2.000 Fms ABYSS Fig. 1. From the outer edge of the continental shelf starts the Continental Slope which is generally taken as stretching down to 900 or 1,000 fathoms. This slope is comparatively steep and may be said to constitute the sides of the ocean basins. Its upper limit, the ioo fathom line and outer edge of the continental shelf, is sometimes known as the Continental Edge. B 4 THE SEAS After a depth of 1,000 fathoms is reached the slope of the sea floor becomes almost imperceptible and is probably in most places not unlike a vast and slightly undulating plain. Although it stretches down hundreds of fathoms deeper, its extent is so great that in most places we should be unable to appreciate any gradient whatever. From about 2,000 fathoms downwards this very gradually shelving ocean bed is known as the Abyss or Abyssal Plain. Although almost level compared with our standards on land, the whole bed of the ocean is not absolutely flat but presents considerable variation in depth over large areas. There is, for instance, stretching north and south through the whole Atlantic ocean a continuous ridge between one thousand and two thousand fathoms in depth surrounded on either side by water down to 4,000 fathoms deep (Plate 3). From this ridge rise the oceanic islands of the Azores. The Saint Paul Rocks, Ascension and Tristan d'Acunha. But it is only in the immediate vicinity of these islands that there is any appreciable slope. Soundings have been taken continuously over great distance by transoceanic cable- laying ships, and their results bear out this point : on one occasion only as much as 250 feet being the extreme range of variation in depth through a distance of 100 miles, in water more than 2,500 fathoms deep. Around the before-mentioned oceanic islands which are volcanic in origin, the slopes may, however, be considerable, being as much as 62 for a short distance off Saint Paul. The regions in which the bottom lies below a depth of 3,000 fathoms are known as " deeps." The greatest depth yet recorded is 5,350 fathoms off the island of Mindanao in the Philippines, in the Pacific. This enormous depth, over six and a quarter miles, is hard to visualize. The reader can perhaps best realize it if he imagine that the highest mountain in the world, Mount Everest, be sunk GENERAL INTRODUCTION 5 upon this spot, when its loftiest peak would be fully covered and lie over half a mile beneath the ocean surface (Plate 2). These deeps are not very numerous and cover only a small proportion of the ocean floor. There are 57 in all, 32 of which are in the Pacific, 5 in the Indian Ocean, 19 in the Atlantic, and one lying partly in one and partly in the other of the latter two oceans. Each deep has been given a name, such as the " ; Murray Deep " and the " Valdivia Deep," after well-known oceanographers and research vessels. The deepest sounding in the Atlantic ocean is 5,227 fathoms, off Porto Rico. Taken as a whole the depth of the oceans is very great, for more than half of the ocean floor lies between two and three thousand fathoms, while well over three-quarters is deeper than a thousand. One realizes how comparatively trivial in extent is the shallow water that lies around our coasts when it is known that the average depth of the oceans and their adjacent seas is over two and a quarter miles Ancient Beliefs The extent of the oceans was not known to the ancients and their ideas of what lay beyond the small world they knew were probably mostly surmise and myth handed down from generation to generation. The Chaldeans imagined that the earth, which floated upon the eternal waters, was surrounded by a ditch in which a river perpetually flowed. The Egyptians likewise conceived ever-flowing around their world a river on the surface of which floated a boat carrying the Sun. But neither of these civilizations can be said to have been maritime, and it was the Phoenicians who were the great navigators of ancient history. Their knowledge of the extent of the sea must have been very considerable, since they ventured often through the Pillars of Hercules, visiting the coasts of Europe and the British Isles. From 6 THE SEAS their accounts of floating seaweed it has been thought that they may have been drifted by easterly winds as far west across the Atlantic as the Sargasso Sea. But few of the Phoenician records have been preserved and we have no maps showing what they imagined the extent of the oceans to be. It seems that the Phoenicians whose business lay on the sea were unwilling to part with their knowledge and kept many of their trade routes secret. Any knowledge handed on from them to the Greeks was small and even that was vague. In the days of Homer it was thought that the world was flat and that surrounding the Mediterranean was the land of the few countries they knew, but outside the land flowed an ever-running river which they called the ocean (Plate 4). In the sixth century b.c. Pythagoras considered that the earth was a sphere and in the fifth century Herodotus recorded further advances in knowledge. He, however, considered that the south and west of the then known world were bounded by the ocean, but could say nothing of the north and east (Plate 4). By the third century the idea of parallels of longitude and latitude had been intro- duced by Eratosthenes, and seventy years later Hipparchus instituted a method of map projection. This ancient era in map-making culminated in the description of the world by Ptolemy in a.d. 150, who imagined that the Atlantic ocean and Indian ocean were great enclosed seas, and that if one sailed into the Atlantic from the westernmost point of land the countries of the east would soon be reached (Plate 7). Thus we see that the world of these ancient civilizations consisted of the lands to which they had access on foot and a few countries separated only by short distances of sea, and the two oceans, the Atlantic and Indian, that bathed the shores of the known land. They bad no know* ledge of the Pacific. Sea surface T. * 1998 ft. Sea f/odr :'■ Deepesf part - of fhe Norm Sea compared with fhe heighh of fhe Woolworfh Buildinq New York (792 fh) Average deprh of fhe Oceans 12,000 Pfc Sea Floor •:«•*.* PENETRATION OF SUNLIGHT. Aflanfic Ocean 3I°20n:35-7w 12,750 feef deep. June, Mid-day. -sea surface -650 ff. -Lower limif of fhe bulk of planf life m fhe Ocean. "3,2»off Photographic plafe blackened affer 80nnnufes exposure. 5,578 ff. Plafe nor aff- ected after I20min. exposure. Greatest deprh yef recorded 32,IO0ff. (Pacific) I ."•••;•. ,: ■;;:•.".•.•. '..:■.•.'■'■'"•'.' •':.••'.•;• '.':;•'■';• ':■'••;'■: Y:V^'A'::;.V-V.'*/. : J $ ea floor '■>'$.'/?"]''!• : &-'-'< *i Alhirude record for Aeroplanes (2J1 f AuguSf 1926) 40,821 ff. **~mMl\AM PL 2. Depths of the Ocean (pp. 5, 227). B6. The limit of commercial trawling shown is the extreme limit; the deepest trawling for hake is only economical down to a depth of about 2000 feet. 1 fathom = fj feet - 1'83 metres. Fl. x. Map showing depths of Atlantic Ocean (p. 4). 1 Fathom = 6 feet = 1'83 metres. B 7 . GENERAL INTRODUCTION 7 From the fourth century a.d. onwards, when civilization suffered at the hands of barbarian invaders, know- ledge of the geography of the world received a great set- back. Fantastic suggestions were made as to the shape of the world, and Isidore of Seville in the seventh century originated the "wheel maps" in which the world was repre- sented as being surrounded by a circle of water as was thought in Homeric times (Fig. 2). In addition the world Fig. 2. — Wheel Map of Isidore of Seville. was cut up into three portions by waters connected with this circle of ocean, an eastern portion, Asia, and two western portions, Europe in the north and Africa in the south. For many years little advance was made save for information obtained by Norsemen in the north and voyages of the Arabs in the east, and in the fifteenth century the maps of Ptolemy reappeared. But the knowledge of the oceans 8 THE SEAS of the world was soon to receive a great impetus, for from 1420 until his death in 1460, Prince Henry of Portugal, known as the " Navigator," did all he could to encourage maritime research and exploration. During his lifetime much of the coast of western Africa and the eastern parts of the Atlantic were explored and charted, and his enthusi- asm may be said to have prepared the way for the great voyages of exploration that were to follow. In i486 Bartholomew Diaz rounded the Cape of Good Hope and thus opened up the connection between the Atlantic and Indian oceans. On October 12th, 1492, Christopher Columbus set foot on the island known by the natives as Guanahani, after having crossed the Atlantic ocean. This island, which he named San Salvador, is generally thought to be that now known as Watling Island. For many years after this, expeditions sailed to the continent of America, but it was still thought that this continent was joined to the most eastern lands then known, as was considered to be the case in Ptolemy's time. In 1497 Vasco da Gama, rounding the Cape of Good Hope, reached India for the first time by sea. But the final link in the chain of knowledge of the great oceans was forged in September, 15 13, when Vasco Nunez de Balbao laid eyes upon the Pacific ocean, a mass of glittering waters that dispelled for ever the idea that India and America were joined by land ; and in 1520 Ferdinand Magellan sailed through the straits that bear his name into the " Great South Sea " that he called the Pacific, and although he himself never lived to see the day one of his vessels at last reached Spain in 1522, having circumnavigated the globe. History of Oceanography Let us pause to consider the true meaning of oceano- graphy. It is perhaps one of the most composite of sciences, GENERAL INTRODUCTION 9 involving as it does the many branches of knowledge that pertain to a study of the life and conditions of the environ- ment in the oceans and seas. The study of the ocean currents, the tides, the temperatures, and the saltness and other chemical properties, is approached through pure physics and chemistry. The charting of the ocean margins and the mapping of the relief of the ocean beds are matters for physical geographers. Intimately bound up with the great ocean currents and the tides are meteorological and astronomical phenomena. The above studies constitute hydrography, although this term is more usually applied to the survey and charting of those features only which have a direct bearing upon navigation. There remains the study of the life in the sea, marine biology, which involves zoology, botany, bacteriology, and includes the study of animal and plant development, the study of the abundance of life, and many other problems; to these must be added the application to the fishing industry of all knowledge of life and conditions of life in the sea, fishery research. This composite whole makes up the science of oceanography, which can be summed up as the study of the world beneath the surface of the sea. It is evident, therefore, that -up to 1522 when Magellan sailed into the Pacific ocean the true science of ocean- ography had not begun ; all these exploratory voyages had but touched the margin of the geographical side. It is true that Magellan made a sounding in the Pacific ocean, but somewhat naively he argued that because he could not touch bottom he had reached the deepest part of the ocean. Soundings are recorded also in ancient times, but as yet there was no systematic attempt to map in relief the ocean bed ; in fact it is doubtful whether, owing to the backward state of other sciences, it would then have been possible. It is only within the last few years that it has been made rapid and simple by the introduction of echo sounding. io THE SEAS The progress of oceanography, therefore, must march side by side with the advance of the sciences of which it is built up. The first true oceanographical expedition sailed when Captain James Cook started on his voyage of discovery in 1768 in the Endeavour. On this expedition both temperature observations and deep sea soundings were made, and included in the ship's company were the Astron- omer Royal and a noted biologist. From this time onwards until about i860 many explora- tory voyages were undertaken on which true oceanographical work was carried out, such as the expedition to the Antarctic in 1839 in the Erebus and Terror under Sir James Ross. At this period also the knowledge of marine zoology and botany grew quickly and new facts were being discovered by naturalists who went out in naval surveying ships. In 1 83 1 Charles Darwin sailed in H.M.S. Beagle ; in 1846 H.M.S. Rattlesnake took with her Thomas Huxley, and in i860 H.M.S. Bulldog went cruising with G. C. Wallich on board. Little was, however, known of the life on the deep ocean beds, and it was generally believed that the condi- tions found there would completely prohibit the existence of living animals. Occasionally organisms had been brought up attached to deep sea sounding-leads, but there was the possibility that these had become entangled while the rope and lead were hauled through the upper water layers. More substantial evidence that life really existed at the great depths came when submarine cables were invented, and the salving of broken cables showed growth of marine organisms upon them. In 1868, H.M.S. Lightning, followed later by H.M.S. Porcupine, proceeded with naturalists on board to settle the question once and for all by deep sea dredging. We can imagine the suspense of the little group of men on board when the dredge was at last nearing the surface ; for deep GENERAL INTRODUCTION It sea dredging is a lengthy business, the dredge taking many hours to be lowered and hauled through the enormous depths. They were not disappointed ; living animals were present at the greatest depths ; and an added excite- ment arose, every haul brought up strange and beautiful organisms that man had never yet set eyes on. The time was now deemed ready for a great expedition to explore all the oceans of the world, to study animal and Fig. 3.— H.M.S. Challenger. plant life and the chemical and physical conditions under which they existed. Accordingly in 1872 Her Majesty's Government commissioned the famous research ship, H.M.S. Challenger, under command of Captain Nares, carrying on board some of the most noted men of science of the day, headed by Sir Wyville Thomson. This ship (Fig. 3) sailed the oceans for three years, travelling in that time 69,000 miles. Soundings were made the world over, temperatures noted, and samples of water taken from all depths ; samples of the sea floor were obtained ; 12 THE SEAS dredgings were made in the great abyss, and nets towed in the water layers between the surface and the bottom. The information obtained gave work to a large body of specialists and resulted in the famous " Challenger " reports, which may be said to form the solid base upon which the superstructure of the science of oceanography has since been built. These results placed the science on a sure footing, and later expeditions went out with a good idea of the conditions to be expected, so that plans could be laid for special problems of interest that had to be tackled. Better instruments were invented and, most important of all, wire came into general use replacing the old bulky ropes, Alexander Agassiz being the first to use it on the voyages of the Blake, from 1877 to 1880. Expeditions sailed from many countries, and since the time of the Challenger some of the more important were the Deep Sea expedition of the Germans in the Valdivia, and the voyages of the National and Deutschland from the same country ; the cruises of the Norwegian vessel, the Michael Sars, under Professor Hjort, and Sir John Murray of Challenger fame ; the famous voyages of Captain Scott and of Sir Ernest Shackleton, and many others. Recently two very important cruises have been com- pleted. That of the Discovery, Captain Scott's old ship, which has been carrying out oceanographical work in the South Atlantic and studying the life-history of the whale under the auspices of the Falkland Islands Dependencies ; and that of the German cruiser, the Meteor, which has been engaged in making a complete survey of the ocean bed of the South Atlantic by means of echo-sounding, and taking many hundred water samples to study the current system of the ocean. In the year 1901 was formed what is known as the GENERAL INTRODUCTION 13 " International Council for the Exploration of the Sea." This council is now composed of delegates from Norway, Sweden, Denmark, Finland, Germany, Great Britain and Ireland, France, Belgium and Holland. It has as its prime object the improvement of the Fisheries of the North Sea and surrounding waters, for these great fishing areas are international in character. These countries undertake to fit out permanent research vessels and to study definite problems in the various areas of the sea allotted to them. Marine Laboratories The foundations of the science of oceanography were laid, as we have seen, by the work of the great marine expeditions. But the work that such expeditions can do is necessarily limited, they are indispensable for studies of the fauna and flora of the open sea, of the nature and inhabitants of the bed of the oceans, and of the ocean cur- rents and the nature of the sea in different regions ; but they are not suited, clearly enough, for examinations of the structure and experiments into the functioning of the animals and plants, or for long continued investigations into the seasonal variations in the constitution of the sea water and its microscopical population, work which, as we shall see later, is of the very greatest importance. For these investigations it is essential that we should have laboratories on the sea coast where marine animals and plants can easily be obtained, where they can be kept in aquaria with running or circulating sea water as near natural con- ditions as possible, and where regular samples of sea water can be obtained throughout the year and the variations in its chemical and physical constitution and its contained life accurately determined by expert chemists and biologists. The need for such laboratories has resulted in the founda- I 4 THE SEAS tion of a series of marine stations in the majority of the civilized countries, the work of which has been of the highest importance and promises, in the future, to be even more important. Expeditions are still necessary, we have still a very great deal to learn about the great oceans, but in the future they will extend and supplement the work of the shore stations rather than, as in the past, be an end in themselves. Among the first and certainly the most famous of these marine stations was founded at Naples in 1872 by a remark- able German zoologist named Anton Dohrn. From the Italian government be obtained a grant of land situated in the Villa Nazionale, a beautiful park lying between the town and the sea, where, with money from the German government and from scientific societies in different parts of the world, but very largely from his own private fortune, he built the famous Stazione Zoologica (Plate 5). Since twice extended, this now consists of three handsome flat-roofed buildings of white stone surrounded by evergreen oaks, palms, cacti and other samples of the beautiful Mediterranean flora and commanding from its upper win- dows a fine view of the wonderful panorama of the Bay of Naples. All tourists to Naples will know it, for on the ground floor is situated the far-famed Aquarium, one of the recog- nized " sights " of the city where as nowhere else the visiter can see something of the wonderful Mediterranean fauna of the Bay of Naples. The water which circulates through the aquarium tanks, and also the research laboratories on the upper floors, is pumped up from the Bay, and allowed to stand in huge storage tanks until the sediment has settled to the bottom when the clear water above is drawn off. Scientists of all nationalities work here, " tables," of which England has three, being rented by the year by scientific institutions in different countries who have the The World according to Homer, B.C. 1000. (p. 6). HAKE A U 3 TH.A I- 1 S PL 4 B 14. The World according to Herodotus, B.C. 450. (p. 6). &'*,.*. *- be said to be of two general types, attached like the sea anemones, and freely swimming like the jellyfish. Many of the attached kinds are not solitary individuals GENERAL INTRODUCTION 19 like the anemones but consist of many individuals united together. Of such are the little Hydroids which have branching stems dotted with many little polyps, like flowers, each one resembling a minute anemone, which are united to one another by a common canal which traverses the stem. The framework is usually of a thin, transparent, horny material, though in a few cases it is of limy substance, the result being a kind of coral. Some of these are not attached at all, have no skeleton, and swim freely in the sea, an example being the Portuguese Man-o'-War. Allied more closely to the anemones are the false corals or Alcvonarians, consisting of many individuals with a common skeleton of thick horny substance or of tiny spicules or other material, and also the true corals (Madreporaria) with massive limy skeletons. The worms are divided into many groups with little resemblance other than in shape. There are the little flatworms or Turbellaria, seldom more than an inch long, very flat and almost transparent, to which are allied many parasitic worms which, though commonly found in marine animals, need not concern us here. There are also the Nemertina, soft-bodied worms with a long proboscis which they can protrude or draw in at will, and without that division of the body into a series of transverse segments which is so striking a feature of the most highly-organized worms. These are known as the Annelida and include the common earthworm (in which the segments we have just spoken of are very easily seen) and the bristle-worms (Polychaetes) which are almost exclusively marine, being everywhere abundant and constituting the great majority of the marine worms. Besides segmentation they are characterized by the presence of long bristles which project from the sides of the body. Some wander about at will and are called errant worms, but others live always in C 20 THE SEAS tubes of lime, sand or parchment-like material which they make for themselves, enlarging them as they grow and being in many cases able to make new ones if they are destroyed. Closely related are the leeches of which there ara marine species which suck the blood of fish, and rather less so the Sipunculids which have a protrusible proboscis and a tough leathery body without any sign of segmentation. They are all marine. One of the commonest constituents of the drifting life of the sea is the little arrow worm, Sagitta, which belongs to a small group called the Ch^tognatha, which has no connection with the other worms. Quite closely related to the bristle-worms are the Polyzoa or sea mats, minute creatures which always live in colonies, some of which are small and branching like the hydroids, and others large and with strong limy skeletons which give them the appearance of delicate corals. In spite of their minute size the individual animals are very complex. The Echinodermata are a diverse group, exclusively marine, which include the starfish and sea urchin. Like the Ccelenterates, they are built on a radial plan though, owing to their mode of life, some have altered this and become bilaterally symmetrical. They all have limy skeletons, some consisting of continuous plates forming a compact shell as in the sea urchins, and others of isolated spicules or tubercles embedded in a leathery skin, as in the sea- cucumbers. They are in most instances either attached or very slowly-moving animals, the mechanism of loco- motion being usually provided by the peculiar " tube- feet," tiny tubes which are worked by water power supplied by a series of canals within the body. There are five distinct groups, the starfish (Asteroidea) with a flat central disc to which are attached a number of arms, usually five, though it may be many more ; the brittlestars (Ophiur- oidea) not unlike the starfish but with the disc more "■ m ^mm Laooratory of the Marine Biological Association, Plymouth, (p. 15). PL 6. C 20. Marine Biological Laboratory, Woods Hole, U.S.A. (p. 17). On extreme left is the Fisheries Laboratory. CI o in Q < s o> o 0- o bo c ■5 r - o o o a o 0> H a; GENERAL INTRODUCTION 21 sharply divided from the arms which always number five, though they may divide and subdivide considerably, and without the groove which always runs along the underside of the arms in the starfish ; the sea urchins (Echinoidea) which are all globular, heart-shaped or disc-shaped, with a firm skeleton or shell covered with spines and usually with definite rows of tube-feet passing through , the sea cucumbers, sea gherkins or trepang (Holothuroidea) with elongated, sausage-shaped bodies traversed generally by five rows of tube-feet ; and finally the feather-stars and sea lilies (Crinoidea) usually attached and with a central disc with attachments below, and above a series of five, often branching, arms which wave about in the water. The Arthropoda include the largest number of species of any group in the animal kingdom. Like the Annelid worms they have segmented bodies, but attached to all, or many, of the segments are the jointed limbs which give the group their name. Of the four great subdivisions, three — the Insect, Spider and Centipede families — are almost as exclusively composed of land animals, as the fourth, called the Crustacea, are marine, one of the few examples of the latter found on land being the common woodlouse. There is no space to go into the many sub- divisions of the Crustacea, so all-embracing a group that it includes the tiny water fleas, the minute Copepods which drift about in countless millions in the surface waters, the barnacles which cover rocks and the bottoms of ships, the sand-hoppers of the shore, the ghost-shrimps, the true shrimps and prawns, the lobster and crayfish, the hermit crabs and the many kinds of true crabs. The greatest difference between the simpler and more complex types is that in the latter the limbs, instead of being very much alike from one end of the body to the other, have become specialized into a number of feeding limbs near the mouth, 22 THE SEAS then a group of walking legs, and finally a series of swimming legs under the tail region. Allied to them are the zoologi- cally mysterious sea spiders or Pycnogonida, resembling the true spiders in little beyond the possession of four pairs of long legs. The Mollusca form another great group. The Gastro- poda or univalved shellfish, such as the limpets, periwinkles and snails, the great majority of which are marine (the chief exceptions are snails and slugs), have usually a shell, always of one piece, though the land and sea slugs have either no shell or else one greatly reduced and covered over with skin. Most of them live on the shore or on the sea bottom but a few, without, or with greatly-reduced shells, swim about near the surface, the sea butterflies being the commonest. The bivalve molluscs or Lamellibranchia have a shell composed of two equal, or almost equal, halv3s united by an elastic ligament which forces the two halves open except when they are drawn together by the powerful adductor muscles. They are all either marine or fresh- water animals, well-known examples being the mussel and the oyster. The third great division of the Mollusca includes the octopus, squid and cuttlefish, and is called the Cephalopoda. Its members are all marine and are very highly organized animals with a head having a pair of complex eyes and surrounded with a series of arms possess- ing suckers and hooks. Some, such as the octopus, have no shell ; others, the cuttlefish is an example, have a broad " bone " down the back which is covered with flesh, while a few, like the pearly Nautilus, have a large shell in which they live. Resembling the bivalves in the possession of a shell consisting of two valves are the Brachiopoda, usually only found in deep waters in our own seas. The two halves of the shell however, are always dissimilar, while the internal GENERAL INTRODUCTION 23 structure of the animal is totally unlike that of the bivalve molluscs. The Tunicata are exclusively marine and com- prise many animals of very different appearance. There are the common sea squirts of our shores and shallow seas, either solitary creatures like little gelatinous or leathery bags fastened on to rocks, or else great numbers of smaller individuals embedded in a common gelatinous mass, solitary and colonial Tunicates respectively. Each individual has two openings which, when the animals are squeezed, squirt out a stream of water, hence their common name. Other varieties of Tunicates, called Salps and Appendicularians, form an important part of the drifting life of seas somewhat warmer than our own, such as the Mediterranean. They are transparent animals, which may be solitary but are often fastened together in long chains. In their early stages the Tunicates resemble little tadpoles with a backbone which later disappears but the possession of which may mean that they are really very degenerate relations of the Vertebrates — distinguished from the Invertebrates, which we have hitherto been considering, by the possession of a backbone. The simplest animals to show certain vertebrate characteristics are the Lancelets, like little white eels which burrow in the sand, while, rather simpler than the true fish, are the Cyclostomata, of which the lamprey is our chief example. They have a larger number of gill openings than the true fish, have round mouths with no true teeth and have no paired fins on the sides of the body. The fish or Pisces are too well-known to need description. They are divided into two principal groups, one, called the Elasmobranchs, having a relatively soft, cartilaginous skeleton and with the gill openings separate — to mention two of their chief characteristics — and including the dog-fish, sharks and skates, and the other, known as the Teleosts 24 THE SEAS or bony fish, with the cartilage converted into bone by the presence of limy matter, and the separate gill openings covered over with a flap known as the operculum. All animals higher in the scale of evolution than the fish are air breathers but a number have returned to the sea and spend their lives in it, some always near or on the surface but others capable of diving to considerable depths though always compelled to return to the surface from time to time to obtain air. Among the Reptilia are the water snakes or Hydrophtd^: which have keeled bodies and tails flattened like those of fish to aid them in swimming, and the turtles which are aquatic tortoises with the limbs flattened to form swimming paddles. In the Mammalia there are the many kinds of whales, dolphins and porpoises which con- stitute the Cetacea and spend their entire lives in the sea, an existence for which they are just as well equipped as the fish themselves, and also the strange sea cows (Sirenia), sluggish, harmless beasts which usually live in shallow water near the mouths of rivers in tropical regions and frequently sit on their tails with only their heads out of water — the origin, probably, of all the stories about mermaids ! Finally there is a group of the Carnivores (which include the lions, tigers and bears) the members of which, exclusively marine, except in the breeding season, are known as the Pinnipedia and include the seals, sea lions and walruses. Plants The sea is far poorer in types of plant life than the earth- The flowering plants have very few marine representatives, the best known being the eelgrass (Zostera marina) which is widespread in sheltered regions round our coasts. The great majority of marine plants are Algae, of simpler structure than the flowering plants and reproducing them- Calcareous algae, Corallina and Melobesia ; and Limpet (Patella vulgata) X A. (pp. 31. 32). PL 8. Smooth Blenny (Blennius pholis) X «j. (p. 36). C24. t \ "^ • ° S a , & <: a %i -^ ft,-*, v~ X V> o C ctf ,^ u k z s - X T3 o o o < pq GENERAL INTRODUCTION 25 selves by " spores " instead of seeds produced by flowers. We may divide them into two types, fixed and drifting. The former are the sea weeds and may be further divided into four groups each of which has a characteristic colour as well as structure. There are (1) the Blue Algae (Cyano- phyceae), (2) the Green Algae (Chlorophyceae), (3) the Brown Algae (Phaeophyceae), and (4) the Red Algae (Rhodophyceae) ; these are found from high-water mark downward into deep water in the order named, ; this is discussed in more detail later. The first-named are of slight importance, they are minute plants which sometimes form a slimy film over rocks, but many of the others are of considerable size, the Brown Algae including, not only the strange floating Sargassum weed which gives its name to the Sargasso Sea in the Gulf of Mexico, but also the largest known plant in Macrocystis pyrifera with fronds 200 yards long which is found off the southern parts of South America. Since plants demand a certain minimum of light, all trace of weed disappears from the bed of the ocean below a certain depth. The deep- est water from which weeds have been taken with any cer- tainty that they were actually growing on the bottom appears to be about 60 fathoms, but normally they are not found abundantly except in shallow water. The drifting plant life, apart from the large Sargassum weed, consists of minute single-celled plants called diatoms which have tiny silica cases, and also the Peridinians which, as stated above, can be considered either plants or animals as they have certain characteristics of both. Both occur in untold billions and are of primary importance in the economy of marine life. CHAPTER II The Sea Shore That narrow strip between the high and low-water marks of spring tides which we call the sea shore is the haunt of a rich and varied collection of plants and animals and has, on account of its unique position at the junction of sea and land, an interest altogether out of proportion to its area. Many books have been written about the sea shore and its inhabitants, for the subject is a big one and teeming with interest, but in this volume with its wider scope we can only devote one chapter to it and endeavour to give some general idea of the many fascinating problems it presents. Those who are sufficiently interested are advised to seek further information in the books recommended at the end of this volume. The extent of shore uncovered at low tide naturally depends on the sharpness of slope, and this depends on a variety of factors, the nature of the land, its configuration and the action of the tides, currents and rivers, being the most important. Anyone who has lived near the sea knows how in some places the outgoing tide uncovers great areas of mud or sand, while in others, with a quickly descending shingle beach, only a comparatively small area is uncovered at the lowest spring tides. It has been estimated that the total area between tide marks in Great Britain and Ireland amounts to some 620,00c acres. We can distinguish between three types of shore, formed of rock, sand or mud, though these may be mixed to a greater or less extent. 26 THE SEA SHORE 27 The waves are the greatest influence at work moulding the shore, breaking with fury against the land, washing away loose material or, with the aid of pebbles and stones which they dash against it, gradually eating into a hard, rocky coast, forming a flat " abrasion platform " at the base often of towering cliffs. Here we find a typical rocky shore. In other regions, on the other hand, the action of powerful cross currents deposits great banks of sand, formed by the breaking down of rocks. At the mouths of rivers or in sheltered creeks and gulleys there are mud flats where the sediment brought from land is deposited. The action of ice and of weathering in general also assists in the wearing away of the land and the formation of the shore, while the influence of plants and animals is not to be neglected. The former often help to bind together sand and mud and convert them into firm, dry land ; encrusting animals, such as barnacles and mussels, may help to protect rocks, but usually the action of animals is destructive, notably that of the various rock borers. A feature of the greatest importance is the variability of conditions on the shore. Not only does the sea cover and uncover it twice daily, but the ranges of temperature are far greater, both yearly and often daily, than in any other part of the sea. An animal in moderately cool water at high tide may be left stranded at low tide in a small rock pool where the temperature rises to great heights under the influence of a hot summer sun. The shallow water near the shore is both warmer in summer and cooler in winter (when ice may form on the shore) than the deeper water further out. The constitution of the sea is also apt to be variable, especially near the mouths of rivers where much fresh- water is mixed with it ; also in the height of summer when, in enclosed areas, evaporation may make the sea more than usually salty. The influence of light is very great, greater 28 THE SEAS than in any other region except the surface waters of the open sea. This has an immediate effect in the distribution of plant life, for plants can only exist where there is light which is necessary for the " photosynthetic " action of their green pigment by means of which the carbonic acid gas in the atmosphere, or in solution, is combined with water to form starch, which, with the addition of salts obtained from the soil or from solution, forms the food of the plant. The influence of this flora of sea weeds on the life of the sea shore is of great importance. The population of the sea shore if it is to withstand these very variable conditions must be extremely hardy — how hardy we realize when we discover that very slight changes in the temperature or salinity of the water will kill animals used to the uniform conditions of the open ocean. The animals must be adapted in many different ways, for pro- tection, food collection, and reproduction, to mention only three of the most important. Yet in spite of these diffi- culties the population of the sea shore is one of the densest and most varied on the surface of the earth. So dense, indeed, that the most striking features of shore life are the perpetual struggle for existence, the constant scramble for food in which the strongest or most subtle are the con- querors, innumerable devices for ensuring the continuance of the race, the never ceasing pursuit by the more powerful of the weaker and smaller, the latter surviving to the extent to which they are able to disguise or hide themselves. We can distinguish very definite associations of animals living on the shore. By this we mean that in different sets of conditions we habitually find the same types of animals, per- haps varying in species from place to place. These animals may be of many different kinds — some of them worms, others starfish, crustaceans, molluscs and so on — but they are all adapted for life under those particular conditions ; they may Acom-bamacles ( Balanus balanoides), X |. (p. 31). PL 10. C28. Periwinkles (Littortna littorea), XL (p. 31). Starfish (Asterias rubens) and young Mussels (Mytilus edulis), X J. ( p# 3J PI. II. C29. Crumb-of-bread Sponge (Halichondria panacea) and Beadlet Anemone (Actinia equina), X A. (p. 32). THE SEA SHORE 29 all be plant feeders and live on weed, they may all be attached forms and live fastened to rocks, or be burrowers into mud or sand, or they may be carnivorous beasts which prey upon the animals composing one of these communities to which they thereby become attached. The most intimate form of association is that of parasite and host (as we call the animal which " entertains " the parasite), or the more equal type of intimate union known as symbiosis, some account of which is given on page 213. Lastly, animals, without being actually united with one another, may always live together, one perhaps upon the other, a condition called commensalism, because the two feed in common and also frequently assist one another. Examples of this are given later in this chapter. We can recognize a series of fairly definite zones as we pass down from high -water mark over the shore at dead low water. On rocky shores we can distinguish these zones by the different levels at which the principal sea weeds grow. The different coloured weeds have a perfectly definite arrangement, the green ones generally growing in pools near high-water mark or even above it where sea water only occasionally penetrates and the water is largely fresh ; the brown weeds are especially common between tide-marks, as no one who has ever walked on a rocky shore can fail to have realized, while below them come the red weeds. These are usually found between tide-marks at the bottom of the deepei rock pools or when the spring tide uncovers an exceptionally large area, but are commonest in shallow water off a rocky shore. Above high-water mark, except at the highest spring tides, there is an area of mixed salt and fresh water — brackish is the term used to describe it — which, if the ground be marshy, is usually characterized by the presence of the little salt-wort, a tiny plant (not a sea weed) which has 30 THE SEAS become adapted to these conditions ; if the shore is rocky and there are pools in this region they are filled with the green weed, Enteromorpha, consisting of long tubular fronds. In the summer the pools are apt to dry up, the weed is killed and only a line of white marks this Entero- morpha belt. About the region of high-water mark there is a belt, in breadth from a few feet to about five yards varying according to the slope of the shore, of the yellow or olive coloured " Channelled wrack " (Pelvetia canaliculata) , distinguished from the other brown weeds by its narrower fronds. (Plate 9.) From the base of this belt to low- water mark the rocky shore is covered with a tangle of brown weeds known generally as the fucoid zone, because most of the weeds are members of the genus Fucus. The various kinds of Fucus are arranged in quite a definite series. Next to the Pelvetia comes the broad fronded Fucus platy carpus, followed by a broader belt of Ascophyl- lum nodosum or " Knotted wrack," with exceptionally long fronds bearing a single row of bulbous air bladders down the centre. This weed is easily to be distinguished because on it invariably grows a small red weed (Polysi- phonia fastigiata). The next belt consists of Fucus vesiculosus or " Bladder wrack " (Plate 9), to be distin- guished by the pairs of air bladders along the fronds which, when dried, " pop " sharply when pressed. Nearest low- water mark is the commonest and most strongly growing of all these fucoids, Fucus serratus, the " Notched wrack " which, as its name tells us, has toothed serrations along the edge of the fronds. These different belts of Fucus are not always present, depending on local conditions, the amount of fresh water, exposure, etc., while there are several less common species which we have not mentioned, but generally speaking the above types will be found on any typical rocky shore. They are terminated, quite sharply, at low- THE SEA SHORE 3i water mark by a broad belt composed of several kinds of Laminaria, the largest of the brown weeds which has long fronds, very broad and without a mid-rib, and is secured to the rocks by a massive " hold-fast." They are never exposed except at the lowest spring tides. A number of other weeds are common on the shore, and can most conveniently be referred to now. The green weeds are especially widespread, the " Sea lettuce " (Ulva lactuca) with broad, delicate fronds and living in pools usually above half -tide level, and Cladophora rupestris of a darker green and bushy compact growth which is common in pools everywhere. An easily recognized brown weed commonly exposed between tide-marks is Himanthalia lorea, which has peculiar cup-shaped attachments to the long narrow fronds, while of the red weeds, the " Dulse " {Rhodymenia palmata) which has flat, irregular crimson fronds, and " Carragheen " (Chondrus crispus) (Plate 127) with thicker dark reddish-brown fronds, which appear blue when seen in certain lights, are the commonest, occurring in the upper and lower regions of the shore respectively. The latter is eaten in certain parts of Ireland. Frequent in rock pools are the pink encrusting corallines (Plate 8), not at first easily recognized as sea weeds for they have limy skeletons like some of the encrusting animals. Now let us consider some ol the animals which live on rocky shores. They may be divided roughly into four categories, those which live exposed on the surface of stones, rocks, or weeds, those found under stones, those which live in holes and cracks in the rocks, and those inhabiting rock pools. Of the first group the most char- acteristic members are the sessile acorn-barnacles (Balanus) which cover the rocks with a carpet of little sharply pointed pyramids, especially near high-water mark where they form a definite Balanus zone (Plate 10). Here too are many 32 THE SEAS marine snails, of which the common limpet (Patella) and the periwinkles (Littorina) (Plate 10) and the top-shells (Gibbula and Calliostoma) are the commonest ; they browse upon the weeds and corallines which cover the rocks. The limpets (Plate 8) have definite " homes " on the rock face to the surface of which their shells are exactly fitted so that, when disturbed, they can, by pulling down the shell, fix themselves so firmly that very great force is needed to disturb them ; it has been estimated that limpets with a basal area about one square inch need a pull of seventy pounds to remove them. In all directions around can occasionally be seen radiating paths cleared of weed showing where the limpet has foraged for its food. There are a variety of periwinkles which live in different regions of the shore, one (L. rudis) always living very high on the shore, often where it is untouched by sea water for weeks together, the common winkle (L. littorea) lives lower on the shore always on rocks, while the smaller and less pointed species (L. obtusata) which varies greatly in colour from black to white, is always found on the fronds of fucus. Resembling the winkles but rather larger and with a thicker shell is the dog- whelk (Purpura), which inhabits the upper half of the shore (Plate 13), preying upon the limpets or smaller top-shells. Other peculiar animals allied to the snails are the chitons of which our largest species are about an inch long and about half as wide ; they are flattened, with the shell arranged in eight parts each slightly overlapping the one behind, and when pulled away from the rocks on which they browse, they curl up like little armadillos. There is a numerous population growing on the weeds, notably many kinds of hydroids, sea mats and small snails. Nearer low-water mark we find the rocks covered with sponges of which the " crumb-of -bread sponge " [Halt- chondria) is the commonest (Plate 11), forming a dense mass Golden Stars Tunicate (Botryllus), y\. (p, 33). PL 12. C32. Gooseberry Sea squirt (Styelopsis grossularia), xf. (p. 33). Dog-whelk {Purpura lapillus) laying eggs ; below : two closed up Beadlet Anemones (Actinia equina) % x|. (pp. 32, 50). I PI 13 C33. Sea slues (Lamellidoris bilamellata) depositing their egg ribbons, x£. (p. 37). THE SEA SHORE 33 which may be several inches thick and vary in colour from a pure white in shaded places to a yellow or green in more exposed situations. The deep crimson Hymeniacidon is often found with it. A common sponge which is attached by a stalk is the purse sponge (Grantia compressa), often found on the sides of rocks where there is no danger of it being dried up, and there are other kinds of sponges too numerous to describe. Growing on rocks or fucoid fronds is the tiny worm Spirorbis, which lives in a flat spiral shell like that of a snail and often occurs in great numbers. Larger worms with limy tubes of irregular shape (Serpulids) are frequently common on rocks. Frequently covering rocks are found sea squirts, especially the compound forms — consisting of colonies of simple individuals — such as the golden-stars Tunicate (Botryllus), which closely resembles a sheet of purple jelly dotted with tiny golden stars (Plate 12) though some simple sea squirts are found, especially the Gooseberry (Styleopsis grossularia), a little reddish lump well described by its common name, and abundant near low-water mark (Plate 12). Other encrusting animals are the sea mats or polyzoa, which form, with the sponges, sea squirts, coralline weeds and other alga3, a dense carpet over the face of the rocks. Finally about low-water mark we may find bivalve molluscs such as the saddle-oyster (Anomia), the common mussel (Mytilus edulis) and one of the smaller scallops such as Pecten varius. These are attached in different ways, the saddle-oyster being per- manently attached by its shell and adhering very closely to the surface of the stone while the mussel and scallop are secured by tough threads known as " byssus " the formation of which is described on page 304. A fresh fauna is revealed when we begin to turn over loose stones. Near high-water mark there is a varied collection of small crustaceans and insects. Of the former the largest 34 THE SEAS is the flat Ligia, sometimes an inch and a half long and closely resembling the common woodlouse ; this is every- where abundant and runs about, especially at dusk, usually just above high- water mark, for it is a beast which is by no means dependent on sea water, a slight moisture being apparently all that it needs. Associated with it are a number of sand-hoppers (Talifrus, Orchestia, Gammarus) which, when a stons or pile of dried weed is moved, go jumping away in all directions. Unlike Ligia, they are flattened not from above below but from side to side, a characteristic which separates the group of crustaceans to which they belong, the Tsopods, from the Amphipods. The insects include some beetles, but especially the little spring-tails. Lower on the shore, where the tide always reaches, are found a great variety of worms, such as Nemer- tines, of which the most conspicuous is the long " boot-lace worm " (Lineus) usually tied in a tangled mass which is almost impossible to disentangle without breaking for it may be many yards long. There are also tiny flat- worms which move like semi-transparent films over the surface of stones or on the under side of the surface film in pools. Here we find great numbers of the more highly organized bristle-worms, examples of which are the bright green Eulalia viridis, which is common all over recks, and the yellow Cirratulus cirratus which lives in patches of sand or mud beneath the rocks with only its filamentous tentacles to be seen. Still more worms are found nearer to low- water mark, the commonest being perhaps the handsome Nereis diversicolor, of varied colours and six inches or more in length. It is frequently used as bait, being known as " rag-worm " in many parts. A variety of little worms, broad and flattened with two rows of large scales on their backs, known as Polynoids, are common everywhere. Of the numerous crustaceans the largest and commonest THE SEA SHORE 35 is the common shore crab (Plate 16), Carcinus, of which examples scurry away from beneath almost every stone we examine. Common also is the velvet fiddler crab (Portunus puber), one of the largest of the swimming crabs, a very pugnacious beast, aptly called by the French fishermen " Le Crabe Enragd," also a great variety of smaller crabs of varying shapes and habits, far too numerous to mention here. Some spider crabs are found, but they are commoner off shore and will be described in the next chapter. The curious little green squat-lobster (Galathea squamifera) may be mentioned (Plate 14), while occasionally left behind by the tide near low-water mark are small lobsters or edible crabs (Plates 14 and 113). Hermit crabs provide one of the quaintest and most characteristic members of the shore fauna. Unlike the other crabs, they have no shell covering the tail region but creep for protection within the empty spiral shells of molluscs holding firmly on to the central column of the shell by means of claw-like appendages at the hind end of the body. They are able to shuffle about quite rapidly carrying the shell, which they can, however, leave if they wish to, a procedure which becomes essential as they grow larger and have to forsake the old shell for another a size bigger. The peculiar little sea spiders, which have long legs and such thin bodies that the stomach for want of space has to penetrate the legs, are found under stones. Starfish are commonly met with near low- water mark, especially the large red Asterias rubens (Plate 11), and, more difficult to see, the little " Cushion-star " Asterina gibbosa of a dull grey or greenish colour which lives on the sides of rocks. Both of these have five arms but those of the latter are much shorter relatively, and are united for almost their entire length. Their relatives the sea urchins especially the small green Echinus miliaris, which is a more typical shore form than the handsome E. esculentus 36 THE SEAS (Plate 15), are also inhabitants of rocky shores. They are covered with spines — which in allied animals from warmer seas may be of great length and, sometimes, thickness — and must be handled carefully. Some shore fishes can alwavs be found under stones. The most common is probably the butterfish (Centronotus gunellus), about six inches long, eel-like, flattened from side to side and with nine or more dark spots edged with yellow down the middle of the back. The smooth blenny (Blennius pholis) (Plate 8), can withstand long periods out of water and is common on the shore, as are the bullhead or father- lasher (Cottus bubalis), with its large head armed with four formidable spines, the five-bearded rockling (Motella mustela) to be recognized by the five barbels under its snout, and various kinds of sucker-fish [Lep ado g aster), whose hinder (pelvic) fins are united to form a sucker by means of which the fish fastens itself to rocks. In holes and cracks in rock live worms of various kinds, also a quaint crustacean called Gnathia and the little sea gherkins (Cucumaria), which can frequently only be removed by splitting the rock with a crowbar. Here, too, are rock- boring bivalves, especially the common Saxicava, though the large " Piddock " (Pholas) and some of its smaller allies are common in some parts. There is no more fascinating or more beautiful spot on the shore than a typical rock pool. Its sides and bottom are usually covered with a many-coloured carpet of weeds, sponges, hydroids, sea mats and sea squirts, amongst which, like flowers, glow the rich colours of anemones (Plate 16). The commonest of these — by no means confined to the pools but common on the higher parts of shore — is the beadlet (Actinia), usually a deep red or brown, but sometimes red with green spots (the strawberry variety) or, less frequently, a bright green. At the base of the tentacles is a row of blue THE SEA SHORE 37 spots. The largest of the anemones is the handsome dahlia (Tealia), with a warty column unlike the smooth one of Actinia, and of many colour patterns, some of them strikingly decorative. This also is not uncommon on the shore at the base of rocks, but usually nearer low- water mark. Especially common in the pools is the " Snake-locked anemone " (Anthea cereus) — appropriately so named for it has long tentacles which wave with the motion of the water and, unlike those of the other anemones, are incapable of contraction. We must pass by many of the other anemones, mentioning only the beautiful little Corynactis very common in some regions, no bigger than a pea and of many colour-varieties, pink, green and white ones being found. Other inhabitants of rock pools are hydroids, of which the handsome Tubularia is the finest, a variety of tube-dwelling worms of which the most conspicuous is Bispira, with its wide parchment-like tube and yellow crown of tentacles in the form of two spiral whorls united at the base, many snails especially the conspicuous sea slugs, such asiEolis (Plate 17), with its back covered with a " fur " of soft grey projections, the sea lemon (Doris) — also very common on rocks everywhere (Plate 17) — and a great number of smaller kinds. Crustaceans abound, especially prawns, both the large Leander and the little iEsop prawn (Hippolyte), so difficult to see because of its remarkable power of colour change. A sandy shore has a very different population. Both sea weeds and encrusting animals are absent for there is no hard surface to which they can attach themselves. Many of the sand dwellers are burrowers, spending all or part of their lives beneath the surface, maintaining contact with the water in a variety of ways. The predominant animals are bivalve molluscs, of a type which do not attach them- selves permanently like the mussel or the saddle-oyster, 38 THE SEAS but can move about and burrow by means of a " foot " — a muscular organ, usually wedge-shaped, which can be protruded from between the two halves of the shell and by its sudden movements enables the animal to progress by a series of jerks on or beneath the surface of the sand. Amongst the commonest of these are the cockle (Cardium edule) , which lives near the surface, several species of Venus, one of the clams, which have ridges round the somewhat globular shell the better to enable them to grip the sand, and a number with more flattened shells, such as Tellina, Macoma, Gari and the beautiful, highly-polished Donax, all of which live deeper down than the cockle. The long razor- shells (Solen) may occasionally be dug at low tide ; in it the two halves of the shell form a cvlinder with the two ends open, the lower one enabling the foot, and the upper one the two siphons concerned with the circulation of water and the supply of food, to be protruded. Worms there are in plenty, usually with a delicate, coloured ring of tentacles round the head end. They live buried with only these tentacles exposed for the capture of their fine food, withdrawing them on theapproacb of an enemy or when the tide retreats. Of such are the Terebellids, to be recognized by their sandy tubes the upper ends of which, fringed with fine filaments, project above the surface, also Amphi trite (Plate 77), a fine red worm with a bush of sinuous tentacles. Other worms of a carnivorous habit, such as Nephthys, are found, while, commonest of all, is the lug-worm, Arenicola, whose castings are so common dotted over the sandy shore (Plate 18). Like the earth worm on land, it spends its time swallowing the sand in which it lives, passing it out in the form of the familiar castings. The head of the animal lies at the bottom of a small depres- sion and the mouth continually draws in sand which, since the burrow is usually U-shaped, is conveniently disposed of Squat-lobster (Galathea squami/era), :.;<>• Sea virehin hinus es, ulentus) under water, \. (p. 35). THE SEA SHORE 39 on the surface. It is extremely abundant in suitable localities, such as the shore round Holy Island where a population of 82,000 per acre has been estimated. An animal which might be easily taken for a worm is the reddish Synapta, also found in sand, which is really a relative of the sea cucumbers. Allied animals — in structure, not appear- ance — are the burrowing sea urchins (Echinocardium) (Plate 20), to be dug near low -water mark, though, as we shall see, they are commoner in deeper water. It is on such sandy shores that the common shrimp (Crangon vulgaris) is found, though it is difficult to see owing to its sandy colour and its habit of covering itself with sand, leaving only the long feelers exposed. With it are a variety of other crustaceans, notably the ghost- shrimps (Mysids), smaller and even more difficult to see. There are several kinds of crustaceans which construct burrows often of great depth so that considerable industry is needed in digging them. Several anemones habitually live buried in sand with only the mouth disc with the sur- rounding tentacles exposed. Where reefs of rock run out into the sand we frequently find large colonies of the peculiar reef -building worm Sabellaria (Plate 18), a creature which forms a sandy tube not in, but above, the sand, and not singly but in great numbers altogether, so that large reefs of hardened sand are formed which, on examination, will be found to be honeycombed with the tubes of the worms which construct them. Various fishes are common in shallow water on sandy shores, being occasionally left behind in pools by the retreating tide ; of such are young flat-fish of various kinds and the sand-eel (Ammodytes). We may consider the associations found in mud and in estuaries together because the mouths of rivers are the great site of mud deposition. The fauna bears many resemblances to that found on sand, with which the mud 4 o THE SEAS is frequently mixed, burrowing forms being by far the commonest. Thus, of the bivalves, the large " gaper," Mya truncata, is the most conspicuous, it possesses a thick sh^ll — some three times the size of the mussel — which has a permanent gape at the hind end where the long, muscular siphons project (Fig. 4). Another characteristic type is Scrobicularia, to be recognized by its extraordinarily flattened shell and by the length of the siphons which, unlike those of the gaper, are free from one another. Where the mud is intermingled with stones the large horse-mussel (Modiolus) is common. Worms are abundant, es- pecially burrowing species, and those living in tubes; of these the common Sdbella pavonia, with its parchment- like tube and widely spread ring of red and white plumy tentacles, and Myxicola infundibulum, with its thick gelatinous tube and shorter, more com- pact tentacles, are the commonest. The former often occurs in such num- bers that at low water the tubes, of which usually some five inches project above the surface, appear like a little forest. Of anemones, the brown Sagartia bellis is the commonest. There are a variety of crustaceans including the ubiquitous scavenging shore crab. Estuaries and creeks with a mixed bottom of mud and stones often harbour oysters, which are fastened to the stones and with these are found their invariable enemies, the boring dog- whelks, Nassaand Murex and the common starfish, Asterias Fig. 4. — The Gaper {Mya truncata), (xi). THE SEA SHORE '4* rubens. A variety of prawns and also sticklebacks pene- trate far up rivers, being apparently indifferent to the change from salt to fresh water — a barrier which effectively prevents the great majority of shore beasts from passing up estuaries. We have dealt with the plants and animals of the sea shore, very briefly, it is true, but in sufficient detail we hope, to give some idea of their variety and of the different types found under different conditions, and it is now time to consider the many peculiarities of the shore beasts and the particular devices they possess to enable them to live and propagate their kind in the strange section of the earth's surface which they have chosen for their home. Methods of Attack and Defence Of the first importance are means of attack and defence. Purely mechanical weapons are not particularly common and are best exemplified by the powerful pincer-like claws of the larger crustaceans, especially lobsters and crabs. In the former the two pincers are. never quite alike and if a lobster be examined it will be found that one of the claws is larger with rounded, irregular teeth — clearly adapted for crushing — and the other slenderer with numerous sharp teeth and is used for cutting. There is no regular arrangement of these claws which may occur on either side ; they are immensely strong and, in the case of the shore crab, have been found capable of supporting a weight equal to about thirty times that of the body, whereas a man's right hand when clenched is unable to support a weight equivalent to that of his own body ! Poison is largely used, especially by the anemones and hydroids and all their allies, which possess batteries of stinging cells, the action of which is described on page 203. All over the 42 THE SEAS surface of the shell of sea urchins are little clawed spines, of various patterns, but usually with three teeth which can open and snap together. These are used to clean the shell, removing any fragment of waste matter, but also for protection, for if an enemy attacks an urchin the long spines on that side turn away exposing the smaller clawed spines beneath, which snap at any part of the enemy touching them, and also produce a poison which passes into the wound. More universal are methods of passive defence. The thick shell of the crustaceans and of the univalve and bivalve molluscs are examples. The last-named are frequently safe so long as their shell can be firmly closed and the two muscles concerned with this — the adductor muscles of the shell — are extremely powerful, though very variably so in different beascs, those of the cockle for example having less than a quarter the power of those of Venus. How the bivalves may be successfully attacked and eaten by whelks and starfish we shall see in Chapter IX. The hermit crabs have found an excellent means of protection by using empty mollusc shells. Then there are the various devices whereby an animal escapes attention by toning with the background. This may be done by having a colour similar to the surroundings — one particular set of surroundings or any surroundings, the animal in the latter case changing its colour to tone with the background. This is particularly common in small crustaceans and flat-fish on the shore. The vivid colours of many sea slugs, which are unable, however, to change colour, usually tone with those of the rocks or weed on which they live. Many spider crabs " mask " themselves by deliberately decking their shells with pieces of weed or sponge which continue to grow there. The dahlia anemone and the smaller sea urchin both cover themselves with stones and pieces of shell. THE SEA SHORE 43 In all these cases we must remember that the conceal- ment is not only from foes but also from the prey, i.e., for attack as well as defence. For protection alone are the limy, sandy or parchment-like tubes of the worms, while the borers probably find the interior of stone or wood safer than a more exposed habitat. The replacement of lost parts of the body is a very familiar occurrence on the shore. Crustaceans have an almost unlimited power of growing new limbs. It is quite common for them to be faced with the alternative of sacrificing one or more limbs or else losing their lives. Fig. 5. — Diagram to illustrate breaking plane of Crustacean limb ; a, muscle responsible for breaking ; b, detached ring of third segment of limb ; /, breaking furrow (after Paul). They do not hesitate to do the former and the limb is cast off at a special line of weakness called the " breaking plane," the fracture being caused by the deliberate contraction of the muscles at this point (Fig. 5) . Owing to the depth of the furrow, the wound is small and blood quickly coagulates and closes the opening. So the animal remains, with a stump in place of a limb, until the next time it moults, when the rudiments of the new limb force their way out quickly before the new shell has had time to harden. At each successive moult the process is carried a little further until the new limb is as large as those which were uninjured. 44 THE SEAS This habit of deliberately parting with limbs which are later regenerated is called " autotomy," and is not confined to the crustaceans, being widespread amongst the starfish and their delicate allies, the brittlestars. Both of these part with their arms very readily and quickly grow new ones ; they may lose all their arms and yet from the central disc there will grow out a complete new set (Fig. 6). The related sea gherkins have the more unique power of casting up their viscera when disturbed or attacked , then proceeding at their leisure to grow new ones. It may well be that, under normal conditions, the attacker is satisfied with the meal of soft entrails thus provided, and will not trouble their owner further. The worms have excep- tional powers of regenera- tion and when cut in two accidentally — or by design Fig. 6.— Starfish regenerating arms. in the laboratory — will Two stages in regeneration of new arms -, A 4-nilo from a single severed arm (adapted from g row new neaaS Or tails Flattely and Walton). with equal facility. Sponges have probably the most remarkable powers in this direction, for a sponge can be broken into tiny fragments which are then strained through fine meshed silk, and yet the isolated pieces will come together and unite to form new individuals ! We are leaving a consideration of parasitism and of the intimate, mutually advantageous union of two animals or an animal and a plant, known as symbiosis, to Chapter IX, but we must say something here of that looser form of THE SEA SHORE 45 association called " commensalism," which may be denned as an external partnership between two different animals usually for their mutual benefit. On the shore the most striking example is furnished by the association between the hermit crabs and different kinds of anemones and sponges. One species of hermit, Eupagurus prideauxi, is always found with its body enfolded by an anemone, Adamsia palliata, which resembles a sausage-shaped bag pushed in at either end, to form, at the upper end the stomach cavity, the mouth of which is surrounded by tentacles, and at the lower end the much larger cavity occupied by the soft body of the crab. A mollusc shell is always present in the first place and to this the crab is attached. The common shore hermit, E. bernhardus, which lives in shells of all sizes up to those of the whelk, usually carries on the shell a large anemone, Sagartia parasitica, while in the upper whorls of the shell there is often a worm called Nereilepas. Yet a third, smaller hermit, E. pubescens, is often almost obscured, by the relatively large masses of a sponge which almost invariably grows on the shell. This definite association between the hermit and the anemones and sponge is clearly not accidental, in the former case the anemone probably helps to protect the hermit which, in turn, provides the anemone with scraps of food, in the latter case the sponge may provide protection by camouflage and itself receive food. Other examples of commensalism are provided by the gall crabs which live in coral. The young female crab settles down between two branches of the coral, which as a result, broaden and finally unite above the crab, forming a gall within which the crab lives, feeding, not on the coral, but on particles brought in by the water. The male of the species remains outside the coral. The pea crab, which lives in bivalves and sea squirts, is a further example. 46 THE SEAS Another is furnished by a tropical crab (Melia tessellata), which carries anemones in each of its claws, first deliberately- removing them from the rocks. It holds them fully expanded, pushing them forward when attacked and taking out the food they capture and transferring it to its own mouth. The claws are used exclusively for carrying the anemones. The advantage to the crab is obvious while the anemone, in spite of losing so much of its food, perhaps gains, as by being moved about in this manner it has so many more opportunities of obtaining food. Adaptations for Breathing The problem of respiration — of obtaining that essential minimum of oxygen, without which no animal can live — is often serious on the shore, where the inhabitants are part of the time in water and the remainder in air. We have not space here to discuss the different organs, or gills, used in different animals for obtaining oxygen, but some instances of shore animals which are able to respire both in air and in water will be of interest. This can be done, in a sense, by a variety of animals which are able to keep their gills moist for considerable periods while out of water, for example, the Crustacea, which have gills covered over by the edge of the shell — as in the higher forms like the lobster and crabs — or in the form of plates beneath the body. Of the different periwinkles, those which live nearest the shore and are often out of water for long periods, have developed the power of breathing air, the walls of the gill cavity having become richly laden with fine blood vessels whereby the blood is able to take up its oxygen from the air. There are crabs which live exclusively on land, some, such as the tropical robber crab, only going to the sea to breed, and having developed a true lung for breathing, while others which have retained their gills have to go down to the sea Sea Anemones, (p. 3ti). 1, Sagartia clegans. 2. Actinia equina. 3 & 4. Sagartia viduata (closed an 1 expanded). Fl. in. D 4'). Shore Crab (Carcinus maenas) easting shell, : '. (p, 53 / }-J .' 1 I M \ V^^J^"- PL 17. '" ••• p.s.R. D47, Sea Slugs. 1, AColis papulosa, 2. Doris tuberctdata, 3. Spawn of Doris on a Sea squirt. (pp. 37, 50, 5] ). Natural size. THE SEA SHORE 47 occasionally in order to moisten them. There are also species of tropical fish, notably the " walking goby," which are able to live out of water and even to climb trees ; thev have a lung-like extension of the gill chamber, while it is also reported that they breathe through their tails, for they sit on the land with only their tails in water ! If held under water for any length of time they are drowned. Locomotion and Migrations As we have seen, many shore animals are attached to rocks or weed or else live in permanent tubes or burrows. The advantage of this mode of life is clear when we consider the perpetual beating of the waves on the shore as the tide comes in and goes out. Of equal advantage is the burrowing habit of many animals, bivalve molluscs and burrowing sea urchins, for example, which are also able to move about beneath the surface. Movement in shore animals takes many different forms. The larger crustaceans clamber over rocks and through gullies by means of their strong, hinged walking legs. The starfish move steadily over the surface by the concerted action of the many tiny " tube- feet " each terminated by a small sucker, double rows of these feet lining the grooves which run down the centre of the underside of the arms. They are connected with a complicated system of canals containing water which can be forced out of or into the tube-feet at will, enabling them in turn to fix and relax their hold. A similar mode of movement is found in the sea urchins, "which also employ their teeth for this purpose ! A mussel may move up the side of a rock by fastening a byssus thread as far as possible above it and then pulling itself up by hauling on to it. Some shore fish, such as the blennies, can crawl over rocks by means of their fins. The shore insects and the crustacean sand-hoppers move by jumping, suddenly straightening 48 THE SEAS out the tail or hind end of the body. Periwinkles, whelks and all their allies really glide over the surface, waves of movement passing over the broad flat " sole " of the large fleshy " foot." Anemones and flatworms glide in a similar manner. Apart from the fish, many animals swim, the lobsters by sudden movements of the tail which propel them quickly backwards through the water, while many worms are able to swim to a greater or less extent by un- dulatory movements of the long body, some, most highly specialised, having paddle-like flaps on the side of the body. Mention will be made of the swimming crabs and of the _ 4 Fig. 7. — Razor-Shell (Solen) ; 1-4 showing successive stages in burrowing in sand (after Fraenkel). swimming scallops in the next chapter. The peculiar movements of boring animals are discussed in Chapter VI. The burrowing bivalves work their way through the sand by the action of the muscular foot, the most specialised case being that of the razor-shell, which drives its pointed foot directly downwards, anchors it there by forcing in blood which makes the end swell out and then, by a sudden cod traction, draws the shell down to the level of the foot, repeating the process as often as need be and burrowing so quickly that one has to drive a spade in very suddenly to capture it (Fig 7.). Burrowing worms, crustaceans THE SEA SHORE 49 and sea urchins all have special devices which enable them to work their way through the sand or mud. Many fishes migrate long distances, usually because their feeding and spawning grounds are far apart. There are also migrations on the sea shore, though on a much smaller scale. The movements of edible crabs have been studied, and it has been found that they move from near the shore into depths of twenty or thirty fathoms about the beginning of autumn and remain there until February, when they begin to return to the shore again. The eggs are laid in the winter in deep water and remain attached to the body of the female, finally hatching out during the summer in the warmer water near the shore. Both temperature and food, therefore, play a part in determining the yearly travels of the crab and also of lobsters and prawns which, though we know less of their habits, appear to migrate in a similar manner. The shore fishes also move into deeper water in the winter, while many of the bigger sea slugs, such as the Sea Hare (Aplysia) and the large Plume-bearer (Oscanius) come on to the shore during the summer specially for spawning. Attention has already been drawn to the more localised movements, apparently concerned with feeding only, of the limpets, and this habit is also found in allied beasts. Spawning In such a difficult region as the sea shore, spawning has to be carefully attended to if the race is to be continued. Spring and early summer are the times usually chosen for spawning, for the water is then teeming with microscopical plant life, which forms ideal food for the newly-hatched young. We may divide shore animals into three classes according to their methods of reproduction ; those which discharge their reproductive products freely into the sea where they float helplessly until swimming organs are 50 THE SEAS developed ; those which deposit adhesive spawn on to stones, sea weed or empty shells ; and those in which eggs and developing young are attached to the body of the parent. In the first division are the sea urchins, starfish, brittlestars, many worms, barnacles and the bivalve molluscs. Since the chances of destruction are excessively great, the number of eggs shed is correspondingly large, a striking example of which is supplied by one of the larger American oysters, which is said to produce 100,000,000 eggs annually ! If a sea urchin or mussel be watched when spawning, it will be seen to discharge a cloudy fluid which, on microscopical examination, will be found to consist of incredible numbers of eggs or sperm. Yet so great are the dangers to which these unprotected young are exposed that the race does no more than hold its own ! If search be made among the rocks during the spring, many kinds of spawn will be found. Common on the under side of rocks are the egg-capsules of the dog-whelk (Plate 13), which look like grains of corn, and are attached by short stalks, occurring in groups of fifty, a hundred or even more. Each consists of a tough case, containing a number of developing embryos, amongst which there is the keenest competition, the weaker being eaten by the stronger, so that finally only the one or two strongest emerge. The sea slugs provide the most diverse and ornamental spawn, covering the rocks with ribbons of jelly, often beautifully coloured, in which lie embedded the developing eggs. The common sea lemons (Plate 17) lay their eggs in a broad band of pure white, always arranged in a triple coil, some fifteen inches long and about one inch wide, the margin being unusually wavy. Over half a million eggs may be laid in a single ribbon, for the young hatch out at a very early and unprotected stage. The spawn of other sea slugs shows other peculiarities, some consisting of spirals THE SEA SHORE 5 i with up to ten whorls, the edges of the ribbon being scalloped or the whole zig-zagged in its course, and usually white or yellow in colour (Plate 17), though that of the common .ZEolis is pink and ropy. The eggs of the crustaceans are usually carried by the female — either in special chambers on the under side of the body called " brood pouches " or, in the more highly- organized crabs, prawns or lobsters, attached to the swim- ming legs — until the early stages of development have been passed, which may take several months, so that the young leave the parent, not as defenceless eggs or embryos, but as actively swimming " larva3," a full account of which is given in Chapter V. A crab or lobster which is " in berry " is probably familiar to everyone, and shows this condition clearly. In the sea spiders the male carries the eggs, which are handed over to him in a bundle by the female as soon as they are laid. The effect of different conditions on the spawning of shore animals is very strikingly shown by the spawning habits of the different kinds of periwinkles. The kind which lives nearest low-water mark lays its eggs in little capsules from which the young hatch out at a very early stage as tiny swimming creatures ; the young of the periwinkle which occurs about the middle of the shore hatch out at a later stage though still requiring water to swim in, while those of the periwinkles which live near, or even above, high-water mark are produced as miniature editions of their parents, and able to crawl about on exposed rocks at once. These differences are connected with the increasing lack of water, for the earlier the stage at which the periwinkles are hatched, the longer they will need water in which to swim before they settle down on the shore. Not all animals reproduce themselves sexually. Some bud off pieces of themselves, divide in two or break up into E 52 THE SEAS fragments, each of which grows into a new individual. The anemones, although some kinds habitually reproduce themselves sexually, often divide (a process which is highly developed in the reef building corals, as described in Chapter VII), or break off small fragments near the base. The little syllid worms break up in a perfectly definite way into fragments of a few segments, each cf which grows a new head at one end and a new tail at the other, and so develops into a full-sized worm. The hydroids have a complicated history. They produce not eggs but little jellyfish, called medusae, which swim about near the surface of the sea and, when fully developed, produce eggs which develop into the hydroids which produce the medusas. This, in common with the production of swimming young by various shore animals, is of great importance in securing the distribution of animals which, because of their slowness of movement or fixed mode of life, would not otherwise be able to spread far from their original home. Growth Length of life in shore animals varies within the widest limits. Some animals, such as many sponges and sea squirts, are annuals or even go through several generations in a single summer, whilst on the other hand anemones may live to a great age ; there are some in captivity known to be over sixty years old. After the early embryonic or, in the case of animals with swimming young quite unlike their parents, " larval " stages, most animals assume the adult shape changing in size only as time goes by. Growth is usually a stead}'' process, varying in speed according to the time of the year, being generally slower in winter and quicker in summer when it is warmer and there is more food. In the crustaceans, however, growth takes place by a series of jumps. Unlike the molluscs, which increase the s* *-..» 4* s ft ft x 03 CO a tr CD -3 I W !» CO ft. a 5- C. &1 On PI. 25. Sea Lily. (Rhizocrinus lofotensis), X|. (p. 71). Sea Spider. (Xvmphon rol'ustum), xf. (p. 71). E 65. THE SEA BOTTOM 65 attaining to a length of a foot and a diameter of three inches, and has a thick brown or yellowish skin. It moves, like its relatives the starfish, by means of five rows of tube-feet, and has the curious habit, when irritated, of shooting out masses of sticky white threads, which swell up greatly in water and completely incapacitate attacking animals. Because of this habit, it is also called the cotton-spinner. On greater provocation it ejects its stomach and entire viscera, later growing a fresh set. Crustaceans are not especially common in mud but a number of small species are found. Shallow Water Zone Many of the animals just mentioned, and also a host of others, are also found in the Sub-littoral or Shallow Water zone, for these two regions contain as dense and varied a population as any on the earth's surface. The bottom is usually soft, of sand, mud or muddy clay frequently mixed with stones and rocks which, together with hosts of empty shells, furnish a foundation for the attached animals. We can here only refer to a few of the commonest or more interesting inhabitants, some of which are also found in shallower water, for the boundaries we make are largely artificial. Carnivores and animals which feed on particles in suspension — animate or inanimate — or swallow the bottom mud and sand, are especially abundant for, though light penetrates, if the water be clear, to depths of 100 fathoms or even more, it is insufficient to support plant life in depths considerably less than this. Down to sixty fathoms, however, there is frequently an abundance of the limy coralline weeds or Nullipores, of which the commonest, Lithothamnion, forms an important constituent of the sea bottom in many parts. 66 THE SEAS Representatives of the Foraminifera, which live in limy shells of cen consisting of a series of chambers spirally arranged, are amongst the smallest members of the bottom fauna. On hard surfaces grow great masses of sponge, such as the yellow Clione and Desmascedon, which produces endless amounts of slime so that when it is put in a bucket of water the whole becomes thick and sticky. Many crustaceans and worms burrow into these sponges or shelter in the crevices. On muddy bottoms the most striking inhabitants are the sea pens, creatures allied to the anemones and hydroids, but consisting of a main, central stem from the upper half of which spring branches bearing tiny polyps, the whole being distinctly reminiscent of a quill pen. Thick growths of hydroids and miniature forests of the beautiful pink coloured gorgonids or sea fans (Plate 26), the numerous branches of which extend all in the one plane, like a fan, are both found on rock or stones. Here also live the beautiful Crinoids or feather- stars (Plate 24), another group of the starfish family, which hold on to the bottom by means of a series of outgrowths from the under side of the body, while their ten delicate, foliaceous arms wave about in the water above. They begin life attached, like the sea lilies of which we shall speak later, but afterwards break off and swim with graceful movements of their arms. The British representative is red, brown or yellow in colour and one of the most beautiful members of our marine fauna. Many brittlestars and a variety of starfish, such as the fifteen armed purple and rosy sun-stars (Solaster) (Plate 15), the red cushion -star (Porania) and the large yellow Luidia occur in different localities, while burrowing urchins (Plate 20) and sea cucumbers are found on soft, and sea gherkins on hard, bottoms. Amongst the many worms are included burrowers in THE SEA BOTTOM 67 mud such as the Sipunculids, which have leathery bodies, one end of which can be drawn out into a long proboscis, a feature also of the beautifully coloured Nemertines, which have soft and very extensile bodies, which often break into pieces when handled. The bristle-worms are commonest of all, and include many of the kinds we have already mentioned as well as others only found in this region. The most remarkable of these is the peculiar tube-dwelling Chaetopterus, which lives in sand or mud occupying a parchment-like tube. In the middle of its body are three broad segments which beat rhythmically — even when de- tached from the rest of the body — and maintain a steady current of water through the tube. Commonest of all are crustaceans of all sizes. One of the barnacles, Scalpellum, attaches itself to hydroids and is remarkable in that the large specimens are all female, the males being minute creatures which live attached to the females. Various ghost-shrimps and many members of the prawn and lobster family are common on the bottom. Of the latter may be mentioned the Norway lobster (Nephrops), which lives on muddy bottoms (Plate 113), and the majestic rock lobster (Palinurus) with its handsome brown, sculptured shell but without the large claws of the common lobster (Plate 114). There are also squat-lobsters, such as the small Galathea and the larger Munida, which have long claws and broad, flattened bodies, the tail, normally bent under the body, being straightened out for use in swimming. Various kinds of hermit crabs scavenge over the sea bottom. Of the numerous crabs, there are the large red spiny spider crab (Maia), the largest and most heavily armoured of the spider crabs, and, in more northern waters, the somewhat similar northern stone crab (Lilhodes), which is stone grey in colour, and innumerable smaller kinds, some of which live in sand like the angular crab F 68 THE SEAS (Gonoplax), which has a reddish-brown rectangular shell and eyes on the ends of movable stalks. A great host of molluscs find a home on the sea bottom, the most conspicuous being large snails such as the whelks, which include the common Buccinum, the smooth-shelled Fusus and Sipho, and Neptuna, with bold ridges running round its shell, the two latter being especially northern beasts. The shell of these, and similar, animals is fre- quently covered with a moss-like growth which on examina- tion proves to be a very interesting hydroid called Hydrac- tinia, which has three kinds of polyps, for feeding, repro- duction and defence, respectively. Our largest sea slug, the Triton (Tritonia), is frequently found at these depths ; it is pale coloured of various shades and browses on Dead- men's-fingers ! The boat-shell (Scaphander) ploughs its way through sand in the search for the small bivalves on which it feeds. Another strange beast living in sand is the Elephant's Tooth, our sole representative of a small but distinct group of molluscs, which, as its name suggests, has a long tapering shell, open at both ends. The empty shells are frequently taken possession of by a small Sipun- culoid worm (never by hermit crabs, which always live in spiral shells), which closes up the main opening except for a small hole through which it protrudes its long proboscis. The varieties of bivalves are legion, from the large, very thick-shelled Cyprina islandica of the North Sea to the many smaller kinds which are often almost incredibly abundant. Thus on the Dogger Bank one such named Spisula, occurs in patches often fifty miles by twenty and at a density of anything from one thousand to eight thousand to the square metre, while another, called Mactra, is found in patches of fifteen to twenty miles in diameter and up to seven hundred per square metre. Everywhere abundant are the many different kinds of Scallops, varying in size from the large THE SEA BOTTOM 69 Pecten maximus, up to five inches across, to species with not more than a tenth of this diameter. The octopus is an inhabitant of this region (and of shallower water in the more southern waters, being found on the shore in the Channel Islands). It prefers rocks, hiding in cracks and holes, for it can flatten its body so that it enters narrow slits in the rock, and darts out on fish and crustaceans as they swim by unsuspectingly. The common British species, found in numbers along the south coast, is the Lesser Octopus (Eledone) (Plate 27), which differs from the larger species (Octopus vulgaris) in its smaller size and the possession of a single, instead of a double, row of suckers on each of its eight arms. The latter is not common on British shores though it occasionally comes into the English Channel from further south during hot summers, doing great damage to the crab and lobster fisheries. In spite of its ungainly appearance, the octopus is capable of rapid and graceful swimming and possesses a larger brain and more highly-developed eyes than any of the other Invertebrate animals ; indeed, in the complexity of the latter organs it is the equal of ourselves. Amongst the animals we have not yet mentioned are various lesser known but interesting creatures. In some parts the sea bottom is covered with luxurious growths of the sea mats, the most conspicuous being the Ross (Lepralia), which forms a massive skeleton consisting of many delicate limy sheets, while two others, Cellaria and Flustra, are frequently taken in great quantities in the dredge. Sea squirts of various kinds are found but are not so abundant as on the shore or in the Littoral zone. Deep-Sea Zone The fauna of the Continental Slope — the Continental Deep-sea zone — is most easily studied in the Norwegian 70 THE SEAS Fjords, many of which descend abruptly from the shore down to depths of five hundred fathoms or more. Many of the animals taken in the dredge are of extraordinary interest and beauty, but we can here only refer to a few representative types. On rocky bottoms there are masses of the stony coral, Lophohelia, which forms branching colonies bearing delicate polyps like orange-flowers on a yellow bush, with it are often great tree-like growths of the giant gorgonid, Paragorgia, which is bright scarlet. Sea pens (Plate 73) are common on soft bottoms, red, yellow or brown in colour, and varying in length between a few inches and a yard or more. Below one hundred and fifty fathoms may be found the big bivalve, Lima excavata, often six inches long and four or five inches wide, which has bright orange tentacles and flesh, the latter being considered a great delicacy by the fishermen. In similar localities the dredge brings up numbers of lamp shells or Brachiopods, representatives of an extremely old, and probably dying, group of animals which have a bivalved shell enclosing the body and hardly to be distinguished by the casual observer from the bivalve molluscs. There are, however, many differences, the shell valves are unequal in size, have a different kind of hinge and the animal is attached to rocks by a small stalk which passes through a hole in the larger half of the shell. Starfish and brittlestars are exceptionally abundant, the latter including the fine specimens of Gorgonocephalus (Plate 28), the long arms of which divide and sub-divide to form a writhing mass of tentacles like the hair of the Medusa it has been named after. Large red and brown sea cucumbers are very common, and also deep-sea sponges some with root-like extensions for anchor- ing themselves in the mud, and others which grow on rock, one such, named Geodia, forming great rounded masses, often many feet across, dead white in colour and of tv CO -n 3 >>: s, z PL 27. Octopus (Eledone moschites), '.. (p. 69). /''7< THE SEA BOTTOM 71 strange shapes like the whitened bones of some prehistoric monster ! Abyssal Zone Owing to the remarkable conditions there prevailing, the inhabitants of the Abyssal zone are quite unlike those of the Continental Shelf, far fewer in numbers and quite different in appearance. The Continental Slope which connects these two regions is seldom traversed by animals passing from the one zone to the other for the conditions in the great depths are such that its inhabitants have to be very specially equipped for life and are unable to adapt themselves to existence in shallower water. Conditions are more uniform in the abyss than on any other part of the earth's surface. There is absolute darkness, an unchanging temperature only slightly above zero and an enormous pressure amount- ing, at depths of three thousand fathoms, to about three tons per square inch. There is no vegetation and no exposed rocks, everything being covered with a layer of soft ooze. The place of attached animals is taken by creatures with long stalks, which lift their bodies clear above the mud. Examples are provided by the sea lilies (Plate 25), closely related to the feather-stars of shallower water but with a long stalk in place of the short root-like outgrowths, and also by the sea pens, here at their most abundant, some alcyonarian corals, sponges, sea squirts and sea mats. Many of the crustaceans have long legs for lifting their bodies above the mud, and so have the sea spiders which may have limbs two feet long (Plate 25). There are relatively few animals with calcareous skeletons, molluscs, especially bivalves, being particularly rare, though some interesting examples of the latter which, unlike their shallow water relatives, have become carnivorous, are found in the greatest depths. The total darkness has resulted, para- 72 THE SEAS doxically, in some deep-sea animals losing their eyes com- pletely, while others have developed especially large ones, sometimes on the ends of long stalks, like telescopes, perhaps capable of detecting any gleam of phosphorescent light. Many deep-sea animals are of graceful and slender build, a result, probably, of the complete lack of movement, for there is no need for a strong skeleton and powerful muscles if water currents and tides are absent. Striped and spotted animals, so numerous in shallower water, are remarkable by their absence, practically all deep-water animals being of a uniform colour, usually white, grey, black or red ; blue and green animals are never found. Many of the fish have great jaws and powerful teeth by means of which they prey upon one another, other animals feed on the dead bodies which rain down from the surface miles above, while others again, such as the numerous many-coloured sea cucumbers, plough their way through the ooze, swallowing it as they go and extracting such nourishment as they are able. Associations In each of the areas we have considered, Littoral, Sub- littoral, Continental Deep-sea, and Abyssal zones, we find definite associations of animals in different localities and on different types of bottom. Just as on the shore, the animals composing the associations are especially suited to the particular conditions and also probably assist one another, for life is a unity and its various constituents are dependent one upon another. Very little was known of these associations of bottom-living animals until recent years, when the Danish Biological Station commenced to study them, using for the purpose, not the dredge which gathers haphazard from the sea bottom, but the more exact grab which takes definite samples of the bottom (see page 263) . THE SEA BOTTOM 73 It might appear at first sight as though this knowledge was of quite secondary importance and not worth the very great labour its collection undoubtedly entails. But this is far from being the case. We have already referred to the unity of life, and here is an example ; the bottom animals, especially the shellfish and worms, form the most important part of the food of the bottom-living fish, such as the different flat-fish, the skates and the haddock, and it has been shown by the Danish investigators that the success or failure of the fisheries in any year is largely dependent upon the numbers of other members of the bottom fauna, especially shellfish, in that year. On the other hand the prevalence of starfish has just the opposite effect for these animals — from the fisherman's point of view — are pests, pure and simple, they are of no use to man while they destroy countless numbers of shellfish. Adaptations Different animals are adapted for life under different conditions, and the bottom-living aniamls possess many structures and peculiarities which fit them for life on the sea bottom. We have already spoken of the manner in which burrowing animals maintain connection with the surface, while in Chapter IX we shall see something of their devices for obtaining food. There is also the vital matter of reproduction. Very many of them produce eggs which hatch out into animals totally unlike their parents. Star- fish and their relatives provide the most striking examples of this, but worms and crustaceans afford instances almost as striking (compare the adult crab in Plate 14 with the young of the same species shown in Plate 43) . These "larva?" do not stay at the bottom but rise towards the surface, where they are for a time part of the drifting life, described in Chapter V, and only settle down to the humdrum life of their parents 74 THE SEAS when they begin to assume the adult form. The young of animals which live in very deep water would have difficulty in rising several miles to the surface for a floating existence of perhaps only a few weeks or even days, and then dropping back that great distance, and as a result we find that the development of deep-sea animals is " direct," i.e., there is no strange " larval " stage, when the young are free in the sea, between the egg and the fixed adult, but the lattei develops directly from the egg. The advantage of the former method in ensuring the wide distribution of the animal concerned will be obvious. The spawning habits of bottom animals are many and various, but, as with the shore animals, can be divided into three main types, where the reproductive products are shed indiscriminately, where protected spawn is laid, and where the developing young are carried about by the parent. Bottom animals do not make long migrations like the fish. This is impossible in the case of the many attached or rooted animals while a large number of the others are too sluggish to move far. Octopuses certainly move from place to place causing, as we saw, great damage when they suddenly invade new areas, but this is probably the result of exceptional temperature conditions and not a seasonal occurrence. Crustaceans, at any rate the larger ones, are active beasts and can move about freely ; many, like the edible crab, coming inshore in the summer and then retiring into deeper water in the winter. In the summer the shore waters are warmer than the deeper water, but in the winter the reverse is true. Another animal which can move about is the scallop, especially the smaller " queen " (Pecten opercularis) which, by the continuous flapping of its shell valves, is able to swim about, hinge side hindmost (a process which is reversed when the animal is alarmed) THE SEA BOTTOM 75 (Fig. it), a habit possessed only by a few other scallops and the delicate Lima hians. The queen lives in large " shoals " which are able to move freely about the sea bottom changing their feeding grounds from time to time. Indeed there is a constant movement on the sea bottom, — > 7777777777777777777777- Fig. 11. — Diagram to illustrate swimming of scallop. Left — Normal mode of swimming, upper arrow showing direction of movement. Lower arrows direction in which water is expelled. Right — Movement when startled, hinge foremost, right arrow showing direction of movement, left arrow direction in which water is expelled (after Buddenbrock) particular animals mysteriously disappearing from a given area and then as mysteriously reappearing, and we know nothing either of the cause or the extent of their move- ments* CHAPTER IV Swimming Animals Over the sea bottom lie the waters of the ocean extending for miles in all directions. Above the surface of the land the air is the medium through which birds and insects, and even man, can make their way with great speed. In the same way the sea water forms a medium through which certain animals can travel rapidly from place to place by swimming. Although many minute animals can swim (in the true meaning of the word), that is, make their way through the water, it is usually agreed that the real swim- ming animals of the sea are those only who can make head- way with sufficient speed to move from place to place against any current they may meet. Those smaller creatures that can swim without sufficient strength to stem the currents, and aie drifted to and fro by them, are usually classed under the heading of drifting life, and will be dealt with at length in the next chapter. The true swimmers are to be found in only a few groups of the animal kingdom, and are the fishes, the whales and seals, and the squids or cuttlefish. Fishes Most sea fishes can be divided into two main groups, the cartilaginous fishes or Elasmobranchs, and the bony fishes or Teleosts. In the group of cartilaginous fishes are to be found all the shark family — dog-fish, tope, sharks, rays and skates. These are characterized by the fact that no 76 SWIMMING ANIMALS 77 true bone is to be found in their skeletons, but that they consist of that curious transparent substance known as cartilage, which, for all its delicate appearance, is in reality very tough. In the bony fishes on the other hand, the skeletons are all made of true bone, which is hard and brittle. This is only a further stage in evolution, bone being actually cartilage within which strengthening deposits of lime have been added. The skeletons of very young animals consist of cartilage, and it is only as they grow that the lime is deposited to build up bone. This being the case it is not surprising to find that the cartilaginous fishes are more primitive than the bony fishes, and appeared first in the course of evolution. In the struggle for existence during the history of the world, however, the fishes with true bony skeletons have been by far the most adaptable and are now far more numerous both in kinds and in numbers than are the shark family. But this is not the place to enter into a scientific dis- cussion on the evolution of fishes or to describe in detail all the many different species of fishes that exist at the present day. It is rather the intention of these pages to show how some of the various fishes behave and live in nature and the part they play in the world under the sea. Spawning Habits In describing the life of any fish, it is reasonable to begin from the day of its birth. Most marine fishes, but not quite all, lay eggs ; only a few are viviparous, that is, are born alive and do not hatch from an egg previously shed by the mother. Amongst these few are the viviparous blenny (Zoarces), one or two kinds of dog-fish and the saw- fish (Prisiis) . But let us consider the majority, that is the egg-laying fishes. Some fish lay their eggs on the sea bottom attached 7 8 THE SEAS to stones, shells and weed. In these cases the eggs when first shed are covered all over, or in certain places, with a. sticky secretion, which soon hardens and glues the egg fast to the stone or shell with which it may be in contact. This method is typical of many of those little fishes which are so common in the rock pools and the tidal zone. The blennies, the gobies and the suckers, all fasten their eggs in little clusters to the insides of empty shells, and under over- hanging ledges or in crevices in the rocks. Egg of Blenny (x 25) Fig. 12. Egg of Goby (x 20) The eggs, which are comparatively few in number, vary considerably in shape. Those, for instance, of the common blenny are rounded or spherical, with a little disc-shaped base that cements them to the rock, while those of the gobies are elongated and flattened (Fig. 12). It is quite a common occurrence for thsse kinds of fishes to guard their eggs. Not infrequently a blenny (Blennius PL 28. F:% GorgonocepJtalus Agassi. zi. > .'.. (p. 70). / / ' PL 29. 1>B£. H.O.B. /.' jg m Fish Eggs and Larva, 25. (pp. 80, 131). 1. Whiting (Gadus merlangus). 4. Dragonet (Callionymus lyraj. 2. Gurnard (Trigla gurnardus). .">. Pilchard (Sardina pilchardus). 3. Whiting (Gadus merlangus). 6. Weever (Trachinus vi : p era). SWIMMING ANIMALS 79 ocellaris) may lay its eggs inside a bottle, and if this bottle is dredged up in a net it is quite likely that the parent will be found inside keeping watch over them. They have even been found inside an ox-bone. The gunnel or butterfish (Centronotus) guards its eggs by coiling itself completely round them, as can be seen in Plate 32 ; the male stickleback and several species of wrasse build nests of weed within which the eggs are deposited. In the case of the pipe-fishes, which live amongst the sea grass, the males carry the eggs in a pouch under their tails. Almost all of our food fishes and many others, however, show no such parental solicitude for their offspring's welfare. They merely cast their eggs forth freely into the water, and these eggs are not heavy and sticky as those mentioned above, but are so nearly of the same weight as sea water that they drift about in the water layers at all depths between the surface and the bottom. Here they are at the mercy of tide and currents and are carried many miles from the region where their parents spawned. The dangers are many ; they are not in sheltered crannies with parents to guard them ; there are enemies all around to devour them ; if their own parents happen to meet them again they will eat them with the greatest relish. What then is the provision for their safety ? There is a well- known proverb, " there is safety in numbers," and perhaps nowhere does this hold better than in the case of these care- free fish. We remarked above that those fishes who fixed their eggs to the rocks laid comparatively few ; by this is meant never more than a thousand and probably consider- ably less. But what is this to the five million eggs a cod will lay, or the eight million laid by a turbot ? And yet such are the dangers to be faced during the life of a fish that it is doubtful whether, at most, as many as ten out of these millions will survive to maturity. 80 THE SEAS These drifting eggs are to the naked eye like little glass- clear balls, varying in size from that of a small pin's head to about that of a radish seed. When first cast out into the water the eggs of many kinds of fishes are indistinguishable save by slight differences in size. But after a few days the young fish begins to form within the egg and there appear upon it flecks of colour, black, yellow or red, generally so disposed on the body of the fish as to make the different species quite distinguishable (Plate 29). There are, in addition, in many eggs, globules of oil which either by their size, number, or colour, make it at once apparent to which species they belong. Thus to the specialist the identification of fishes' eggs becomes no harder than does that of the birds' eggs on land, the only difference being that it gener- ally has to be carried out under the microscope. The eggs of the Angler Fish (Lophius piscatorius) present a striking contrast to these single drifting eggs. In this case the eggs are kept together in a large ribbon of jelly, which may float at the water surface. Egg masses of the Angler Fish have been reported several square feet in ex- tent, flat expanses of jelly carrying millions of eggs. Practically the only one of our food fishes which does not lay drifting eggs is the herring. The herring's eggs are deposited on the sea floor, in certain localities only, on stony ground (Plate 112). The eggs are sticky and cling together in clumps in the crevices between the stones among which they fall. They are much enjoyed by other fish as food, especially by the haddock (Gadus ceglefinus) ; indeed, some of the spawning grounds of the herring have been located by the fishermen owing to the presence in their trawl catches of what they call " spawny haddock," that is, haddock whose stomachs are packed full with the herring's eggs. Many skates and dog-fishes have curious eggs, which are SWIMMING ANIMALS 81 very familiar to those accustomed to rambling along our beaches. They are the so-called " mermaids' purses," little horny capsules with spines or tendrils at the four corners. The smaller, narrower type, with long curling tendrils at its corners, is that of the dog-fish (Scy Ilium), while the large, broad ones with the four spines, belong to the skates (Plate 31). The dog-fish eggs are attached to sea weeds by the parents, who wind the tendrils securely round the stem of the weeds ; these eggs are occasionally to be found on weeds at low tide with the yolk and small developing fish inside, because the " mermaid's purse " is of course merely the case in which the egg lies. The skate's eggs, on the other hand, are deposited in deeper water, where they are buried in the sand with the two spines projecting above the surface, and through these a current of water is drawn into the case by the developing fish for breathing purposes. These eggs of the dog-fishes and skates take many months to hatch, although most other fishes hatch out within about a fortnight after the eggs are laid. One of the most surprising cases of spawning instinct is exhibited by a little fish that abounds on the shores of California. This fish, the Grunion (Leuresthes tenuis), belonging to the Atherine family, deposits its eggs in the sand at the high-tide mark along the sandy beaches. This it does one to three days after the highest spring tide, burying the eggs two or three inches beneath the surface of the sand. Here the eggs remain and develop, until in a fortnight's time, when the high tides once more cover them, they are ready to hatch. Now notice the providence of nature. If those eggs had not been laid just after the highest spring tide there is quite a possibility that the next high spring tide a fortnight later, might not have been quite so high, thus preventing the eggs from being reached by the water ; and in this case, being ready to hatch, they could 82 THE SEAS not survive another fortnight. Also, by being laid after, instead of before, the highest tide, they would be unlikely to be washed out of the sand before they had fully developed, since each successive high tide would be slightly lower until the spring tides started once more. Early Life In most cases when first the baby fish hatch they are extremely small. A young whiting (Gadus merlangus), for instance, which is typical of many marine fishes, is only about a quarter of an inch in length. It carries on its under surface an oval sac of yolk on which it is nourished for the first few days of its free existence (Plate 29). When this supply is exhausted the young fish starts to feed. At this stage in its life the fish is drifting freely in the water layers above the bottom, and because of its small size and feeble- ness it can make no headway against the currents, but is carried along by them. All around is a community of other drifting animals and plants, and on these the little whiting makes its meals. After drifting thus for a fortnight or longer the fish begins to assume its adult character and appears as a true miniature of its parents. It is then about an inch in length and can swim with considerable agility, and seeks out new grounds in its search for food ; some kinds of fish at this stage take to the sandy bottoms, while others seek the rocky coves and bays along the coasts. In the case of the whiting, however, a very interesting stage in its life- history has yet to be passed through. The little fish seeks out those big blue jellyfish, the Cyanea (Plate 30), which abound at the same time of the year. These beautiful animals vary in colour from a deep brown red to the most heavenly ultramarine blue. They can be found of all sizes, from that of a small mushroom upwards to as much as a foot in diameter, and they have even been recorded in the SWIMMING ANIMALS 83 cold northern waters to reach the immense size of seven and a half feet across, with tentacles 120 feet in length. These tentacles are armed with batteries of stinging cells. Undei the shelter of these jellyfish the small whiting rind a temporary home. On a calm day it is at times possible to see one of these jellyfish floating near the surface of the water, and all around within a radius of a few feet can be seen numbers of these little whiting darting about picking up their food. A sudden splash with the oar will drive them all beneath their curious shelter where they rest secure, trusting in the stinging powers of their host as a protection against their enemies. And the amazing thing is that the jellyfish allows it and does not attempt to capture them. In European waters the baby horse-mackerel (Caranx trachurus) also seek this floating shelter, while in American waters young butterfish (Poronotus triacanthus) do likewise, as well as young haddock and cod from both sides of the Atlantic. All our flat-fishes, such as plaice and soles, spend the first days of their lives drifting freely through the water. But at this period they are rather unlike their parents. The full- grown fishes, as we all know, have both eyes on the same side of the head ; but not so the very young fishes, which are quite symmetrical, with one eye correctly placed on each side of the head. After two or three weeks, however, the eye on one side begins to move and slowly travels over the top of the head until it reaches the other side. At this stage the fish turns over on its side and .seeks the bottom where it lies, with eyes both pointing upwards. This shows that many of the flat-fishes are flattened sideways, and are, indeed, living on the bottom on their sides. Such fishes are the plaice, the sole, the brill, the turbot and many others. Some fishes, however, such as the skate and the Angler Fish, may be flattened from above downwards, 84 THE SEAS in which case they swim over the bottom truly on their stomachs. The young stages of many fish are totally unlike the adults. The Angler Fish, for instance, so shapeless and cumbersome when grown up, is a most beautiful object soon after hatching (Plate 37). Its fins are drawn out into filaments of fairy-like delicacy. Like other young fish, the Angler spends its early days drifting in the water layers above the bottom, and the long-drawn-out filaments of the fins help to keep it suspended in the water. Migrations It is obvious that, if, for several weeks in the early stages of their lives, most fishes are going to drift freely about at the mercy of tide and current, they will be carried far from where their parents shed the eggs. The greatest importance therefore attaches to the position of the spawning ground, from which after a definite time the young fishes must have been carried to suitable grounds for feeding and growing. For this reason many adult fishes undergo spawning migrations, that is, they move off all together to a chosen locality to shed their eggs. The plaice, for instance, in the southern North Sea move further south (Fig. 61, p. 323), so that their eggs and young are drifted by the prevalent currents on to the so-called " nurseries " in the shallow, sandy bottomed regions along the coasts of Holland. The herring, too, move in from deeper water to deposit their eggs on the bottom in certain regions, and it is then that the fishermen set to work to catch them in the drift nets. But most surprising of all spawning migrations is that of the common fresh-water eel. For years men in all countries of Europe have wondered how it is that the eels in our streams thrive and multiply, and yet nobody had ever seen PL 30. A Jelly fish (Cyanea capillata), Y.\. (p. 82). £84. PL 31. (pp. 81, 8-2). DHL. VV.R. 1 . Egg-ease of Dog-fish (Scyllium canicuia). 2. Egg-case Of Skate (Rain. naevus). Natural size. G 85. SWIMMING ANIMALS 85 their eggs or their very tiny young. Izaal- Walton in his Compleat Angler says : " Some say that they breed, as worms do of mud ; as rats and mice, and many other living creatures are bred in Egypt, by the sun's heat when it shines upon the overflowing of the river Nilus ; or out of the putrefaction of the earth, and divers other ways . . . " " and others say, that as pearls are made of glutinous dewdrops, which are condensed by the sun's heat in those countries, so eels are bred of a particular dew, falling in the months of May or June on the banks of some particular ponds or rivers . . . which in a few days are, by the sun's heat, turned into eels." But just as we know that pearls are not made from dew- drops, so are we now certain that eels do not spring from mud, or putrefaction, or drops of dew. For in the year 1922 a great Danish oceanographer, Dr. J. Schmidt, found the true spawning place of the eel and cleared up the mystery once and for all. He disclosed the amazing fact that when the eels in our rivers become mature they set off on one of the longest journeys as yet known to be under- taken by any fish. When the common fresh-water eel of our rivers has reached a length of about a foot, it changes its appearance and puts on what is known as its " spawning livery." Instead of its usual yellow colour it becomes quite silvery in appearance and hence at this stage is known as the " silver eel." In this condition the eels work their way down to the mouths of the rivers. In the late summer and autumn this migration takes place and the eels push on out of the estuaries and into the open sea. Then starts the long, long journey out into the deep water and so into the Atlantic Ocean. Of the speed at which' the silver eels go on their journey we know little. Once into the deep water they become lost to our observation. In the Baltic 86 THE SEAS they have been captured and marked. Their recapture on their way to the North Sea shows that they had travelled at the rate of about nine miles a day for some three months, but they were only a very short way on their total journey, for the next we know of them they must have arrived in the deep central part of the North Atlantic, known as the Sargasso Sea (a distance of some two to three thousand miles), although none of the spawning eels have themselves been seen there. There is, however, irrefutable evidence that they must have been to those regions, because at the end of winter and during spring the baby eels are there. The young eels are very unlike their parents ; in fact so unlike that the first time one was seen it was considered to be a new species of fish and given the name of " Leptocephalus." They vary from a quarter of an inch to three inches in length according to their age. They are flattened sideways so as to resemble a leaf, and are quite transparent (Plate 35). These baby eels now start on the return journey and we can in this case get some idea of the rate they travel, for Prof. Schmidt has, by measuring very large numbers, shown that they take three years to reach the European shores once more. During the long journey across the Atlantic they grow, and it is by their growth that their birthplace has been located ; catches made between our shores and the central Atlantic exhibit these eel larvae in ever decreasing size (Fig. 13). Near the coasts they are about three inches long, but down in the locality of their birth they are only a quarter of an inch in length, and indeed, in this region, eggs have been taken that are without a doubt those of the parent eels themselves. The eggs are about the size of a pea, quite transparent, and drift in the water layers at depths of about a hundred fathoms, just like many of the other fishes' eggs mentioned earlier in this chapter. When the eel larva? have reached their destination on the SWIMMING ANIMALS 8? coasts of Europe, a very remarkable change comes over them. From their leaf -like shape they gradually assume a true eel-like appearance, becoming narrower, rounder, and at the same time slightly shorter in length. After this change has taken place they are the typical little eels, known as " elvers," that ascend some of our rivers in such countbss numbers. Here they remain in fresh water, feeding and growing. After a course of time, that may vary — ~ 8 ■p »J * I 100 »o j K ■* JuV in ^"40 ■ ■> 7 y \\x\ ¥\\^fi S ■ ^k 1 f*Ss**^ Jl.- ^ 1 _^>*%=^--. /^\S ^y\^ 7 ■>-. /^ "-**-/ — : — — ~~r^~^i " ^« oL \ ^u 5 1000 2000 miles f ^0 u 4*^. 30 1; • a.". '0 :j Fig 1 3. — Distribution of larvae (dotted area) and of adults (black stripes along coasts where species occurs) of the European Eel. The contours show the limits of occur- rence of the different sizes, i.e. larvae less than ten millimetres long have only been found inside the ten millimetre curve, u.l. is the limit of occurrence of unmetamor- phosed larvae (after Schmidt) . from five to twenty years, the eei puts on its silver spawning dress and once more sets out on the return journey to that deep part of the Atlantic where it spawns and probably dies. There is in America an eel which is very similar in appear- ance to the common European fresh -water eel, only differing in certain- minute anatomical details. This eel undergoes 88 THE SEAS a similar life-history to that just described for our eel, with the only difference that the spawning ground is slightly to the west of that of the European eel, and the leaf-shaped larvae take only one, instead of three years before they are ready to ascend the American streams as typical elvers (Plate 35). This difference in the life-histories of the European and American eels is of great interest in showing the speeding up or slowing down of development to suit the environment. If the larvae of the European eel took only one year to reach metamorphosis they would still be far from their destination on the European coasts, when they had assumed the shape at which they normally ascend the rivers ; while the American eels by a reduction of the period of larval existence are ready for metamorphosis by the time they have arrived at the American coasts. A somewhat similar, but reversed, life-history is that of the Atlantic salmon (Salmo salar). In the case of this fish, while the adults undergo most of their growth in the sea, they move up into the rivers to lay their eggs. The young live for two or three years in the fresh water before migr iting down to the open sea to feed on the large food supplies available there and to make that very great growth that distinguishes them from their relatives the trout, who spend all their lives in the rivers. Distribution It can be seen that, because of the large journeys carried out by many fishes, the area of their distribution must vary at different times of the year and be rather widespread. Nevertheless there is noticeable, in the seas of the world, quite a definite zoning of the distribution of different species of fishes. There are, of course, differences in distribution to be found between such fishes as live always in the tidal Above : " Pen " of Squid (Loli-o Forbesi) Below: "Bone" of Cuttlefish (Sepia officinalis), x^. (p. ]Q6). '^X^.^f By pe y7>i ission of E. II'. Gud^er. PI. 32. G 88. Gunnel or Buttevfish (Centronotusgunneiius) guarding its eggs in an empty oyster shell, xf. (pp. 36, 79). O x •v. « a to 8 S a I — ■ a 4! 43 H SWIMMING ANIMALS 89 zones or in the vicinity of rocky coasts, as opposed to those of more open water. Such differences can be noticed in comparatively small areas ; but there are, as well, differ- ences to be found over very large regions. Everyone knows that if he visits tropical latitudes he will see fishes that he never sees m more northern waters. A visitor to such widely separated fish-markets as those of Grimsby in England, Trieste in Italy, and Colombo in Ceylon, will at once be convinced of this. Each market will display its own characteristic fish, and it is doubtful whether any fish would be found common to all three markets. Such localization of faunas is easily understandable on land, where such barriers as deserts, high mountain ranges or large areas of water, limit the different animals to their own special regions. But in the sea, with all the oceans of the world connected, there are no such purely mechanical barriers present to limit the dispersal of the different kinds of fish and prevent them from presenting a perfectly uniform distribution from ocean to ocean and sea to sea. How is it then that one finds nevertheless that certain widely separ- ated areas each possess their own characteristic population of fishes ? It is evident that there must be some barrier, and a general exploration of the chemical and physical properties of sea water has shown that in all probability the chief factor in the distribution of the fishes is the temperature of the water itself. It is generally to be noticed that in our northern waters, wherever the temperature of the water is less than ten degrees Centigrade, our typical northern fishes will be found, cod, halibut, haddock, herring, and many others. On our south-western coasts, those of Devon and Cornwall, we find that we are on the southern limit of the distribution of these northern fishes ; and, at the same time, here occur the northern limits of certain southern warm-water loving go THE SEAS species. It is, so to speak, a meeting ground of the two great areas and certain fishes common to both occur. Besides the herring and the cod, both northern repre- sentatives, we have, for instance, the pilchard, and occasion- ally the red -mullet and the anchovy, fishes which are typical of the warm waters of the Mediterranean and Atlantic. Within the large areas, also, it is to be noticed that there are further sub-divisions ; certain fishes which live in the very cold part of the Norwegian Sea, and in the Arctic waters, are quite characteristic of those regions. In the tropics, again, the fish population is quite character- istic, and here are to be found many of the most brilliantly coloured fishes in the world. In the geographical distribution of fishes, apart from these differences caused by temperature, there are also differences that are occasioned by the depth of the water. There are shallow-water fishes, deep-water fishes, and abyssal fishes, that is, fishes who live on the very flat plains in the deepest parts of the oceans known as the abyss. It is natural that those fishes which will be limited to certain areas by depth boundaries are those that live most of their time swimming on, or close to, the sea bottom itself. Fishes such as herring and mackerel (so-called pelagic fishes), can have a much wider area over which to roam, because, swimming as they always do in the upper water layers, they can keep to the depths they most prefer, irrespective of at what depth the actual bottom of the sea may be. As examples of fish whose distribution is limited by depth, we can name the plaice, which lives in the com- paratively shallow flat areas such as the North Sea ; the hake, that roams along the deep-water shelf from one to five hundred fathoms, that constitutes the continental slope ; and that curious fish of the cod family known as the SWIMMING ANIMALS 91 Macrurus or rat-tail, who spends the greater part of its life in the cold dark depths over the abyssal plain (Plate 33). These remarks on the distribution of fishes will help to explain also the distribution of some of our large sea fisheries. Certain fish, such as the mackerel (Scomber scomber), which prefer warm water, will be found around the north coast of Scotland and also in the North Sea. This, at first sight, appears rather contradictory, but the explanation is simple when we realize that there lies the course of the Gulf Stream, and it is to the warm waters of this oceanic current that the mackerel are keeping. It is evident that all the fishes that live most of the time on the bottom will be limited by the depth barriers to coastal waters or the regions of comparatively shallow banks. There are however many fishes practically unknown to most people that may be termed oceanic. These fishes roam about all through the water layers out in the open oceans. They are mostly very small, the largest being a few inches in length. Many of them are very bizarre (Plate 34) in appearance and possess most interesting organs for the emission of phosphorescent light. The distribution of these small oceanic fishes is also of great interest. They, like most other fishes, are apparently limited in their geographical distribution by those unseen temperature barriers. At the same time just as the bottom fishes appear to be restricted to areas within certain depths, so in the open waters of the ocean these fishes are to be found living, each species inside a definite limited range of depth above the ocean floor. There will bs, for instance, those that occur mostly between the surface and one hundred fathoms ; others, again, will never be met with in these water layers, but are only found below perhaps two hundred fathoms ; and yet others may only b3 captured at very great depths, such as a thousand fathoms. 92 THE SEAS Shape To the average person the word fish summons up in the imagination a silvery, wriggling, slippery animal of a certain definite torpedo shape. This characteristic spindle or torpedo shape is admirably suited to the habits of most fishes. If we take, for example, a fish like the mackerel, we realize that not only is it beauti- ful and elegant, but that it is efficient ; the fish is made to swim and nature has made no mistake. Just as man has learnt many lessons from the forms of birds to aid him in the c designing of the most efficient aeroplanes, so 1 1 1 1 can much be learned ////tis^* from the mackerel about \ \ the best shapes for rapid I j motion through the B water. Its shape is " stream-lined " ; that is, it is adapted so that m its progression through the water the least possible friction is set up and it can cleave its way without hindrance. If a square block of wood is placed in a flowing stream there will be considerable resistance on the front surface and at the same time many eddies will be created just behind it, which act as a suction and impede its passage through the water (Fig. 14). If now the front surface be rounded off the water becomes parted with a minimum of resistance ; the water can now be seen to flow round the block in two streams which meet again some little way behind. Between the back surface of the block, the two surfaces of the divided stream, and Fig. 14. — A, B, and C, are diagrams showing the successive building up of an oblong block into a stream-line shape to reduce resistance caused by eddies. x r ft 8 X £ crte ft I to 3* a v! to Pi 3 4 1 p> 8 ^ .ft a 8 CD ?s a CO *«• CD ~ ja ~ '1 > 3 s •£. CD* ■-i sa s ■s.. ^^ "

(). 5. Font ell 'na ■ n uiata. \ ' DRIFTING LIFE in in his Voyage of a Naturalist noted such discolouration, and mentions passing through two patches ot reddish-coloured water, " one of which must have extended over several square miles." " What incalculable numbers of these microscopical animals ! " he exclaims. " The colour of the water, as seen at some distance, was like that of a river which has flowed through a red clay district ; but under the shade of the vessel's side it was quite as dark as chocolate. The line where the red and blue water joined was distinctly denned." " Stinking water " is another expression used by fisher- men, and in this case also a characteristic odour is given to the sea by the presence of hordes of certain organisms. Many of these small drifting creatures, in fact almost all, are capable of emitting a phosphorescent light. It is these that make the sea sparkle with little glowing points of fire when we dip our oars into the water on a dark night. Occasionally, some phosphorescing animals will swarm together in such countless numbers that on calm still nights the whole sea surface seems to glow with a pale cold light: Darwin again describes such a sight in picturesque terms. He says, " While sailing a little south of the Plata on one very dark night, the sea presented a wonderful and most beautiful spectacle. There was a fresh breeze, and every part of the surface, which during the day is seen as foam, now glowed with a pale light. The vessel drove before her bows two billows of liquid phosphorus, and in her wake she was followed by a milky train. As far as the eye reached the crest of every wave was bright, and the sky above the horizon, from the reflected flare of these livid flames, was not so utterly obscure as over the vault of the heavens." Nearly fifty years ago the word " plankton "* was used by a German professor to embrace all this drifting life, * (Gk. 77 A ay to s, wandering). ii2 THE SEAS and the word is now in general use among those interested in the science of the sea. Plankton Plants One naturally wishes to know what kinds of creatures these are that make up the almost infinite multitudes that drift freely throughout the water layers. The plankton is now known to play a part of the greatest importance in the economy of the sea, and in this respect the organisms that deserve our first consideration are the microscopic plants. These are not like the plants on land, but consist generally each only of a single cell ; nevertheless, they are plants in the true sense of the word, because each contains within its cell colouring matter closely similar to that so characteristic of our land vegetation. These little plants are known as " diatoms " ; they are so called because they have, sur- rounding their cell-walls, glass-like protective shells com- posed of two halves — two lid-like structures that fit one into the other, and thus enclose the body of the plant in a little box. In order to catch these diatoms it is necessary to use a net made of the finest muslin or silk, as they are, mostly, so small that they will pass through the meshes of ordinary coarse muslin. The catch, to the naked eye, will look like a greenish -brown scum, and if placed under the microscope will be seen to consist of a jumble of interlacing spines amongst which may be noticed green oblongs, squares and circles. To find out the true nature of the catch it must be diluted with sea water and only a drop examined, when it will be found that the diatoms are much fewer in number and separated one from the other so that their true structures can be made out. Some will be like little circular discs, others oblong with little spines or horns projecting from each corner, and others strung together to form chains of tiny DRIFTING LIFE 113 boxes covered with delicate, interlacing, hair-like pro- jections. On Plate 88 are given drawings of some of the commonest diatoms that would be found in any catch from the northern and more temperate regions of the Atlantic Ocean and the seas around its border. There are many thousands of different species occurring in the world, and this is no place to confuse the reader with a medley of Latin names. For those who may take a deeper interest in making the acquaintance of the different kinds of diatoms and who find delight in observing the marvellously delicate and beautifully designed structures of these minute plants through the microscope, a list of literature will be found at the end of this book. In addition to the diatoms there are other single-celled organisms that help to swell the plant life of the drifting community. They are especially remarkable because, be- sides containing the colouring-matter of plants, they possess two tiny structures like the lashes of a whip which by vigorous waving motion serve as a means of propelling the creature through the water. Now, a true plant derives all its nourishment from gases and dissolved salts that are absorbed through the cell walls ; it never takes in solid particles of food as an animal does. Many of these little creatures, which are called "peridinians," have no coloura- tion and are able to swallow solid particles of food through a small depression on their cell surface. There are, however, a few which possess colouring matter and also swallow solid particles ; because they are able to feed, like plants, and at the same time utilize solid food like animals, they are a continual source of bickering in scientific circles, the botanists claiming them as plants and the zoologists maintaining that they are animals ! Many of these peridinians consist only of a little naked cell with its whip-like lashes, and are in consequence, n 4 THE SEAS extremely delicate and easily destroyed by the net ; others* however, possess wonderfully designed skeletons made up of little plates which may carry spines and wings, giving the plants a beautiful appearance (Plate 40). Some of these peridinians are the cause of the coloured water mentioned above. The Goniaulax figured in Plate 40 has been known to occur in such profusion as to colour the water red, and another species has been reported to be so thick that when they died and decayed they caused the death of great numbers of fish. There is yet another group of plankton plants known as Coccospheres. These also are unicellular but are charac- terized by the presence of numerous calcareous plates embedded in the cell, the different shapes of which serve as a means of identification of the numerous species. These three groups, the Diatoms, the Peridinians, and the Coccospheres, are the most important constituents of the drifting plant life. The Diatoms are most abundant in the colder waters of the temperate and polar seas, while the two latter are characteristic of the warm waters of tropical and sub-tropical regions. These plants are at times extremely abundant ; they have often been reported to be so thick in the Baltic that a thimbleful of water would contain more than a thousand individuals. Were it not for these countless myriads of small plants we can safely say that the great oceans and seas of the world would be valueless to us as reservoirs from which to draw much of our food in the form of fish and other edible marine animals, for the small plants of the plankton are indeed the pasturage of the sea. They form the food of millions of small animals living in the drifting community on which larger animals prey. On land, man and beast alike are ultimately dependent on the grass and herbs of the field for food, the animals which feed on the DRIFTING LIFE 115 grass being eaten by man ; so also in the sea, the ultimate food supply is to be found in the drifting microscopic plants. But, whereas on land the plants are substantial and can be directly eaten by large animals, in the sea they are minute and are first eaten by the small drifting animals, which are in turn swallowed by larger creatures, and so on until, eventually, the fish forms food for man. In fact certain chemical constituents of our food have been traced to these tiny plants. To most the word " vitamin " is well-known. It is the name given to certain chemical bodies, which, although present in minute quantities only in our food, appear to be essential to our health and well- being. Their absence is thought to give rise to such diseases as scurvy ; and fierce controversy has raged over the desirability of eating white or wholemeal bread. Cod-liver oil also contains a certain vitamin which is thought to be partly responsible for its great medicinal value. The presence of this vitamin has been traced from the liver of the cod to the insides of the capelin (Mallotus villosus), a little fish that forms a large portion of the cod's food, and the swarms of which bring the cod together in vast shoals on the Newfoundland banks. From the capelin it has been traced to the minute animals on which it feeds, and so to the diatoms which nourish them. It is in these little plants — the diatoms — that the vitamins are made. Plankton Animals We have mentioned above that the drifting plants form the chief food of the small animals of the same community. Of what is this animal population chiefly composed ? Actually almost every group of the invertebrate animal kingdom has representatives in the plankton. In addition, the young of many kinds of fish live for a shorter or longer period a free, drifting existence. But of far the greatest I n6 THE SEAS importance in the animal plankton is a group of crustacean organisms known as " copepods " or " oar-feet." They are all small, the largest being under half an inch in length. While there are very many species included in the group of copepods there is one that stands out far before all others in numbers and in importance in the chain of food organisms that links the fishes with the drifting plants. This little animal is unfortunately unknown to most people, because it can only be caught with the aid of a tow-net, and when captured is so small that it is regarded as insignificant. But if ever an animal merited attention it is the small copepod which goes by the Latin name of Calanus fin- marchicus (Plate 45). It is unfortunate that it has no real popular name of its own ; but we shall call it in these pages " Calanus " for short, in the hope that some day Calanus will be just such a common every-day expression as shrimp, crab, or prawn. Calanus is an inhabitant of the cold northern waters, where it forms one of the chief items of food of that most important of all food fish, the herring, and is even sufficiently abundant to aid in the building up of the enormous bodies of two of the Atlantic species of whales, one of which has been described as having been seen " in still weather, skimming on the surface of the water to take in a sort of reddish spawn or brett, as some call it, that at times will lie on the top of the water for a mile together." The spawn or brett is, of course, the little copepod, Calanus, which occurs at times in such swarms as to colour the water, whence it has acquired the name from fishermen of " red feed." It will give you some idea of the numbers of these little creatures if two examples are given of exceptionally heavy catches. In the Gulf of Maine, by towing a conical shaped net with a circular opening of one metre diameter behind the boat for fifteen minutes, over 2,500,000 Calanus Portion of whalebone showing fringed margin (Balaenoptera borealis), y.\. (p. 103). PL 42. Bv pe> mission of J. T. Nichols. I Il6. Sperm Whale, (p. 102). * # si ■<» -» A * /V. 43. DHL. M.v.r.. /u 7 . Zoea Larva of Edible Crab (Cancer fagurus), 10. (pp. 7a, 12;). DRIFTING LIFE 117 have been caught, or enough to fill ten pint-tumblers solid. Again, near Iceland, 200,000 have been recorded from a five minutes' tow. There are many other species of copepods but none to compare with Calanus in importance, although many far excel it in beauty, especially some of those that come from warmer and more tropical climes. Some of these are equipped with the most beautiful array of feathery spines ; others are iridescent and shine with all the colours of the rainbow (Plate 41). But next in importance as food for other marine animals, if not perhaps the most important, are shrimp-like animals that, like Calanus, are rarely seen and almost completely unknown to the world in general. These are known as euphausiids, or " krill," as they are called by the Nor- wegians. They are about an inch and a half in length, but are so abundant that they form a large part of the food of many of the northern fishes, and are the chief food of nearly all of the whalebone whales. Their bodies are quite transparent except for the presence of minute red spots, and they possess enormous black eyes and on this account the fishermen of the west coasts of Scotland call them " Suil dhu " or " black eye " (Plate 45). But they are most remarkable because along the sides of their bodies are numerous little organs that can blaze up into brilliant phosphorescence at will. More will be said about this phosphorescence under Chapter VIII, but suffice it to say that it has been recorded that with -six of these little animals in a jar of water, flashing on their lights, it is just possible to read newspaper print ! There are, besides, many microscopic, single-celled animals dwelling in the drifting community. Of these, perhaps, the group of animals known as Radiolarians are of the greatest interest. These little unicellular creatures nS THE SEAS are noteworthy for possessing solid skeletons on which their protoplasm is supported. Although these skeletons are so minute, they are fashioned in the most beautiful and symmetrical patterns. Almost every conceivable shape is to be found among them, and a few forms from bottom deposits are figured in Plate 21. Another unicellular animal that is very commonly found is the Globigerina, which builds a calcareous shell made up of a number of connecting compartments (Plate 21). So abundant are these two groups, the Radiolarians and the Globigerinas, in certain parts of the ocean, that when they die their skeletons, sinking to the bottom, form characteristic deposits. These are the Radio- larian and Globigerina Oozes mentioned in Chapter III ; in certain localities also Diatom Oozes are formed from the rain of siliceous frustules, the skeletons of the dead diatoms. Enough has been said of the most important members of the animal plankton. Let us now consider some of the more grotesque and unusual forms. Ordinary shellfish or molluscs are heavy lumbering creatures ; yet there are some members of this group of animals that are delicate enough to drift about in the water layers amongst the plankton community without fear of sinking rapidly to the bottom. They are the sea butter- flies, a most fascinating group of marine organisms. Perfectly transparent, some carrying a delicate paper-like shell, they move through the water by the rapid flapping of what appear to be wings. These wings are in reality Fig. 23. Pteropods or Sea butterflies. 1. Creseis acicida. 2. Clio pyrimadata. 3. Limacina ret rover sa. DRIFTING LIFE 119 modifications of the " foot," that solid uninteresting mass on which most shellfish creep (Fig. 23). These animals are also so numerous in some parts of the ocean that their empty shells form deposits on the sea floor, the Pteropod Ooze (see page 56). It is a characteristic of nearly all the plankton animals that live in the upper layers of the sea, that they are almost transparent. If one looks at a tow-net catch that has been & Fig. 24. Sagitta (x 2). Fig. 25. Tomopteris (x 2) placed in a glass jar full of sea water, it is at first very hard to see the various animals on account of their transparency. A very common creature in the catch is the " arrow worm," or Sagitta (Fig. 24) ; this is thought to be a relative of the true worms, like the rag-worm, although it is very unlike them in appearance. It looks just like a little glass rod about three-quarters of an inch in length ; but for all its apparent delicacy it is a very voracious creature. Sur- 120 THE SEAS rounding its mouth, are a number of powerful hook-like teeth, and with these it can seize on its prey, which it rapidly devours. Often it can be seen to have within its stomach one or two of the copepod Calanus, and, when very young herring are abundant, it will capture them and eat them even though they be as long as itself. Another very beautiful plankton worm is the Tomopteris, which has a row of wing-like feet down either side of its body, with which it paddles its way through the water with a curious wriggling motion (Fig. 25). In warm ocean waters there are commonly to be found numbers of animals known as Salps. These are closely allied to the common sea squirts, which lead a sedentary existence fixed to rocks and piers ; but unlike its relative, the Salp lives a free drifting existence in the open waters. It is an inch or more in length, shaped like a barrel, and perfectly transparent. They are not usually to be seen in northern waters, and when found are a fairly reliable indication of the presence of Gulf Stream water. Belonging to the same group of animals is the Pyrosoma, so remarkable for its phosphorescence. The Pyrosoma suffers a curious indignity at the hands of a small crusta- cean, the Phronima, a glass-like creature, all head and eyes. The Phronima eats off all the living portion of the Pyrosoma (which is really a colony of animals) and then retires inside the barrel-like skin that is left (Fig. 26). This interesting beast is sometimes to be seen within its gelatinous home, surrounded by a brood of young, and has been observed to navigate its home about in the water. But the members of the plankton are too numerous to mention here. The best book is nature itself, and therefore at the irst opportunity one should make a small tow-net, take a small boat and row out about a mile from the shore on a calm day, drop the net overboard until it is a few feet ' f / • PL 44. DEL. M.V.I.. / I2Q Megalopa Larva of Edible Crab (Cancer fagurus), X30 (p. 123). The Crustacean Copepod (Calanus finmarchicus), x C a. 12. (p. 116). F/ioto. by Fleming. PI 45- I i2i. Krill, a Crustacean Euphausiid < Meganyctiphanes n->rvegica). Natural size. (p. 117). DRIFTING LIFE 121 below the surface, and then tow it slowly along for a few minutes. The catch will be sufficient reward for the trouble and the sight of the delicate creatures will open one's eyes to the amazing abundance and diversity of this drifting life that peoples the sea in every region of the globe. The few animals that have so far been mentioned con- stitute part of what is known as the " permanent plankton," that is, they live as drifting organisms for the whole of their lives. There are yet other members of the plankton which have adopted this mode of life for only a short period in their life -history. In the coastal regions nearly every Fig. 26. — Phronima in dead Pyrosoma (x 3). animal we find has at some time drifted freely and aim- lessly about in the water layers above the bottom. The young of all the smaller marine animals are, when first they hatch from the egg, extremely small, and in con- sequence have insufficient swimming power to cover large distances, and are unable to cope with the tides and cur- rents. At this stage, they rise up from the bottom, if hatched there, and become members of the plankton. Here they will remain for shorter or longer periods, growing and developing until such time as they have assumed their adult characters 122 THE SEAS or are large and strong enough to seek for themselves their natural home on the bottom. Worms, starfish, crabs, lobsters, oysters, sea squirts and even the majority of fishes in the sea, spend the first days of their lives drifting about in this manner (see Plate 91). One of the main advantages of this mode of life is that it ensures complete dispersal of the young. Many marine animals live fixed to the rocks and bottom, such as hydroids and mussels, or are at most very sluggish and do not move far afield It is obvious, therefore, that if their offspring were born and hatched " at home " the parental abode would soon become so crowded with young and half-grown children that there would be no room to turn. But if the children are sent out to fend for themselves in the upper water layers they will be carried far and wide by the currents, and those that survive will settle to the bottom many miles from where their parents lived. These drifting young of many marine animals are very different in appearance from their parents ; indeed, it is safe to say that if they were shown to a novice it would be impossible for him to say into what they would grow. Because of these extraordinary differences between adults and young, the early stages of many marine animals, when first discovered by naturalists in the plankton catches, were regarded as new species of animals and described and given names of their own. It was only by rearing them, or piecing together successive stages in the life histories from the catches, that the adult into which they were going to develop could be discovered. Who, for instance, without knowing beforehand, would dream of suggesting that the httle animal figured in Plate 43 was the young of the common edible crab ? When first discovered it was not recognized as such and was given the name " Zoea," on this account it is novv known as the zoea stage of the crab. After DRIFTING LIFE 123 hatching, the young crab remains in this stage for some time during which it undergoes several moults. Finally, it suddenly moults into a stage quite unlike the zoea and more like the adult crab ; albeit when first found it was not recognized as such and was labelled " megalopa " (Plate 44). From this stage it moults into a perfect little crab. So we see that in the life-history of one animal, the crab, the successive stages are so different that no less than three animals have been thought to exist, the zoea, the megalopa and the crab, which were in reality all crab. These animals which appear for only a short period in the drifting community form the temporary members of the plankton, and their appearance in the upper water layers depends, of course, on the times at which the parents spawn, and this gives rise to the seasonal changes in the plankton mentioned on page 244. It is chiefly in the coastal regions that these temporary drifters are found because of the great wealth of bottom life in the shallower regions. Distribution of Plankton Having given the reader some idea of what composes this drifting life or plankton, let us now examine what is known of the distribution of the plants and animals that form it in the sea. To begin with it must be realized that besides the immense horizontal area of all the oceans and seas of the world, all of which are inhabitable, the waters also have a considerable depth, the average of which is as rnuch as two miles. Seeing then that organisms that live in the water layers themselves are free to move up and down, as well as in a horizontal direction, it is evident that we have two types of distribution to deal with, namely, horizontal or geo- graphical distribution, and vertical or depth distribution. Much work has been done on the geographical distribution 124 THE SEAS of the animals and plants of the plankton by large oceano- graphical expeditions which have been sent out by different countries during the last fifty years, and it is fairly estab- lished that all the waters of the globe are inhabited by them. Just as on land, the various species of plants and animals have their own distribution, some living in tropical climes and others in the far north or south, so it is noticed that many members of the plankton are to be found only in certain regions and may give the catches made in those localities a characteristic appearance. Amongst diatoms, for instance, there are species that are normally only found in coastal regions, others that occur only in the open ocean waters of tropical and sub-tropical regions, and others again that characterize the catches of northern waters. But while species of these little drifting plants are to be found almost anywhere in the oceans all over the world, the cool temperate and the cold arctic and antarctic waters are now known to carry this diatom life in quantities far exceeding the other regions. The waters that bathe the shores of the British Isles, of Holland, Sweden, Denmark, Norway and Iceland, on the east, and of Greenland, New- foundland and the Gulf of Maine in America on the west, possess a richer pasturage of microscopic plant life than is to be found in any other locality in the North Atlantic Ocean. The significance of the plankton plants in the general economy of the sea will then become at once apparent, when it is realized that it is precisely these regions, the North Sea, the Baltic, the Norwegian and Greenland Seas, and the banks of Newfoundland, that give rise to the greatest fisheries in the Atlantic, and in fact in the whole world. Dependent on the diatoms are, of course, the small animals of the plankton, and it is natural to suppose that where the plant life is most abundant there will be found Dinoflagellates (Ceratium tripos), xca. 20. (p. 114). / 124. Spring Diatom Plankton X44. Chiefly Coscinodiscus and Biddulphia. (p. 113). n. 47 Chart showing distribution of Plankton in North Atlantic. Darkest green, most abundant. Blue, scarcest. (p. 126). DRIFTING LIFE 125 the greatest quantities of animal life. Such indeed is the case. The abundance of that little creature, the Calanus, and of the shrimp-like euphausiids, has already been dwelt upon and they too are to be found chiefly in the regions outlined above. But, while the plankton is present in greatest quantities in these northern waters, it is remarkable that compared with the plankton of the warm and tropical regions the numbers of different kinds of animals are extremely few. The catches made in the northern waters can almost be described as monotonous in composition, that is, although they are so large, they will be made up of only comparatively few species of animals. To the collector then the catches made in warmer regions prove vastly more interesting on account of the wealth of different species to be found there, even though they be present only in small numbers. One example is sufficient to show how marked this difference is. Of roughly three hundred species of copepods, while eighty per cent, are to be found in the warm regions, only five per cent, are found in the cold northern region and only two per cent, in the southern. This phenomenon holds good for all the different groups of animals represented in the plankton. Apart from the excessive abundance of drifting life in the colder waters, it is to be noticed that everywhere the coastal regions are considerably richer than the waters of the open ocean. The central regions of the North and South Atlantic oceans are the poorest in plankton life, that is the areas lying a little north and^a little south of the equator. Actually in equatorial regions the plankton is a little richer. The deep blue Sargasso Sea, in the centre of the North Atlantic, is probably as barren as any region in the world. There are several factors that together are responsible for the abundance or poverty of plankton life, but of greatest importance is the presence or absence of 126 THE SEAS certain nutrient salts dissolved in the water on which the drifting plants depend for their growth. In localities where these bodies are richest the plants will be most abundant, and, consequently, the animals which depend on them for their food supply. A discussion giving the basic principles that underlie this plankton production will be found in Chapter XI ; it would be out of place to enlarge on the problem here until the reader has made himself acquainted with some of the properties of sea water that are outlined in Chapter X. In Plate 47 is given a chart of the North Atlantic Ocean and surrounding seas, in which the density of plankton life is shown diagrammatically. The richest areas are shown by the green colouration, the regions of greatest density of life being the deepest green. The green colour gradually shades off into blue which can be taken to represent a poverty of plankton organisms. Having dealt with the geographical distribution of the plankton, let us turn now to consider at what depths this drifting life is to be found and where it is most abundant. Taking the plants first, consisting of the Diatoms and the Peridinians, collections made by research vessels from different depths show that it is only the upper water layers of the sea, from the surface down to about one hundred fathoms, that contain drifting plant life in any quantity. A moment's thought will satisfy anyone that this must of necessity be the case, since all plants are dependent on the sun's light for their life and the deeper we go into the water the less light is there present. In fact it has been shown that at little more than ten fathoms in the English Channel off Plymouth the amount of light present is already similar to that in the heart of an English wood. In the clearer open waters of the ocean the light can perforce penetrate deeper, but it is certain that at a depth of one DRIFTING LIFE 127 hundred fathoms it is already so dim that few plants can live in a healthy condition. More will be found on the subject of the light beneath the sea surface in Chapter X. Although there is a depth limit at which the normal life of the plant becomes impossible, yet at times collections may show their presence at still greater depths, the likelihood is however that these plants will be in a dying condition and will have sunk to those depths under their own weight. Now animals, as we know, are not so dependent on light directly ; in fact there are many animals on land that seem to prefer darkness and are nocturnal in their habits, shunning the light of day. The same applies to the sea animals, there are many that live in the dark deep layers ; in fact, recent research has shown that there is no depth at which some plankton organisms do not exist. All the animals are not, however, found evenly distributed in the water layers from top to bottom. Each animal seems to show a definite preference for some particular depth region ; for instance, there are some that live always in the upper fifty fathoms or so, rarely penetrating to deeper levels ; others again will never be caught between one hundred fathoms and the surface, but always live deeper. On account of these differences in depth distribu- tion, exhibited by the various animals of the plankton, we find that catches made from different levels are each quite characteristic and distinct one from another in their composition, in the same way that collections from different geographical regions are each characterized by the presence of those organisms that are prevalent in the locality in which the catch was made. It is a general fact that in the daytime the animals that live in the layers quite near the surface are very few, this is especially the case in the summer months. For instance, 128 THE SEAS off our coasts it is necessary in the daytime to fish at a depth of ten to fifteen fathoms in order to get the largest catches. In the open ocean the depths at which the greatest assem- blages of plankton animals are found are generally consider- ably deeper. Now, as has been said, this is the case in the daytime ; but it is a remarkable fact that at night matters are quite different, and a curious change comes over the plankton distribution. On the approach of dusk, just after the sun has set, all the animals begin to swim in an upward direction, so that by about nine or ten o'clock, in the summer months, the surface layers, so barren in the daytime, have been enriched by animals that have swum up from the deeper levels. In Plate 49 is shown the actual results of collecting at different depths in daylight and at dusk. It can be clearly seen that whereas in the daytime the layers down to about five fathoms are very poor in plankton compared with the deeper levels, at dusk the two upper collections are quite as large as the deeper ones. It will be noticed that the bottom catch at dusk is also greater than that taken in the daytime, this is because of the addition of large quantities of animals that have come up from deeper levels quite near the sea bottom ; in fact, there are certain small creatures that live actually on the bottom itself in the daytime which move upwards towards the surface at night ; they may never actually have time to reach the surface itself, because at dawn all the animals begin to move downwards again to take up their abodes at their usual day levels (Plate 48) . We all know that the herring fishermen only shoot their drift nets at night. This is because at night, the herring, like the plankton animals, also come to the surface. This phenomenon of upward movement at night, or vertical migration, throws considerable light on why the different animals prefer certain depths in the daytime. It seems almost certain that the causes of the up and V- JQ r SB n CD CD (a n a 3 Co' CD - OQ (A, O rj ^ '\ p 1SI !-<■ <-> o J> H 1") In "X. w o 3j 5 MJ n 4 i — i 3 ^" ** r» CD O Ifl 2^ ~ n ** P Ov > to 3 CD p 3 c M p c (/! c+ CO • e-f Di c> X"S p P e+ si f ~ <-i 00 CD ^. * to en ?! •— S u *. t— fv, p o 2C ho > 2 > ir > 2 V x x- x --* ;-> $7 wV - " x-.f "- r -) --:. > > Z 2 fev w --> j.: jT >.- : v.-:-i-:--- y -» IOa.m sid %\ \ v ->-^.V J -*---*--."*--;v - . z o o z S v * >^iv-: •.v.-.-f'-* ~l 3 — ; - CO *" ■* iq ^ h y cN « « CO to CC CI 0> o CO O XI CD a >4 4-= - 1 — i ctf 01 Pi — • C a O >, =t-C 3 •"9 o " n c a .— •^H fl w c3 etf d : f, CO s •i— « 5- - C CD — ■< cd cd G £ o -» •*-> o ^ cd pW fl ■f r-» OS *^ ce - CO Is Pi <►* 3 >5 cd r\ tc "0 4-> ... CO CD ' | P 4-= ~ ^^ ^, z rt •rt z « - *_ i o r— i *■■* > .r - ? M o j: *d 4= — « cd P CO S3 ft DRIFTING LIFE 129 down movements are the changes in the strength of light experienced by the animals ; this is further confirmed by the fact that many animals live deeper in the bright sunny days of midsummer than they do earlier in the year, when the light entering the water is not so strong. From this, then, we see that most of the plankton animals tend to avoid strong light and prefer the dimly lit conditions of the deeper layers ; at the same time all the evidence goes to show that each animal shows a preference for a certain strength of light to which it is adapted. Towards the evening they follow the " optimum " strength of light towards the surface as night draws on, but in the dark there is no light stimulus and they are free to move anywhere (see Plate 48) ; at dawn they once more pick up their optimum intensity and move downwards as the daylight strengthens. It is also a curious fact that the older an animal becomes the more it shuns the light, and it is generally the younger stages that are found near the surface. This is not, however, always the case ; the early stages of the Velella or " By the wind sailor " are found at very deep levels, while the adult, as mentioned below, floats right on the surface of the water, where it is blown hither and thither by the winds. Adaptations for Suspension In considering this drifting life it may have struck the reader that it is a curious thing that all these organisms, many of which, such as the diatoms, are practically in- capable of any independent movements, should remain suspended in the water. Why do they not sink rapidly to the bottom of the sea ? The answer to this question rests in the demonstration of some of the most remarkable structural adaptations, which fit most of the plankton organisms to the conditions under which they live. Their main requirement is that by some means or other 130 THE SEAS they should be able to maintain the level at which they are drifting. The means by which this capacity is attained are many and varied, the general aim being to reduce the organism's specific gravity directly until it is the same as or less than, that of the surrounding sea water, or to obtain a similar effect in a more indirect manner. Species that have achieved the power of becoming lighter than water are comparatively few in number. The chief examples occur in a group of jellyfishes known as Siphono- phores. The name of the Portuguese Man-o'-War, a stinging jellyfish, is well-known to all. This animal possesses a specially designed " float " into which gas is secreted by a specialized gland. This gas-filled reservoir projects above the surface of the sea and acts as a sail, by means of which the wind blows the jellyfish along, trans- porting it from place to place with tentacles extended in all directions ready to seize any unwary prey that they may touch, instantly paralysing it with their batteries of stinging cells. Another closely allied form is the Velella, or " By the wind sailor " (Plate 50). This likewise has a small gas-filled sail. The animal, when seen alive, is of a fairy- like delicacy, possessing this transparent, papery sail, situated above the centre of the body : on the under surface is the " mouth," centrally placed and surrounded by delicate mobile tentacles of a sky-blue tint. These little creatures, which reach a size of one or two inches in length, are natives of the warmer ocean waters and the Mediter- ranean. However, they are occasionally to be found stranded along the western and south-western shores of the British Isles after prolonged southerly winds, that have wafted them speedily along the surface of the sea from the warmer latitudes. In order to reduce the specific gravity as nearly as possible to that of sea water, many animals make use of fats and DRIFTING LIFE 131 oils formed in their bodies. These oils, being lighter than water, tend to diminish the weight of the animals that contain them. Those round globules of oil mentioned on page 80 as distinguishing characters in the drifting eggs of so many fishes probably tend to help in keeping the egg suspended in the water (Plate 29). But of all the modes of obtaining buoyancy the indirect methods of producing the same effect as a reduction in specific gravity are the most wonderful. If we drop a stone into water, and watch it sinking, we shall notice that its sinking speed is considerably less than if it were falling through air only. The speed is reduced by the frictional resistance set up by the stone as it moves through the water. This property of resistance to the movement of a body is known as " viscosity." Some liquids are naturally more " viscous " than others ; they are said to have a high viscosity. Treacle is a very viscous fluid ; many oils also are highly viscous. Thus the frictional resistance to a falling body would be greater in treacle than in sea water. Now, of course, the total frictional resistance experienced by a falling body depends on the amount of area exposed against the fluid through which it is moving. We know that a slate takes longer to sink flatways than if it is on edge. Clearly then, if the frictional resistance can be made infinitely great compared with the actual weight of the body itself, a stage will be reached at which it would counteract the force due to gravity and the body would no longer sink, but become suspended in the water. The structure of many plankton organisms shows an attempt to increase the frictional resistance, which evidently succeeds in keeping the organism almost suspended in the water. Although they cannot completely counteract the effect of gravity they can, nevertheless, come very near it so that sinking is extremely slow. K 132 THE SEAS The effect is brought about either by greatly increasing the surface area by means of long spines or feather-like projections, or by extreme flattening so that the creature is like a leaf and when lying horizontally in the water is prevented from sinking except at a very low speed. The spiny and hair-like processes on many of the plankton diatoms depicted in Plate 88 serve this purpose in nature. The diatoms themselves are so very minute that the many spines must increase their surface area comparatively to an enormous degree, and they become literally suspended in the water so slow is their sinking speed. The rate at which a diatom will sink through the water has been measured, and, of course, varies for the different species according to their shape. Let us take as an example a species of Chaetoceros (Plate 88, fig. 3) ; this takes on an average about four and three-quarter hours to sink three feet, or eight minutes an inch. This was in still water, but in nature, near the sea surface, owing to wave action there are continual little swirls and eddies which will bring it up or down at a much faster rate than it sinks. Now, the animals, of course, on account of their larger size and weight will sink considerably faster than plants, but they also possess the power of locomotion, by which they can regain their level. The presence of spines and feathery processes on the animals' bodies, by slowing down the rate at which they sink, prevents them from having to be continually on the move to keep up in the water. If a living plankton animal is watched it will be seen to make a rapid upward movement by swimming and will then rest while it sinks only slowly through the distance that it so rapidly moved up through. An extreme example of flattening in an animal is to be found in the case of the larva of the common crawfish or rock lobster. This is, of course, a different species from the O \vj 3 P« \ V) P 1— '• O • "\ a s to 9= CD o W ^ V H I CO KH *<; &> '/) X r. — « J X CO 00 X £ a x £< 0) — C/l O - 6 o K o > i- od •-J 85 DRIFTING LIFE 133 lobster that is most eaten for food in the British Isles. It is the " Langouste," which is, however, much preferred as a delicacy by the French. The larva is a most curious sight (Plate 51). The body, measuring about a quarter of an inch across in the oldest individuals, is quite transparent and flattened like a piece of paper, while the long feathery legs and projecting eyes stick out all round and effectively aid the purpose of the flattened body in preventing the animal from sinking through the water. It is not necessary to resort to experiment to prove that these structures are an aid to suspension in the water. It is known that the viscosity of water varies with the tem- perature, a rise in temperature lowering the viscosity to a marked degree, so that the resistance the water offers to a body sinking in it is considerably lessened. If, then, an animal or plant lives normally in warm water it will need to be extra well equipped with hairs and spines compared with its cousins who live in cold waters. Now, it is a remarkable fact that by the side of organisms from cold and temperate regions those from the tropics exhibit to a much greater degree these structural excres- cences, making many of them bizarre and grotesque in appearance. It has even been found that the same species of Peridinian may differ from place to place in form. For instance, in two individuals of the same species, the one from tropical seas has very much longer horns than those of its relative from our own colder waters. In these few pages an attempt has been made to put before the reader some of the main features of that remark- able community of drifting life, the plankton. Their im- portance cannot be over-estimated and plankton study 134 THE SEAS forms a considerable part of modern investigations into the resources of the sea. This chapter has been devoted mainly to describing what the plankton is and where and how it lives, but in a future chapter will be given further information of the part played by these small organisms in the watery world, in the light of recent research. CHAPTER VI Boring Life Of all the creatures which inhabit the sea, few are more interesting than those which bore into wood or stone and live within the burrows they construct. None certainly do so much damage as the wood borers, notably the dreaded Shipworm which, since the dawn of history, has been re- corded as the cause of grave damage to wooden ships. The galleys of Greece and Rome in classic times, those of Venice in the Middle Ages, and Drake's famous Golden Hind, all were riddled with the burrows of the Shipworm which has even threatened, by its attack on the dykes, the very existence of Holland. Although in modern times the steel hulls of the majority of ships have nothing to fear from it, the Shipworm still does great damage to wharves and piers made of wood ; so great indeed that both in this country and in the United States extensive investiga- tions have been set on foot in the hope of discovering some means of combating its ravages and those of its accomplices. Wood Borers The wood borers may be divided into two groups, those which are molluscan and those which are crustacean. The former are the more important and we will consider them first. The Shipworm is the most outstanding of these for, in spite of its common name and naked, worm-like body, it is really a bivalve mollusc which has taken to a very extra- ordinary mode of life and become, as a result, very unlike i35 136 THE SEAS its relatives such as the cockle and mussel. When taken out of its burrow and examined, the Shipworm, as shown in Figure 27, is seen to consist of a long, naked body with at one end a pair of small, peculiarly shaped shell valves, and at the other a pair of delicate tubes or " siphons." The former is the front end of the animal, it lies at the inner end of the burrow and possesses the boring organs and the mouth. The siphons are the only part of the animal which projects from the burrow ; they are instantly withdrawn when they are touched and the opening of the burrow — not much larger than a pinhead — is closed by the pushing forward of a pair of shelly, club-shaped "pallets" which are fastened to the hind end of the animal about the base Fig. 27. — Shipworm (Teredo) out of burrow (slighly reduced) ; e.s. and i.s., Siphons for taking in water; /., foot; p., pallets for closing opening ; s., shell. of the siphons. Within the body the organs are greatly extended as a result of the elongated shape of the animal. The principal organs are near the front end, but along the entire length there stretches a cavity divided down the centre by a delicate lattice-work of tissue. Water enters the body of the animal through one of the siphons and passes into one of these divisions, it is then filtered through the lattice-work, leaving behind it any food particles which are carried to the mouth, passing into the second chamber from which it is expelled by way of the other siphon. In this way the animal obtains both oxygen for respiration and a certain amount of food. BORING LIFE 137 Now let us consider how it bores its way through the wood. To understand this it is necessary to refer to Figure 28 which shows the position of the boring organs while they are in operation. Although many theories have been advanced on the subject, it is now certain that boring is carried out by the action of the small shell valves, which are specially adapted for this purpose. As shown in Figure 29, they are globular and can be divided into three regions, a hinder portion in the form of a broad wing which is known as the auricle, a middle portion which forms the t Fig. 28. — Head end of Shipworm (Teredo) lying in burrow ; b. body of worm : /., foot ; f.s., fold of skin above shell for gripping wood ; s., shell t., edge of burrow. major part of the shell, and a portion in front of this which only extends for less than half the width of the middle portion and is then cut away sharply at right angles. The surfaces of these two latter regions arexovered with sharply- pointed ridges, those on the former passing diagonally across the anterior third of its surface, while those on the latter run parallel to the sharply cut lower margin. Trans- ferring our attention to the inner surface of the shell, we see that there are two knobs, one at either extremity of the middle region, while from the upper of these there hangs i 3 8 THE SEAS down a long process known as the apophysis. The two halves of the shell are attached by the two knobs so that the valves are able to rock backwards and forwards upon these two points, first the hinder and then the frontal regions coming closer to one another. This rocking process takes place regularly in life, the motive power being supplied by two pairs of muscles, one of which runs between the hinder regions of the shell and the other between the frontal regions. From between (JL.CL CL.CLcL. p. cut V.CL. f'iG. 29. — Shell ol Shipworm (Teredo). Inner and outer views. a.ad., and t>.ad., attachments of muscles; ap., apophysis; a.L, m.l., and p.l., parts of shell ; d.a. and v.a., articulating knob. the valves in front there projects a little round sucker, which corresponds to the " foot " or organ of movement in such animals as the cockle. The muscles which work the foot are attached to the apophysis and by their con- tractions enable it to grip the wood at the head of the burrow as shown in Figure 28. At the same time a flap of skin which overlaps the shell above presses the animal firmly against the wood in that region. With the shell pressed tightly against the head of the burrow in this manner it is easy to see how boring takes place. By the contraction of BORING LIFE 139 the hinder muscle, the frontal portions of the shell are drawn apart so that the sharp ridges with which they are covered scrape off the surface of the wood while at the same time the ridges on the middle lobe widen the opening behind. The small muscle between the frontal lobes now comes into play and by contracting in its turn brings the frontal lobes together again, which is a much easier operation because the surface of the wood offers no resistance to movement in this direction. The foot then loosens its hold and moves a short distance to one side, when the same process is re- peated. This goes on time after time until the shell and front half of the body have twisted completely round — the hinder end cannot do this because it is attached to the burrow in the region of the pallets — when the movement is reversed. As a result of this continuous working of the shell and twisting of the front end of the body first in this direction and then in that, the inside of the burrow becomes perfectly smooth and circular in cross section. Immediately behind the shell the surface of the wood becomes covered with a layer of shelly substance which the animal produces in the naked region of its body. The whole process of boring is a model of mechanical efficiency which is not to be surpassed by any member of the animal kingdom. The mouth of the Ship worm lies just above the sucker- like foot. Into it pass, not only the tiny particles collected from the sea water which enters through the siphons, but also the minute fragments of wood which are cut away by the boring organ. All of these have to pass through the gut of the animal before they can be discharged into the sea water by way of the second of the siphons. It has long been a matter of dispute whether the Shipworm can actually digest the wood or whether this passes through the body unchanged, but recent investigations seem to show quite definitely that the animal does obtain a considerable 1 40 THE SEAS amount of nourishment from the wood, the fragments of which are acted upon by digestive juices, and sugars produced which can be absorbed by the tissues. There is, moreover, a large extension of the stomach which is always filled with shavings of wood and seems to be well adapted for the storage of such a slowly -digested substance. Once encased in its burrow no Shipworm can ever leave it. As we have seen the opening of the burrow is not much larger than a pinhead whereas, within, the burrow widens out quickly and may, in the common species, be some half an inch in width. Moreover, the animal is actually attached to the edge of its burrow in the region of the pallets. If a Shipworm is taken out of its burrow, no matter how carefully this is done, the animal cannot make a new burrow for itself. Since this is the case how does it happen that new wood becomes infected, as it does very quickly if conditions are favourable for the growth of the Ship- worm ? During the spring and summer especially, the eggs and sperms are discharged in immense numbers into the sea by way of the second of the siphons. There the young Shipworms develop and at this early stage of their existence they are exactly like young mussels or similar bivalves. After a short time a pair of tiny shell valves appear which entirely enclose the body and from between which a crown of small hairs or " cilia " can be protruded by means of which the little animals are able to swim about, in exactly the same manner as the oyster (Fig. 59). This freely swimming ' larval " Shipworm is shown in Figure 30. It is not known for exactly how long the Shipworms remain in this state, but they can probably do so for several weeks, and during this time they may be carried for great distances by ocean currents or wind drifts. This early stage in their existence is the only time when the Shipworms are able to move about freely in the sea and BORING LIFE 14 1 when they are able to infect new timber. It has been shown by experiments that these larval Shipworms are attracted by wood or by an extract of wood made in alcohol or ether, and, as a result of this attraction, they remain on its surface should they chance to drift there, whereas they do not remain on any other hard surface such as stone. Very soon after it has alighted on the surface of the wood, the larval Shipworm begins to change ; first of all it loses the crown of tiny hairs whereby it swims, developing in its stead a long, tongue-shaped organ or foot by means of which it moves about on the wood until it finds a suitable place to begin boring. This having been found, it com- mences operations, first, it is said, covering itself with fragments of wood or other particles. , 1, Both shell and foot are quickly con- vnXuVAHiiIi l// verted into those of an adult Ship- \||wj worm and the animal begins to bore its way quickly into the wood, the pallets and the siphons are formed and remain attached to the burrow near its opening while, as the burrow grows longer and longer, the naked body elongates until the long, worm- like appearance of the adult is gained. Larva of Shipworm (Teredo) It is usual for them to enter the wood at right angles to the grain but they soon turn in the direction in which this runs and excavate a long burrow. However many animals there may be -in a piece of wood, the burrows never run into one another, to avoid this they will twist and turn and interlace with one another in the most intricate manner, as an X-ray photograph of a piece of heavily infected timber shows extremely clearly (Plate 54). Owing to the small size of the openings of the burrows the wood may be heavily infected and show no in- i 4 2 THE SEAS dication of this fact ; finally, however, it crumbles away (Plate 53) . Shipworms do not live long, a year or perhaps two years. When they reach a certain age or when there is no further wood to bore into, they continue the shelly casing over the front end of the burrow and remain quiescent within this, taking such food as they can obtain from the water, until they die. The action of the Shipworm in our own waters is much slower than that of the tropical species. Here, a piece of untreated wood is seldom attacked until it has been in the water for at least a year, whereas in the South Seas wood will become infected in a few weeks and after six weeks show a similar degree of infection to wood which has been exposed here for eighteen months. There are many different species of Shipworms but they all belong to two genera, one called Teredo and the other Bankia. All the British Shipworms belong to the genus Teredo, of which three species are found in our seas, and none of them construct tubes longer than about eighteen inches, the majority much less, but there are tropical Shipworms which are much larger, one, the " giant Teredo," being reported to attain a length of two yards and become as thick as a man's arm ! The members of the genus Bankia are inhabitants of the tropics and are easily dis- tinguished from Teredo by the structure of the pallets, which are paddle-shaped in Teredo but are long and feather- like in Bankia. There are two other bivalve molluscs which live in wood. One, known as Xylophaga, is commonly found in floating timber in temperate seas, it has a shell very like that of Teredo and it bores in the same manner, but there is no elongate body, the siphons project from the hind end of the shell which completely encloses the body (Plate 52). The burrows are short, seldom more than one and a half inches BORING LIFE 143 deep, and are not lined with shell. Although the burrows are apparently made in the same manner as Teredo, by- means of the shell, there are no pallets and the animals cannot digest the wood. The same is true of the third type of Molluscan borer, Martesia. This is a native of the tropics and is very like a small mussel in appearance ; its burrows are not generally more than two and a half inches long and one inch wide, i.e., the size of the animal which lives within them. Neither of these animals has attained the efficiency of Teredo as a borer and both seem to seek the wood mainly as a means of protection, and not also as a source of food as does the Ship worm. There are a number of Crustaceans which habitually bore into wood. One of these stands out pre-eminent, like the Shipworm amongst the Molluscan borers, on account of its ubiquity and the great damage that it does. It is the Gribble, Limnoria lignorum, a little creature resembling a miniature woodlouse, usually between one eighth and one sixth of an inch long and with a semi-cylindrical body divided into segments (Plate 52). It has seven pairs of short legs each ending with a sharp, curved claw by means of which the animal holds on to the sides of the burrow. Beneath the hinder end of the body there are five pairs of legs each carrying two broad plates which act as gills, these keep up a continuous movement during the life of the animal and so constantly renew the water needed for respiration. The animal bores into the wood by means of a pair of stout " mandibles, " one on either side of the mouth. These are not identical, for the one on the right has a sharp point and a roughened edge which fits into a groove with a rasp-like surface in the left mandible, the whole providing a " rasp-and-file " combination, as shown in Figure 31. Unlike the burrows of the Shipworm, those of the Gribble are always the same width throughout, so that it appears 144 THE SEAS as though the animals must come out of their burrows as they increase in size and start new ones. Another way in which the Gribble differs from the Shipworm is that it is always the fully -grown animals which start new burrows. To do this, they first of all hollow out a groove along the surface, keeping in the soft part of the grain, and then pass by a very easy incline into the wood. Usually the Fig. 31. — Mandibles of the Gribble, Limnoria, showing " rasp-and file combination, greatly enlarged (after Hoek). burrows are not deep ; the need for obtaining a constant supply of fresh sea water probably controls this, and, though they have been found as deep as three-fifths of an inch below the surface, they do not usually penetrate foi more than a third of this distance. The burrows are often three-quarters of an inch or more in length. It is easy to follow the course of a burrow from above be- mm /Y. 52. A;i44- Left: above, Boring Crustacea, Lintnorla (slightly enlarged). below, Boring mollusc, Xylopkaga. Right: Wood bored by Limneria; notice how the hard knot has not been bored. Natural size. (pp. 142, 143, 146). I £ 'jk-fc-Vtij in Pi- 53- A'i45- Wood bored by Shipworm {Teredo norvegica), x\. (p. 142). BORING LIFE 145 cause the roof is perforated by a regular series of fine holes (Fig 32) like minute " man-holes " which probably help the animals within to maintain the necessary circulation of water within the burrow. The female appears to do most of the work, for, though there are usually a pair of Gribbles in each burrow, the female is invariably at the head end. When the males are touched they will crawl backwards slowly out of the burrow, but the females on similar provocation will brace themselves firmly against the side of the cavity by means of their broad tails and successfully resist attempts to pull them out. Probably Entrance to burrow Respirators AfaJe Sea, P'ts FemoJe water Fig. 32. — Diagram to show method of burrowing of the Gribble, Limnoria. Slightly enlarged. they brace themselves in this manner when boring, for essentially the same purpose as the Shipworm uses his sucker-like foot. Instead of the myriads of minute eggs which the Ship- worm discharges, the female Gribble only produces about twenty or thirty eggs but she takes very good care of these and incubates them in a special brood-pouch " beneath her body. There the developing Gribbles remain till they have reached a relatively large size and when they finally hatch out they are about one-fifth the size of their parents and are fully formed animals capable of proceeding immediately about their life's business of boring. They i 4 6 THE SEAS do this by hollowing out little burrows from the side of the parent burrow, and never by leaving the burrow and beginning a new one in fresh timber. As we shall see later, the fact that fresh timber is always infected by adult and not young Gribbles is unfortunate from the point of view of the protection of wood. The ravages of the Gribble are always clearly apparent on the surface of the wood which is gradually rotted away until the outer layer falls off ; the Gribble is then able to penetrate still deeper and so, layer on layer, the wood is destroyed (Plate 52). The Gribble is always especially abundant in such structures as pier piles about low-water mark and here it eats deepest into the wood, which tapers away and finally breaks through at this point. So numerous are they in badly infected wood, that between 300 and 400 Gribbles have been collected from a square inch of timber. There is no evidence that the Gribble can actually digest the wood though it certainly swallows large quantities of the fragments which are bitten off by its powerful mandibles. It has been found boring in the insulating covering of submarine cables so that clearly it does not depend on wood to the same extent as does the Shipworm which has never been found anywhere but in wood. Some of the other crustacean borers and the mollusc Xylophaga have also been found in the insulation of cables and all these creatures appear to bore for the protection it affords them and not for the purpose of obtaining food. There are several other crustacean wood borers. The most common of these is a creature known as Chelura terebrans (it has no common name, unfortunately) which is slightly larger than the Gribble, and flattened from side to side, being a relative of the common sand-hoppers of the shore. It usually works along with the Gribble, but nearer the surface of the wood, and is almost as world- n. 55. / 147. Boring Molluscs and Sponge.