Читать книгу The Open Sea: The World of Plankton - Alister Hardy - Страница 7

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 and Plate II; 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 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).

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.

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.¹

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, 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.²

Like the correction of the idea of a zero of life, the destruction of another early myth, the mystery of the ‘Bathybius’, is also amusing history. In 1857 H.M.S. Cyclops had made a line of soundings across the Atlantic with an improved modification of Brook’s apparatus for collecting samples of the sea-bed. The deposits of ooze were found to contain a strange gelatinous substance which was supposed to be a primitive form of life. Carpenter and Wyville Thomson again found it in their deep-sea dredgings. To give an idea of the universal interest it aroused at the time I will quote from ‘The Depths of the Sea’; Wyville Thomson is here referring to his deepest haul from 2,435 fathoms.

“In this dredging, as in most others in the bed of the Atlantic, there was evidence of a considerable quantity of soft gelatinous organic matter, enough to give a slight viscosity to the mud of the surface layer. If the mud be shaken with weak spirit of wine, fine flakes separate like coagulated mucus; and if a little of the mud in which this viscid condition is most marked be placed in a drop of sea-water under the microscope, we can usually see, after a time, an irregular network of matter resembling white of egg, distinguishable by its maintaining its outline and not mixing with the water. This network may be seen gradually altering in form, and entangled granules and foreign bodies change their relative positions. The gelatinous matter is therefore capable of a certain amount of movement, and there can be no doubt that it manifests the phenomena of a very simple form of life.”

“To this organism, if a being can be so called which shows no trace of differentiation of organs, consisting apparently of an amorphous sheet of a protein compound, irritable to a low degree and capable of assimilating food, Professor Huxley has given the name of Bathybius haeckelii. If this has a claim to be recognised as a distinct living entity, exhibiting its mature and final form, it must be referred to the simplest division of the shell-less rhizopoda, or if we adopt the class proposed by Professor Haeckel, to the monera. The circumstance which gives its special interest to Bathybius is its enormous extent: whether it be continuous in one vast sheet, or broken up into circumscribed individual particles, it appears to extend over a large part of the bed of the ocean …”

The ‘Bathybius’ however came to an inglorious end. It was shown by the naturalists of the Challenger to be a precipitate thrown down from the sea-water associated with the deposits by the alcohol used in their preservation, and T. H. Huxley made a public retraction of his earlier ideas.

Towards the end of the century came the founding of the famous marine stations, the first at Naples in 1872, and then those at Plymouth and Millport in this country and Woods Hole in America; in these laboratories and many more to be founded later, researches into the structure, development, physiology, life and habits of marine creatures of all kinds have been continued to the present day.

It then began to be realised that progress in oceanography was essential to a better understanding of fishery problems and to the development of a more rational exploitation of the sea. The rapid development of trawling, with the introduction of steam power and the replacement of the old beam-trawl by the much larger and more efficient otter-trawl, gave rise to some concern as to the possible depletion of the stocks of fish; this led a number of nations, our own included, to set up fishery investigations. As we shall see, those fears were indeed well founded. In 1899 King Oscar II of Sweden invited all the nations of Europe interested in sea fishing to send representatives to a conference in Stockholm; the discussions which took place led to the foundation in 1901 of the International Council for the Exploration of the Sea. The different nations began a series of investigations to form part of one great plan. In spite of the temporary suspension of activities in two world wars the work of the Council still goes on.

The scientists of the various fishery departments are not only enquiring into the natural history of the fish themselves, their life-histories, food and feeding habits, migrations, growth, birth-rates and so forth; but with continually improved equipment they are studying the distribution of the different planktonic forms upon which they depend, the conditions under which they live, the flow of the ocean currents, the physics and chemistry of the sea and the varying nature of the sea-bottom and its life. It was wisely realised from the start that in order to provide answers to such questions as: ‘Why are fish sometimes plentiful and sometimes scarce?’; ‘Can the future of a fishery be forecast?’; ‘Is this or that area being overfished?’ and so on, the natural history of the sea must be investigated in all its different aspects.

Everything the naturalist wants to find out about the conditions under which fish live he must grope for in this unseen world, often below a storm-tossed surface; he must always remember too that the sea is a very big place—he must work with a sense of proportion and perspective. He stops his research ship at intervals to let down his instruments on wires and ropes: some to take the sea’s varying temperature and to collect samples of water from different depths for analysis, others to measure the amount of light reaching different levels, and others again to record the speed and direction of ocean currents. He samples its life with all kinds of nets to estimate not only the varying quantities of the plankton but of the eggs and fry of the fish themselves. Building up a picture of life in the sea is like putting together a huge jig-saw puzzle made up of tiny pieces, but much more difficult. Not only have we a very imperfect idea of what kind of picture will emerge, but all the pieces to be fitted together are not on the table before us; they are lying about somewhere underneath it and we must feel about for them in the darkness. It is certainly a fascinating pursuit, but full of disappointments. Some bits of the puzzle—perhaps a stage in a life-history or some evidence of a migtation—can only be picked up during a short period of the year; before the missing pieces can be found, stormy weather may intervene and we must wait a whole year before we can try again.

In spite of these obstacles the picture of life in the sea is continually growing: the chapters which follow will endeavour to sketch an outline of what has been achieved. The amateur naturalist should not be discouraged by these difficulties for it is because of them that there are so many gaps in the story yet to be filled in. There are still many original discoveries to be made. And the difficulties add spice to the game; a golf-course would indeed be a dull one if there were no bunkers on it.

¹ In the “Summary of the Scientific Results of the Challenger” Part 1, p. 79, Sir John Murray refers to the deep dredging of the Erebus and Terror. He says: “Sir James Ross was an indefatigable zoological collector, but it is to be regretted that the large collections of deep-sea animals, which he retained in his own possession after the return of the expedition, were found to be totally destroyed at the time of his death. Had they been carefully described during the cruise or on the return of the expedition to England, the gain to Science would have been immense, for not only would many new species and genera have been discovered, but the facts would have been recorded in journals usually consulted by zoologists instead of being lost sight of as was the case.”

² Sir John Murray also wrote an excellent little introduction to oceanography: The Ocean (1913). For those who wish to make a fuller study of the development of our science, and particularly of the early plankton investigations by the German Hensen school, there is J. Johnstone’s important book on Conditions of Life in the Sea (1908). Two other valuable introductions should be mentioned: Fowler and Allen’s Science of the Sea (2nd edition, 1928) which gives much practical advice on collecting specimens and the working of gear, and Russell and Yonge’s charming general natural history of The Seas (1928) which deals with the life of tropical waters as well as of our own.