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The Open Sea: The World of Plankton
Alister Hardy
The New Naturalist editors believe this to be the greatest general work on the subject ever written.Professor Alistair Hardy is truly obsessed by animals of the sea – devotedly enthusiastic about the nature of their adaptations and life histories, brilliantly critical in the examination of their mysteries, acutely lucid (and at the same time highly artistic) in his descriptions of them in his arresting plates.To describe the relatively unknown and mysterious world of plankton is a task that the greatest of marine zoologists might boggle at. Yet the plankton is to the sea what vegetation is to the land. The study of plankton is a complex discipline which few amateur naturalists have had the privilege to enjoy. Never before has such a synthesis of knowledge been attempted in a community of animals so mysterious, yet so important. Professor Hardy has grasped this problem in a new and exciting way; and at least the common reader can discern the pattern of life that dominates two-thirds of the world’s surface.
Collins New Naturalist Library
34
The Open Sea – Its Natural History:
The World of Plankton
Alister C. Hardy
EDITORS: (#ulink_e6e54ff0-6414-5b73-9650-9f68a5f32df3)
JAMES FISHER, M.A.
JOHN GILMOUR, M.A.
JULIAN HUXLEY, M.A., D.SC, F.R.S.
L. DUDLEY STAMP, G.B.E., D.LITT., D.SC.
PHOTOGRAPHIC EDITOR:
ERIC HOSKING, F.R.P.S.
The aim of this series is to interest the general reader in the wild life of Britain by recapturing the inquiring spirit of the old naturalists. The Editors believe that the natural pride of the British public in the native fauna and flora, to which must be added concern for their conservation, is best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research. The plants and animals are described in relation to their homes and habitats and are portrayed in the full beauty of their natural colours.
Table of Contents
Cover (#u82c38694-9eb0-577d-9fa4-3ab644beba53)
Title Page (#ua3d0bf02-d61d-5172-9d69-2d2f0063da7b)
Editors (#u33b5931d-34cc-591f-8e03-74f51ec6208b)
Editors’ Preface (#u15a5dc3c-e074-5bd3-b86f-385281f19702)
Author’s Preface (#ucb71242a-0b06-57a9-80b3-2ac33fa25869)
CHAPTER 1 (#ubcb89914-6a12-5f9a-961d-a32f7a64e31a)
INTRODUCTION
CHAPTER 2 (#u3397ab9e-de11-5382-badf-93137316dee4)
THE MOVEMENT OF THE WATERS
CHAPTER 3 (#uf28d2542-dfd2-59a4-8652-470397ba0dae)
PLANTS OF THE PLANKTON
CHAPTER 4 (#uf9e2ac0e-843a-5efb-963b-16fe015b8f29)
SEASONS IN THE SEA
CHAPTER 5 (#ub9d1ef9b-f5a9-51ce-96c7-37d3c4e2a8de)
INTRODUCING THE ZOOPLANKTON
CHAPTER 6 (#litres_trial_promo)
LITTLE JELLY-FISH AND LESSER FORMS OF LIFE
CHAPTER 7 (#litres_trial_promo)
SIPHONOPHORES AND THE LARGER JELLY-FISH
CHAPTER 8 (#litres_trial_promo)
MORE ANIMALS OF THE PLANKTON—BUT NOT THE CRUSTACEANS
CHAPTER 9 (#litres_trial_promo)
THE PLANKTONIC CRUSTACEA
CHAPTER 10 (#litres_trial_promo)
PELAGIC LARVAL FORMS
CHAPTER 11 (#litres_trial_promo)
THE PUZZLE OF VERTICAL MIGRATION
CHAPTER 12 (#litres_trial_promo)
LIFE IN THE DEPTHS
CHAPTER 13 (#litres_trial_promo)
PHOSPHORESCENCE AND PHOTOPHORES
CHAPTER 14 (#litres_trial_promo)
SQUIDS, CUTTLEFISH AND KIN
CHAPTER 15 (#litres_trial_promo)
PLANKTON AND THE FISHERIES
Glossary (#litres_trial_promo)
Bibliography (#litres_trial_promo)
Index (#litres_trial_promo)
Colour Plates (#litres_trial_promo)
Plates in Black and White (#litres_trial_promo)
Copyright (#litres_trial_promo)
About Publisher Page (#litres_trial_promo)
EDITORS’ PREFACE (#ulink_472ac1a3-e0a2-5cc4-97be-95d34706cd70)
PROFESSOR HARDY began his marine biologist’s life over a third of a century ago on his return from service in the first world war. After Oxford and a scholarship to the Stazione Zoologica at Naples, he soon became a member of the Fisheries Department in the Ministry of Agriculture and Fisheries; and, in the middle ‘twenties, served as Chief Zoologist to the R.R.S. Discovery expedition, to the Antarctic seas, making a special study of plankton. His subsequent professorships—first at University College (now the University of) Kingston-upon-Hull; next at Aberdeen; and since 1945 at the University of Oxford—have brought him the highest academic status and honours, but have not kept him away from his beloved sea. In the closing stages of the writing of this volume, as the editors well remember, he was correcting the typescript, and completing his unique and wonderful colour illustrations, on the deck of the latest Royal Research Ship, Discovery II, scanning the contents of each netting or dredging, sketching new or rare creatures of the sea before their colour faded, applying himself to his research with an enthusiasm excelling that of most naturalists of half his age.
If the editorial board were asked to select from Professor Hardy’s many scientific qualities that which has contributed most to the creation of this extraordinary book, they would perhaps settle for enthusiasm. Throughout The Open Sea it is quite apparent that he is devotedly obsessed by, and interested in, animals; he is eternally curious about the nature of their adaptations and lives, brilliantly critical in the examination of their mysteries, acutely lucid and at the same time highly artistic in his depiction of them in his remarkable plates. It was a welcome burst of enthusiasm that caused Professor Hardy to write so much and so well of the life of the sea that he has written us two books instead of one. It is the first of these, concerned with the general natural history of the open sea and the world of its plankton, that we here welcome. The second part of The Open Sea concerns the open sea’s fish and fisheries, and will be published some time in 1957 or early 1958; like the present book, it will be illustrated by Professor Hardy’s own colour paintings, which represent what no colour-camera has yet been able to catch, and by black and white photographs by that most distinguished marine biologist and skilful photographer, Douglas P. Wilson.
To most readers the subject of this first of Professor Hardy’s two contributions to our series—the world of Plankton—will be relatively unknown and mysterious; but here the enlightened amateur naturalist is shown how, with modest equipment he may investigate it himself. The world of plankton is a world of complex anatomy, much of which can be understood only with the lens of the microscope. The life-histories of the animals are also complicated; some of them are extraordinary. To describe the plankton of our seas, and to set it in its pattern of community, climate, sea-scene and season is a major task. Professor Hardy has brought vast knowledge and experience and scholarship to a synthesis never before attempted.
THE EDITORS
AUTHOR’S PREFACE (#ulink_e7956feb-8941-5258-be07-4c5f44a5e981)
ORIGINALLY it had been intended that the whole natural history of the sea, apart from that of the sea-shore and of the sea-birds already dealt with in the New Naturalist series, should be treated in one general volume. As the writing proceeded, however, it became clear that to do justice to the subject it would be impossible to include all its different elements within a single cover. There is the life of the plankton in almost endless variety; there are the many kinds of fish, both surface and bottom living; there are the hosts of different invertebrate creatures on the sea-floor; and there are those almost grotesque forms of pelagic life in the oceans depths. Then there are the squids and cuttlefish, and the porpoises, dolphins and great whales. In addition man’s fisheries now play such an important part in the ecology of our waters that they also must form a part of any general natural history of the sea.
Certainly there is too much material to go into one volume. There occurs, however, a fairly natural division between the teeming planktonic world and the other categories of life it supports: the fish, the whales and the animals of the sea-bed. This first book on the open sea deals mainly with the plankton; it aims at giving the general reader a non-technical account, save for the necessary scientific names, of its many remarkable animals and showing how, with only a little trouble, quite a lot of them may be seen and studied alive. Perhaps to some it may introduce a new world of life—a world so unusual that few of its inhabitants have homely English names at all. It is hoped, too, that it may be a guide to the plankton for university students who are beginning their studies in marine biology. The book also deals with the water-movements and the seasons in the sea; and it contains an account of the squids and cuttlefish, and of those queer creatures, including the deep-water (and often luminous) fish, swimming in the great depths only a little way beyond our western coasts. It will conclude by showing how a study of the plankton is helping us to have a better understanding of the lives of our commercially important fish. Later, and before very long, will come the sequel: a separate volume devoted mainly to fish and fisheries, but also including whales, turtles and other marine animals which are likewise, directly or ultimately, dependent on the plankton for food.
Before going any further I must thank the publishers and editors, not only for all the trouble they have taken over the production of this book, but also for the patience they have kindly shown over my delay in its completion. I accepted their invitation to write it in August 1943, some twelve years ago; it has, however, meant more than the writing. My excuse for its late arrival will be offered after I have made my main acknowledgment.
The value to the book of the remarkable collection of photographs by Dr. D. P. Wilson of the Plymouth Laboratory will be clear to all, but just how wonderful they are and consequently how lucky I am to have them as illustrations, may not at once be fully appreciated by those who are not yet familiar with the living plankton of the sea. Douglas Wilson has long been recognised as the leading photographer of marine life and his beautiful pictures in black-and-white and in colour which graced Professor C. M. Yonge’s The Sea Shore in this series of volumes will, I am sure, have been seen and admired by most of my readers. I, too, am showing some of his studies of the larger forms of life, such as those of cuttlefish or his unusual view of that strange jelly-fish, the Portuguese-man-of-war, taking a meal; it is, however, his photographs of the tiny plankton animals to which I particularly wish to draw attention here. Though they are taken through a microscope, they are photographs of creatures swimming naturally, very much alive and certainly kicking. Never before has such a series of photomicrographs of living members of the plankton been published; they are unique and will, I believe, be of immense value not only to marine naturalists but to all students of invertebrate zoology. They are the fortunate result of a remarkable combination; Dr. Wilson has brought his skill and artistry to work with that very modern invention the electronic flash. For the first time this device has made possible such instantaneous pictures at a very high magnification. It is not only that invention, however, which makes these pictures unique; while others will follow him, Dr. Wilson’s photographs will always have a quality of their own, because he is an artist as well as a scientist. He is not satisfied until he has produced a photograph that has an appeal on the score of composition as well as on that of scientific value. All his photographs except two (the stranded jelly-fish and squid) are of living animals. A few excellent black-and-white pictures by other photographers are included in some of the plates and these are acknowledged in the captions or the text.
It was my hope, and that of the editors, that in addition to his black-and-whites Dr. Wilson would have been able to contribute a series of colour photographs of the living plankton and especially of the richly pigmented animals from the ocean depths. At that time the electronic flash was only just being developed and he felt unable to attempt them. The movement of the ship at sea, he said, would prohibit the use of a long enough exposure to enable the deep-water animals to be photographed in colour by ordinary means; they quickly die and fade, and so must be taken as soon as they are brought to the surface. I had already had some experience in making water-colour drawings of living plankton animals on the old Discovery during the Antarctic expedition of 1925–27; the editors kindly allowed me to undertake a series of such studies to form the accompanying twenty-four colour plates. To obtain and make drawings of the full range of animals which I felt to be desirable, meant a considerable delay and this was added to an earlier postponement of my start on the book caused by my being appointed to the chair of Zoology at Oxford soon after I had accepted the invitation to write it. For several years the work of my new department and research to which I was already committed took all my attention.
All save seven of the 142 drawings in the plates were made from living examples or, in a few cases, from those taken freshly from the net when some deep-water fish and plankton animals were dead on reaching the surface. The seven exceptions, which are noted where they occur, were drawn from preserved specimens but with memories or colour-notes from having seen them alive; I should like to have drawn these too from life, but I could delay the book no longer. It may be of interest to record how the drawings were made. All the animals, except the larger squids and jelly-fish, were drawn either swimming in flat glass dishes placed on a background of millimetre squared paper where they were viewed with a simple dissecting lens, or on a slide under a compound microscope provided with a squared micrometer eyepiece; in either case the drawings were first made in outline on paper which had been ruled with faint pencil lines into squares which corresponded to those against which the specimen was viewed. In this way the shape and relative proportions of the parts could be drawn in pencil and checked and rechecked with the animal until it was quite certain that they were correct. The outline was then gone over with the finest brush to replace the pencil by a permanent and more expressive water-colour line; next all the pencil lines, including the background squares, were rubbed out and the full colouring of the drawing proceeded with. If rough weather at sea made such a course impossible, the living animal would be sketched in pencil, and painted, in perhaps one or two different positions, to give life-like attitudes and colouration without attempting to get the detailed proportions exactly right; it would then be preserved in formalin for accurate redrawing by the squared-background system when calmer conditions returned. The animals I have selected for illustration are mainly either those which are not included in the black-and-white photographs or those for which colour can add supplementary information. I have, for example, drawn some of the transparent but iridescent comb-jellies, but not the transparent and colourless arrow-worms or salps. I am most grateful to the Sun Engraving Company, who made the blocks for the colour plates, for the great care they have taken in making such excellent reproductions.
I must now make special acknowledgments in regard to these drawings. First I must record my thanks to Dr. N. A. Mackintosh, the Deputy Director in charge of the biological research of the National Institute of Oceanography, for kindly allowing me to accompany the R.R.S. Discovery II on two of her biological cruises in the Atlantic in the summers of 1952 and 1954. It is to the Discovery, with all her equipment of deep-water nets, powerful winches and laboratory accommodation, that 71 of these drawings are due, including all those representing the remarkable animals which live in the great depths over the edge of the continental shelf to the southwest of Britain. Without such facilities they could never have been made; actually three of them date back to earlier days when I had the honour of sailing south in the old Discovery in 1925. Next I must thank a number of kind helpers who have sent me living specimens of plankton in specially protected Thermos flasks from many parts of the coast: Mr. J. Bossanyi from Cullercoats, Dr. E. W. Knight-Jones from Bangor, Dr. Richard Pike from Millport, Professor J. E. G. Raymont from Southampton and Mr. R. S. Wimpenny from Lowestoft. Although I made many visits to different places to draw my specimens, there were still a number I could not get myself in the time available; these were supplied by these kind friends who were on the lookout for what I wanted at widely separated points. I am most grateful to Dr. Marie Lebour and to the Council of the Marine Biological Association of the United Kingdom for kind permission to reproduce some of her beautiful drawings of living plankton animals capturing their prey; these, which form my text-figures 26, 27, 35, 40 and 41, were originally published in her papers in the Association’s Journal in the years 1922 and ’23. Then I must thank Sir Gavin de Beer and members of his staff at the British Museum (Natural History), particularly Dr. W. J. Rees and Mr. N. B. Marshall, for kindly allowing me to make many of the black and white drawings in the text from specimens in the museum collections. I am similarly indebted to Dr. J. H. Fraser of the Marine (Fisheries) Laboratory at Aberdeen who has let me draw some of the beautiful plankton animals he has caught to the north and west of Scotland; and to Dr. Helene Bargmann and Mr. Peter David who have also kindly given me much help on looking out specimens from the Discovery collections for me.
With no less gratitude, I must make acknowledgments regarding the text. Apart from the more normal editorial comments and suggestions I particularly want to say how much I am indebted to my old friend—and once Oxford tutor—Dr. Julian Huxley, who read the whole book with the greatest care and made many valuable suggestions for its improvement. My typescript—how reminiscent of my undergraduate essays of 1919 and ’20!—was covered with his pencilled scribblings in the margin: “Surely you should refer to—, this might be made more emphatic” and the like; not all were adopted, but certainly a great many. The chapter on water movements was read by Dr. G. E. R. Deacon, and that on squids and cuttlefish by Dr. W.J. Rees; I am indeed grateful to them for a number of helpful suggestions they kindly made.
Finally I wish to draw the attention of those who are not scientists to a glossary at the end of the book giving a simple explanation of the few technical terms which have been unavoidably used; and for the zoologist I would point out that the authorities for the different specific names will be found quoted after these names in the index and not in the text where they are left out to avoid undue elaboration.
A.C.H.
CHAPTER 1 (#ulink_79bd5715-643a-5479-aa34-2f4332f09b89) INTRODUCTION
THERE is a very simple fact about the sea which makes its inhabitants seem even more remote from us than can entirely be accounted for by their being largely out of sight. To make my point allow me to imagine a world just a little different from our own.
Suppose for a moment that we live in a country which is bounded on one side by a permanent bank of fog. It is a grey-green vapour, denser even than that often known as a London particular, and it has a boundary as definite as the surface of a cloud so that it is like a curtain hanging from the sky to meet the ground; we cannot enter it without special aids except for a momentary plunge and as quickly out again for breath. We can see into it for only a very little way, but what we do see is all the more tantalizing because we know it must be just a glimpse—a tiny fraction—of all that lies beyond. We find it has life in it as abundant as that of our own country-side, but so different that it might be life from another world. No insects dwell beyond the barrier, but other jointed-legged creatures take their place. Unfamiliar floating forms, like living parachutes with trailing tentacles, show their beauty and all too quickly fade from view; then sometimes at night the darkness may be spangled with moving points of light—living sparks that dart and dance before our eyes. Occasionally gigantic monsters, equal in size to several elephants rolled into one, blunder through the curtain and lie dying on our land.
To make a reality of this little flight of fancy all we have to do is to swing this barrier through a right-angle so that it becomes the surface of the sea. How much more curious about its unfamiliar creatures many of us might be if the sea were in fact separated from us by a vertical screen—over the garden wall as it were—instead of lying beneath us under a watery floor. Who as a child has not envied the Israelites as they passed through the Red Sea as if marching through a continuous aquarium: “and the waters were a wall unto them on their right hand, and on their left.”? What might they not have seen? Because normally our line of vision stretches out across the sea to the skyline and carries our thoughts to other lands beyond, many of us tend to overlook this perhaps more wonderful realm beneath us, or we seem to think it must be too difficult of access ever to become a field for our exploration or delight.
The aim of this book is to give the general reader an account of the natural history of the open sea around our islands and at the same time show how he may, with only modest equipment, see something of this strange world for himself. The amateur naturalist afloat—whether on a yachting cruise, on a fishing vessel, or just out in a rowing boat—may see much if he has the right kind of quite simple gear and knows how to use it; he may perhaps also be lucky and make original observations which will be a contribution towards finding an answer to one of the many unsolved riddles of the sea. The book will also give a sketch of some of the factors upon which the success of our great sea fisheries depend. The lives of the different fish are like threads woven in a web of life—a network of inter-relationships between many various creatures large and small, as complex as any on the land. The story of fishery research, which belongs mainly to our subsequent volume, is so closely linked with this unseen web, that it is hoped an account of these less familiar animals may be as interesting to the fishermen as to the naturalist; indeed many fishermen are naturalists and have much of importance to tell the scientist.
As our title indicates, the book will deal with the open sea—the sea beyond the coastal waters. The life of the intertidal zone has already been beautifully treated in this series of volumes by my friend Maurice (C. M.) Yonge (1949). The sea-shore can be studied by direct observation as the tide recedes and has long been a happy hunting-ground for the naturalist; he can lift up the fronds of seaweed, turn over stones, probe into rock-pools and dig into the sand and mud. Our methods of studying the life of the open sea must be very different; it is far from ‘open’ to the investigator, being in fact a hidden world, but this makes its exploration all the more exciting. Deep-sea photographic and television cameras are important new developments which promise much for the future; they, however, as also submarine observation chambers like the bathysphere, must for some time to come be regarded as very costly and specialist equipment giving us here and there direct confirmation of what we usually have to find out by other means. The diving helmet and the aqualung may help us to see something of this enchanting world in shallow water, but for the discovery of what is happening over wide stretches of the underwaters of the open sea we must devise more indirect methods.
The fact that we can see only a very little way below the surface indicates a property of water, and particularly of the sea, which is of fundamental importance to the life it contains. Held up in a glass, water appears so very transparent that we are at first surprised to find how quickly light is absorbed in the sea itself and what a little distance its rays will penetrate. Measurements made in the English Channel off Plymouth show that at a depth of five metres (just over 16 feet) the intensity of light is less than half that just below the surface, while at 25 metres it is only an insignificant fraction, varying between 1½ and 3 per cent. This at once tells us that the green plants, which must have sunlight in order to live, will only be found in the upper layers of the water.
The one real difference, of course, between animals and plants is a matter of their mode of feeding. We know that an animal of any kind, whether mammal, fish, shrimp, or worm, must have what we call organic food: proteins, carbohydrates (sugars, starches and the like) and fats, which have been built up in the bodies of other animals or plants. One animal may feed upon another kind of animal which in turn may have lived upon other kinds, and perhaps these upon yet others, but always these food-chains, long or short, must begin with animals feeding upon plants. Only the green plants, with that remarkable substance chlorophyll acting as an agent, can build themselves up from the simple inorganic substances by their power of using the energy of sunlight (photosynthesis); they split up the molecules of carbon dioxide, liberate the oxygen, and combine the carbon with the oxygen and hydrogen of water to form simple carbohydrates, which are then elaborated into more complex compounds by being combined with various minerals in solution. On the land we are all familiar with this elementary fact of natural history; my reason for recalling it is to emphasise that it is of universal application. The plants are the producers and the animals the consumers as much in the sea as on the land. Indeed ‘all flesh is grass’.
Where then in the sea, we may ask ourselves, are all the plants upon which the hordes of animals must depend? They cannot grow in the darkness or dim light of the sea-floor, and the seaweeds, forming but a shallow fringe along the coasts, are of no real importance in the economy of the open sea. From the deck of a ship, or even from a rowing boat, we can see no plant-life floating near the surface; yet we know it must be there. Another little flight of imagination will, I think, help us to get some idea of the extent of this elusive vegetation.
Let us suppose for a moment that the herring is not a fish, but a land animal. We know that some three thousand million herring are landed every year at ports in the British Isles; these, together with all those landed in other countries, must be only a small fraction of their total number, for we also know that herring are the food of so many other abundant animals of the sea. For simplicity let us consider them to be feeding directly upon plants—and let us imagine them in their unnumbered millions sweeping across the continent. If we do this it needs no imagination to see that the countryside would be stripped of vegetation as if by locusts. Now let us think of the other fish in the sea besides the herring: the cod, haddock, plaice, skate and such that fill our trawlers (as distinct from the herring-drifters) to the extent of more than a million tons a year; then also think of the crowded invertebrate life of the sea-bottom. If all these animals were on the land as well, what an immense crop of plants it would take to keep them supplied with food! There are indeed such luxuriant pastures in the sea but they are not obvious because the individual plants composing them are so small as to be invisible to the unaided eye; we can only see them through a microscope. Their vast numbers make up for their small size.
As an introduction to all that follows let us consider the natural economy of the sea in its simplest terms. We have the sun shining down, its rays penetrating the upper layers of the water; we have the gases, oxygen and carbon dioxide, dissolved in it from the atmosphere; we have also the various mineral salts—notably phosphates and nitrates and iron compounds—continually being brought in by the erosion of the land, and there are minute traces of some essential vitamin-like substances. These are ideal conditions for plant growth. Just as these are spread through the water, so is the plant life itself scattered as a fine aquatic ‘dust’ of living microscopic specks in untold billions. In a shaft of sunlight slanting into a shaded room we have all watched the usually invisible motes floating in the air, floating because they are so small and light; these tiny plants remain suspended in the water in just the same way. Many of them are provided with fine projections like those of thistledown to assist in their suspension.
Feeding upon these tiny floating plants, and also like them scattered through the sea in teeming millions, are little animals. Crustacea, little shrimp-like creatures of many different kinds, predominate; mostly they range in size from a pin’s head to a grain of rice, but some are larger. There are hosts of other animals as well: small worm-like forms, miniature snails with flapping fins to keep them up, little jellyfish, and many other kinds which surprise us with their unexpected shapes and delicate beauty when first we see them through the microscope.
All these creatures, both animals and plants, which float and drift with the flow of tides and ocean currents are called by the general name of plankton. It is one of the most expressive technical terms used in science and is taken directly from the Greek πλavktov. It is often translated as if it meant just ‘wandering’, but really the Greek is more subtle than this and tells us in one word what we in English have to say in several; it has a distinctly passive sense meaning ‘that which is made to wander or drift’ i.e. drifting beyond its own control—unable to stop if it wanted to. It is most useful to have one word to distinguish all this passively drifting life from the creatures such as fish and whales which are strong enough to swim and migrate at will through the moving waters: these in contrast are spoken of as the nekton (Gk nektos, swimming). Actually when they are very young, the baby fish are strictly speaking part of the plankton too, for they are also carried along at the mercy of the currents until they are strong enough to swim against them. Photographs taken through a microscope of some typical planktonic plants and animals are shown in Plate I (#litres_trial_promo) and Plate II (#litres_trial_promo); they have been caught by drawing a net of fine silk gauze through the water. Their natural history will be dealt with in later chapters.
The simple sketch in Fig. 1 (#litres_trial_promo) shows this general economy of the sea in diagram form. A number of fish, including the herring, pilchard, sprat, mackerel and the huge basking shark, feed directly upon the little plankton animals; and so also, curiously enough, do the great whalebone whales, the largest animals that have ever lived. From this world of planktonic life, dead and dying remains are continually sinking towards the bottom and on the way may feed other plankton animals living in the deeper layers. For this reason the zoöplankton (animal plankton—Gk zoön, an animal) is not confined to the upper sunlit layers as is the phytoplankton (plant plankton—Gk phuton, a plant). On the sea-bed we find a profusion of animals equipped with all manner of devices for collecting this falling rain of food. Some, rooted to the bottom, spread out their branch-like arms in umbrella fashion and so look like plants; others, such as many shellfish, have remarkable sieving devices for trapping their finely scattered diet. Feeding upon these are hosts of voracious crawling animals. These and their prey together—worms, starfish, sea-urchins, crustaceans, molluscs and many other less familiar creatures—in turn form the food of the fish such as cod, haddock and plaice which roam the sea-floor in search of them. Finally comes man: catching the herring and mackerel with his fleets of drift-nets near the surface, hunting the great whales with explosive harpoons, and sweeping the sea-bed with his trawls for the bottom-living fish.
FIG. 1
A diagrammatic sketch illustrating the general economy of the sea.
We see how all-important the plankton is. All the life of the open sea depends for its basic supply of food upon the sunlit ‘pastures’ of floating microscopic plants.
Our knowledge of life in the sea has been built up step by step by many pioneer naturalists. Oceanography is still quite a young science; its beginnings were made only a little over a hundred years ago. It is worth while looking back.
The vast community of planktonic animals and plants was unsuspected till it was discovered by the use of a very simple device, the tow-net: a small conical bag of fine silk gauze or muslin, usually with a little collecting jar at its end, towed on a line behind a boat. In nearly all the text-books of oceanography it is stated that the tow-net was first used in 1844 by the German naturalist Johannes Müller, and I have myself been guilty of repeating this error. It is certain that Müller’s researches excited the scientific world and led many others to follow him; but our own great amateur naturalist J. Vaughan Thompson, when serving as an army surgeon in Ireland, was using a tow-net to collect plankton from the sea off Cork as early as 1828. It was there that he first described the zoëa, the young planktonic stage of the crab. A little later, 1833, he discovered the true nature of the barnacles and so solved an age-long puzzle. These enigmatic creatures, fixed to rocks or the bottoms of ships, had been thought to be aberrant molluscs. Thompson caught little undoubted crustaceans in his tow-net and found that they settled down to be transformed into barnacles. His classical discoveries were described in privately printed memoirs which he published in Cork; they are among the rarest items of biological literature. He showed that the plankton consisted not only of little creatures permanently afloat, but also of the young stages—larvae, as the scientist calls them—of many bottom-living animals; these latter more sedentary forms throw up their young in clouds to be distributed far and wide by the ocean currents, just as many plants scatter their seeds in the wind for the same purpose. Charles Darwin also used a tow-net before Müller, on his famous voyage in the Beagle; in his Journal of Researches (1845) under the date of 6 December 1833 he writes: “During our different passages south of the Plata I often towed astern a net made of bunting and thus caught many curious animals.” Today many forget that our famous T. H. Huxley, champion of Darwinism, began his career as did Darwin before him, as a great field naturalist; in 1846 he sailed for the South Seas as surgeon in H.M.S. Rattlesnake and by his use of the tow-net laid the foundations of our knowledge of those remarkable composite jellyfish-like animals, the siphonophores, which we will later discuss (see here (#litres_trial_promo)).
Another simple device, the naturalists’ dredge—a coarse netting bag on a rectangular iron frame—dropped and dragged along the bottom of the sea revealed another new world of life. It was first used by two Italian zoologists, Marsigli and Donati, in the middle of the eighteenth century, but it was another of our own great marine naturalists, Edward Forbes, who became the leading pioneer in this work; he began his dredging in 1840, both in British waters and in the Aegean Sea.
How deep in the sea can life exist? This became the subject of much controversy among scientists following the discoveries made by the use of an ingenious device invented by just a boy—a brilliant young midshipman in the U.S. Navy—J. M. Brook. He hit on the idea of attaching a quill to the sounding lead used in plumbing the ocean depths and so bringing to light a sample of the ooze from the bottom into which it had penetrated. It gave only a tiny sample—but how exciting! That was in 1854, and soon from all over the Atlantic basin, from any depth over 1000 fathoms, came samples of oozy sediment containing minute calcareous shells. These were shells of animals belonging to the group of the Protozoa (single-celled animals) known as Foraminifera and nearly all belonging to one genus, called Globigerina on account of the spherical form of their shells. This form of deposit has consequently become known as Globigerina ooze. Did the creatures which made the shells actually live at these great depths, or did the shells fall from near the surface when their floating owners died? That was the problem. It is amusing for us now to recall that most of those who held the latter and correct view did so on quite false grounds: they believed that it would be quite impossible for life to exist at these great depths and that therefore the shells must have fallen from above. A drawing of a living Globigerina is shown in Plate 2 (#litres_trial_promo).
Edward Forbes had considered there was what he called a zero of life at about 300 fathoms—a boundary below which no life could stand the great pressure of the depths. This fallacy was soon to be exposed. The laying of submarine cables was just beginning. In the Mediterranean one of these after a little use had parted and was hauled up for repair in 1858; it came up encrusted with bottom-living animals, some of them at points on the cable which must have lain at a depth of over 1000 fathoms. Once it is pointed out, the truth of the matter seems obvious: an aquatic animal should feel no ill effects of pressure provided it has no spaces or bubbles filled with air or gas inside it. All liquids are only very slightly compressible. A body made up of fluid or semi-fluid protoplasm, and covered with a flexible or elastic skin, will contract only very slightly even under the greatest pressure; its contents too will be of course at the same pressure as the surrounding water. With the stresses inside and outside the body perfectly balanced in this way, the animal can have a most delicate structure and make the finest movements just as well in the great depths as can one living nearer the surface. Even the seemingly rigid armour-platings of such animals as crabs are in fact made up of a number of parts separated from one another by thinner flexible joints, so that changes of pressure are equalised inside and out; the same applies to the starfish and sea-urchins, whose armour is actually not strictly on the outside of the body, but just below the skin.
Simple as the explanation seems to us now, the discovery of these animals living under great pressure came as a real surprise to most people. This was all the more extraordinary, for actually there was in existence a thoroughly attested instance of a remarkable starfishlike animal (one of the Gorgonocephalidae with branching arms) being brought to the surface from a depth of 800 fathoms; it came up entangled round a sounding line on Sir John Ross’s expedition to Baffin Bay in 1818. It had been forgotten or overlooked by the naturalists of a later generation, who also did not appreciate the significance of the dredgings reported by his even more famous nephew Sir James Clark Ross. Accompanied by the young Joseph Hooker, he had made a number of rich hauls from depths down to 400 fathoms during those great south polar voyages in the Erebus and Terror from 1839 to 1843. Unfortunately these important deep-sea collections, which contained marine invertebrate animals in great variety, were subsequently lost to science.
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The waters immediately to the north and west of the British Isles may perhaps be regarded as the cradle of oceanography; they became the scene of the pioneer deep-sea dredging expeditions in the naval surveying ships Porcupine and Lightning led by Dr. W. B. Carpenter and Professor (later Sir) Wyville Thomson. During the summers of 1868–70 they made nearly 200 dredge hauls over a wide area and reached a depth of 2,435 fathoms; as far down as they went they revealed a wealth of life and opened up a new world to the naturalist. Thomson’s great book The Depths of the Sea (1873) is still fascinating reading. It was their remarkable discoveries, together with the interest taken in the new venture of laying transoceanic cables and the consequent need for a more accurate knowledge of the ocean floor, that led in 1872 to the dispatch by the British Government of H.M.S. Challenger on her famous expedition; under the leadership of Sir Wyville Thomson she sailed on a three and a half years’ voyage to explore all the oceans of the world. The results of this magnificent venture filled more than 50 large volumes with a wealth of information not only of the life of the ocean and of the nature of the sea-floor as revealed by tow-net and dredge, but also about the physics and chemistry of the sea at different depths. Oceanography as an organised branch of science had come into being. Other nations followed the example of the Challenger and sent out similar expeditions.
Having mentioned The Depths of the Sea I must also refer to another great book of similar title which I believe will always be a classic in the literature of Oceanography: The Depths of the Ocean by Sir John Murray and Professor Johan Hjort, published in 1912. Murray was on the Challenger with Wyville Thomson and later, when Thomson’s health failed, directed the Challenger Office, seeing to the completion of all the work and the editing of the great series of Reports; Hjort, who died as recently as 1948, was the great Norwegian marine biologist and Director of his country’s fisheries research. I shall be referring to this book again, particularly in Chapter 12 (#litres_trial_promo), for it contains the results of a very successful expedition which the two authors made in 1910 in the Norwegian research ship Michael Sars to study the deepwater life of the North Atlantic. I draw attention to it here, however, because it is also a splendid introduction to our science in general, with a valuable chapter on the early history.
