Life in the sea

Front matter

J. JOHNSTONE

The Cambridge Manuals of Science and

Literature

Contents

CHAP. PAGE

I. The Categories of Life . 1

II. Rhythmic Change in the Sea .... 28

Chapter I

It is probable that there are at least about 500,000 distinct species of plants and animals in the sea. Each species is an assemblage of individuals which differ from the individuals composing other species or assemblages, in form, in life-history, and to some extent in habits. Above all, the individuals of each species differ from those of other species in that their mode of reproduction has been so special¬ ised, that the offspring born from them resemble the parents more than they resemble the individuals of any other species. Now it is the task — as yet an incompletely performed one — of marine biology to classify all these organisms so as to display their inter¬ relationships to each other and to the species of organisms which have inhabited the earth in past geological periods. Even to indicate the main prin¬ ciples of such classification would be beyond the scope of this book, but if we assume a general knowledge of the main types of plant and animal life it will not

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be difficult to devise a grouping which will be at once convenient and philosophical.

Let us suppose that it is possible to start from some place on the west coast of Ireland and walk out along the sea-bottom, underneath the North Atlantic Ocean, until we reached the opposite shore of North America. As we did so we should witness a re¬ markable series of changes in the physical condition of the sea-bottom, and of the water over our heads ; and we should also witness a similar change in the nature of the life of the sea ; such a change as would enable us to associate the variations in the physical conditions with certain variations in the abundance and nature of the animal and plant life of the ocean. We will suppose that we were to start out from some stretch of beach covered by stones and gravel: on the higher parts of such a beach we should find a particular kind of life — large sea-weeds, such as the bladderwrack, clinging to the stones; mussels, barnacles, periwinkles, dog- whelks and small Crustacea such as the sandhoppers. The animals and plants of which these are the most familiar examples inhabit the sea beach between the upper and lower tide marks and we may call them the Littoral Benthos. Now descending the beach to near the limit of low water of high spring tides we should find a more varied and luxuriant fauna and flora, which would consist of the large-fronded sea-weed Laminaria,

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small fishes such as the blennies, gobies, and gunnells ; many Crustacea such as lobsters, crabs, hermit-crabs, and prawns ; molluscs such as the whelks, horsemussels, clams, scallops, and brightly coloured seasnails (Nudibranchs) ; starfishes, sea-urchins and brittle stars ; many kinds of crawling worms ; sea-anemones, sponges, and numerous species of plant-like zoophytes and polyzoa encrusting or growing on the stones or larger algae. This fauna and flora inhabiting the zone of shore only exposed by exceptional ebb¬ tides we may call the Laminarian Benthos. If our initial point of departure had been a rocky coast we should still find much the same distinction, for the upper parts of the rocks would be covered with the barnacles, periwinkles, etc., while the lower rock-pools would contain much the same organisms as we found on the lower parts of the gravelly beach. If we had started from a flat, sandy shore the nature of the life would, however, be quite different, for the sand between the tide marks would probably contain such molluscs as the cockle, animals like the lugworm burrowing beneath the surface, and small Crustacea such as the Copepods. The larger rooted algae would be absent, but their place would be taken by micro¬ scopic plant life. Here and there on the surface of the sand, on the ripple marks, or in the little gutters left by the retreating tide there might be patches of yellow-green slime, and on examining these we should

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find them to consist of masses of Diatoms ; and almost everywhere the sand would contain these though they might not be visible to the naked eye. The impression we should get would be that the sandy shore con¬ tained less life than that found on the stones or gravel, and this impression would probably be an accurate one : nevertheless the sand would contain far more life than one would at first imagine it to hold.

As we walked out to sea to greater depths we should witness many changes in the nature of the sea-bottom. Here and there it would consist of bare rock, or of stones and gravel, and the nature of this rock or gravel would depend on that of the adjacent land. But as we went further the more common deposit on the bottom would be sand, although there might be patches of mud, particularly in the deeper parts or channels. The general arrangement would be first of all rock and stones or gravels, then sands, and then muds; for all these materials result from the erosion of the land, and the gravel is first deposited or rolled down, then the sand, and lastly the mud. Out beyond a depth of about 50 fathoms the muddy deposits would be more abundant than closer inshore, for the finer particles of which they are composed are carried for a greater distance in the water, while much of the fine particles carried in suspension by the water of the rivers is precipitated by the salts of the sea-water and only settles down very slowly.

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This mud would form in the deeper depressions where the run of tide is weakest, and we should never find it on ridges of rock standing up from the sea bottom. Now all these materials come from the wasting away of the land, but we should also find sea-bottom deposits which are formed by the agencies of living creatures. There would be patches of hard, sandy substance formed by the tubes of the worm Sabellaria; here and there deposits of the coral-like skeletons of calcareous algae, or Corallines ; and perhaps also extensive deposits of broken shell fragments. So far we should have got down to a depth of perhaps 50 fathoms.

The life inhabiting the sea-bottom would not differ much from that which we had already observed in the Laminarian zone, but there would of course be greater variety, especially in regard to the fishes. The latter would consist of plaice, dabs, soles, gur¬ nards, skate and ray, codling, whiting, dogfishes, and others. There would be shrimps on the shallower sandy bottoms, accompanied by hosts of invertebrates such as various species of crabs, starfishes, ophiurids, sea-urchins, hermit-crabs, polyzoa and many others. In the deeper places near the 20-fatliom line there would be other species of fishes not found in the shallower water, such as turbot and brill, and there would also be a difference in the invertebrates : the larger squids and cuttle-fishes and many others would

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be found here. The rocky bottoms would harbour many species of fishes not found on the sand — the brilliantly-coloured wrasses for instance, with perhaps pollack, ling or saithe. The muddy places would contain many kinds of burrowing Crustacea and worms, and perhaps the Norway lobster (Nephrops). All this part of the sea we might call the shallow water region, and the bottom life inhabiting it the Shallow Water Benthos. Life here is richer and more varied than in any other part of the sea, not only because of its permanent fauna but also because hosts of young fishes inhabit this region for the first year or so of their lifetime. As we proceed further out to sea there would be a progressive change both in the nature and the abundance of life. The algae would disappear entirely. The shallow water fishes such as the plaice and sole would be replaced by the witch and megrim, and the cod, whiting and haddock by the hake. Invertebrate life would also become rather less abundant. Between the 50-fathom and the 100-fathom contour lines the region of sea may be called the deep-sea area and its bottom life the Deep-Sea Benthos. It would all be familiar to us.

So far we have been travelling under relatively shallow water, over the narrow selvage of sea-bottom which fringes the margins of the continents, and extends out to sea for a variable distance from the dry land. The British Islands themselves are situated

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on this margin, which is called the ‘ Continental Shelf.’ The slope downwards would be a fairly gradual one so far, but at depths of about perhaps 500 fathoms it would begin to become much more steep, and in some parts of the ocean it would be represented by a series of terraced precipices. Beyond these, and at depths of over 1000 fathoms, would be the permanent abysses of the ocean.

Very strange and unlike anything on the dry land would be this ocean bottom. Beyond the edge of the continental shelf there would be few hills, or valleys, and the slopes downward to greater depths, or upwards to oceanic islands, would be so gradual as to be hardly perceptible to us. Sailors, visualising the sea-bottom from their knowledge of it acquired by soundings, speak of knolls, deeps, etc., but they usually exaggerate the degree of slope actually existing. It is said that there are some fissures on the ocean bottom which resemble the deep river gorges of the land, or bare pinnacles and ridges of primeval rock uncovered by any deposits, but these must be quite exceptional features, and the configuration of the ocean floor probably resembles nothing so much as that of the great prairie lands of the western conti¬ nents. But nowhere on the land would there be such immense tracts of level surface as those of the beds of the Atlantic, and the greater part of the Pacific, Oceans.

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The materials forming the sea floor at shallow depths would mostly be familiar to us, but near the edge of the continental shelf they would become quite unlike anything that we know on the land. All the coarser detritus washed away from the land surface would have settled down, and everywhere the sea-bottom would be covered with soft, fine, muds or oozes, varying in colour through tints of blue, green, red or grey. These are the terrigenous muds and they consist of the very finest particles of mud borne in suspension by rivers and currents, and floating in the water for immense distances. Nearly everywhere they would be modified in com¬ position by the action of decomposing organic matter resulting from the dead bodies of marine organisms. Here and there would be volcanic debris, scoriae, fine dust, or even fragments of lava, either ejected from submarine volcanic vents, or settling down on the ocean floor after having been drifted about in the sea or in the atmosphere. Beyond this area of terrigenous muds we should encounter that of the pelagic deposits ; and nearly everywhere at depths of 1500 fathoms to rather over 3000 fathoms the sea bottom deposits consist mainly of the shells or skeletons of animals and plants which live in the upper layers of the sea, and dying sink down to the bottom to form the abyssal oozes.

In many parts of the world the characteristic

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deposit at depths of about 500 to 1500 fathoms is that known as Pteropod ooze. This consists to a large extent of the shells of the pelagic mollusca known as Pteropoda. We should not encounter any patches of this ooze in our traverse across the North

Radiolarian ooze Globigerina ooze

Fig. 1. Deep-Sea Deposits.

Atlantic where the greater part of the sea-bottom, outside the limit of distribution of the terrigenous muds, consists of Globigerina ooze. This is a soft white or grey mud containing gritty particles and composed largely of carbonate of lime. The picture

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on page 9 represents it after the finer amorphous sediment which makes up the greater part of its bulk is washed away. The rounded shells of which it is composed are those of pelagic Foraminifera, protozoa which live mainly in the upper parts of the sea, and which dying there sink down to the bottom, where their shells accumulate to form the deposit. How deep it is we have no opportunity of knowing, but telegraph engineers believe that it forms at the rate of about an inch in ten years in some places. It is the characteristic deposit of the North Atlantic ocean floor, and only here and there the deeper parts of the latter are covered with material of volcanic origin. At depths over 2500 fathoms the Globigerina ooze tends to disappear, for the lime becomes dissolved by the sea water at these depths. The deposit where the sea is about 3000 fathoms in depth elsewhere than in the Atlantic consists of either volcanic material or the mud called Radiolarian ooze. This latter formation consists largely of the skeletons of the pelagic protozoa known as Radiolaria. It contains very little carbonate of lime, for the skeletons of the Radiolaria are composed of silica. We should also find the skeletons of these organisms in the deposits in shallower parts of the ocean, but they are not so evident because of the greater mass of the limy shells of the Foraminifera. The siliceous skeleton of the Radiolarians is better able to withstand

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the solvent action of the sea water. This ooze is not the only one which is composed of siliceous ma¬ terial, for in the Antarctic there is an area of ten and a half millions of square miles covered with the remains of diatoms. These organisms live in the superficial layers of the sea in such enormous numbers that their dead shells form the major part of the mud covering the floor of the Antarctic.

In the greatest depths of all, 3500 fathoms and over, we find no longer any obvious indications of organic structures in the mud on the sea-bottom. The deposit in these great depths is called the Red Clay, and it is a very soft brown mud consisting of exceedingly fine particles, which are the insoluble remains of the shells of pelagic plants and animals, and volcanic dust settling down from suspension in the water, or from the atmosphere. Among it we find cosmic dust — that which has originated in the combustion of meteorites entering the atmosphere of the earth from outer space. It accumulates with exceeding slowness, for we ought to find meteoritic dust in every part of the sea-bottom, but it can usually be recognised only among the red clay, for everywhere else it is hidden by the more abundant materials. In the red clay we also find the earbones of whales and the teeth of sharks. All the rest of the skeletons of these great creatures have been dissolved by the action of the sea water.

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Nothing in our ordinary experience gives us any adequate idea of the physical conditions at the bottom of the ocean. As we descend deeper and deeper the temperature falls more and more, and at a depth of about 400 fathoms the annual variations in the temperature of the sea due to the seasons fade away, and uniformity reigns. From about 400 fathoms downwards the temperature would fall very slowly, and everywhere in the North Atlantic it would not be much higher than that of the freezing point of fresh water. In the circumpolar seas it would fall still further and in some places would be about two degrees below zero of the Centigrade scale.

The darkness would be greater than that of which we have any ordinary experience. Even at a depth of about ten fathoms the illumination would be that of faint twilight only, and at fifty fathoms it would be still less. The light there would be no longer white, for sea water absorbs first the red, orange, and yellow rays, and then the green, blue and violet. At about 50 fathoms the violet light would still be fairly strong, and it would be present even at a depth of 200 fathoms, but there would be absolutely no trace of the red rays. At about 400 to 500 fathoms even the violet light would have disappeared entirely and only the ultra-violet rays — light which is invisible to our eyes, would remain. At a depth of about 900 fathoms the last traces of ultra-violet radiation

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would have disappeared and a delicate photographic plate exposed here for over two hours would not be affected. Absolute darkness exceeding that of the photographer’s dark room would prevail.

We can easily imagine the cold, and the darkness, and the great plains of semi-liquid mud, but not the extreme pressure of the water. At the surface of the sea an organism is exposed to a pressure of one atmosphere, that is 15 pounds per square inch of its surface, and a rise of even one atmosphere produces very unpleasant effects in ourselves. But for every ten metres (5 fathoms) that we descend, the pressure due to the weight of the water rises by one atmo¬ sphere, and at a depth of 3000 fathoms the pressure on every square inch of the body of an animal amounts to about three tons. If we did not know that no part of the sea-bottom — however deep — is utterly devoid of animal life we might conclude — as the older naturalists did — that this enormous pressure was inconsistent with a belief in the existence of living organisms. Even now it is difficult to under¬ stand how the tissues of an animal are able to withstand it. As it is, the pressure exerts a profound influence on the structure of the animals inhabiting even such moderate depths as 200 fathoms. A deep-sea fish is easily recognised by its large eyes, its long and attenuated tail, and the softness rof its flesh. One’s finger leaves a dent in the flesh of such a fish

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as a hake if pressed against it. But the pressures in the abyssal regions are very much greater than those which a hake has to withstand. Even the thermo¬ meters at first used in deep-sea exploration were frequently shattered to fine dust by the pressure of the water ; and this is the great difficulty in the construction of all instruments for the investigation of the oceanic depths.

Just as we recognise a littoral, a shallow water and a deep-sea Benthos, so we may speak of an Abyssal Benthos inhabiting the whole sea-bottom deeper than 2000 fathoms. Before the time of the great oceanographic expeditions there was much speculation as to the nature of the animals which might be found in the greatest depths of the sea. It was thought that exploration might disclose unfamiliar and bizarre forms of life, perhaps gigantic animals, perhaps remnants of former geological faunas persisting under the uniform conditions of the abysses, while all the rest of the earth was undergoing physical change. These expectations have not been realised, for all the animals captured from the deep belong to well-known groups of life ; and with the exception of some stalked Crinoids, animals closely allied to the Crinoids which formed so characteristic a feature of the fauna of the Carboniferous period, most of the species belong to recently evolved groups. One cannot say that there are no monstrous or gigantic

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animals still living in the abysses, for no form of fishing gear yet used would enable us to capture a very large abyssal animal, and one may perhaps still attach some consideration to stories of seaserpents. Nevertheless the general experience is that the animals obtained from the great depths of the sea are as a rule smaller than their shallow water allies. There is, of course, much that is strange in the abyssal Benthos, but though a zoologist accus¬ tomed only to shallow water animals would fail to recognise most of the deep-sea forms, he would still be able to refer them to their respective families or classes.

Deep-sea animals have certain general characters. Most of the great invertebrate groups, the molluscs, Crustacea, sponges, echinoderms, alcyonarians, etc., are represented in the deep. The Crustacea are often long-legged creatures coloured a uniform red. Most of them are smaller, or at least no larger than are the shallow water forms, but some are relatively gigantic — one of the Isopods, for instance, a group represented by the little ‘slaters,’ is about nine inches in length. Some of the crinoids, tunicates and alcyonarians (seapens) are carried on long stalks — apparently an adaptation to life in the soft oozes. There are echinoderms, starfishes and sea-urchins which do not differ in any essential respect from those living in shallow water except that spines are rather more pronounced in the abyssal forms.

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The fishes are very unfamiliar to us though they all belong to well-known groups, and it is very probable that they have been evolved from shallow water species. They have as a rule either very large eyes, or very small ones, or they are quite blind. This is what we might expect if the fishes had spread into the deep from the shallow water, for evolution might proceed in various directions : either an organ which was inefficient under the new conditions would become larger by the accumulation of small variations, or being unused and useless it would disappear. In most of them the tail is long and slender; the mouth and teeth are very large so that the animal has an eminently predatory character, which would be a consequence of the difficulty in procuring food ; some abyssal fish are able to devour others almost as large as themselves. Their bones are fragile and have less than the usual proportion of lime ; a character which is common to most abyssal animals which have calcareous external or internal skeletons. The flesh is soft and watery. There is a general absence of the pronounced colour markings of the shallow forms ; they are not silvery as a rule ; and there are no indications of the obliterative counter¬ shading of the fishes usually known to us whereby the upper parts are darker than the lower ones so as to destroy the appearance of solidity, and render the animal nearly invisible to another in the water

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at the same level as itself. The colour is usually a monochrome — a black or dark red: in the deep, in the absence of red light, such a coloured fish would appear black ; if indeed one can speak at all of colour in such surroundings.

The great majority of abyssal animals are phos¬ phorescent : they possess luminous organs similar to eyes in structure, and often the fishes of moderately deep water have rows of such ocelli along their sides. Sometimes deep-sea animals secrete a sort of phos¬ phorescent slime. Why they should produce light it is hard to guess : we must remember that it is only to our eyes that they appear luminous. Very strange indeed would be the appearance of these animals if we could see them in the deep. In the absolute darkness of the abyss they would appear as ghostly silver-blue shapes glimmering like an electric lamp through dense fog on a dark moonless night. Of all the characters of the deep-sea fauna this almost universal phosphorescence is the strangest.

All the animals and plants which we have so far examined belong to the category of life called the Benthos — a collective term applied to those organisms living at the sea-bottom attached to stones or other fixed objects; or burrowing in the sand or mud ; and which may also be extended so as to include all those animals which, though free to move about, are sluggish and inert, and which frequent only a

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comparatively small area of sea-bottom. Among the attached forms we find the algae, the zoophytes and polyzoa, the sponges, the reef-building and solitary corals, the sea-anemones, the branching corals and alcyonarians, the crinoids, some Crustacea of which the barnacle is the most familiar example, many molluscs such as the cockle, mussel and oyster and most of the tunicates. Among the burrowing forms are some protozoa, many worms, some sea-urchins, and sea- cucumbers ( Hoi othurians), some sea-anemones, many Crustacea, and many molluscs such as the clams or razor-shells. The sluggish, slowly moving, bottom living animals include the starfishes and most of the sea-urchins, crabs, lobsters and hosts of other Crustacea, many worms, many molluscs such as the scallops, periwinkles and whelks. Some fishes are also to be placed in this category. Benthic animals thus include representatives of all the great groups of life. When we remember that distinct faunas are to be found on sea-bottoms of different depths, and of different physical conditions, we see that it is convenient to speak of littoral, shallow water, deep water, and abyssal benthic populations. In the wider economy of the sea all these categories of life are distinct from each other.

Now the great majority of marine animals are benthic in habit but there are a number which possess powerful locomotory organs and which are thus able

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to migrate over wide areas of sea. These are all grouped together to form the category of life known as the Nekton. Comparatively few marine animals in addition to the fishes are nektic in habit. The whales, seals, dolphins and porpoises are the next largest group, and in addition to these and the fishes only a few invertebrates, such as the squids, the cuttle-fishes, and the giant calamaries are actively moving animals; and perhaps some pelagic worms and medusae may also so be described. According to their habits the fishes may be further subdivided into demersal and pelagic species. The demersal fishes are those which live on the sea bottom and obtain their food from animals which also live there. Most of the commoner fishes belong to this category ; the sole, plaice, whiting, haddock, cod, skate and ray, turbot and brill and many others are examples. The pelagic forms are the minority, and the best-known examples are the herring, mackerel, sprat, pilchard, and probably also the salmon while it inhabits the sea.

As a rule the distinction between demersal and pelagic fishes is a very real one, and it is exceptional for a fish which lives at the sea bottom to approach the surface ; if it does the habit may be regarded as an abnormal one. If we trawl demersal fish such as the cod or whiting from even the moderate depth of 20 fathoms, and take care that the animals are not injured by the action of the fishing gear, we see

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that they have the greatest difficulty in again going down to the sea-bottom, and for a time they struggle helplessly at the surface belly upwards. This is because of the expansion of the gas contained in their swim-bladders, and it is only after this has been absorbed by the blood stream that the fish regains its former specific gravity and is able to go below the surface. It used to be the habit of the Grimsby trawlers, who often brought cod alive into the markets, to prick the fish behind the shoulder with a long needle and so allow the gas in the swim bladder to escape. On the other hand fish like the plaice and sole, which have no swim bladders, are able to be transferred directly from water of moderate depths to quite shallow tanks without any apparent bodily disturbance, even although the difference in pressure may be more than one atmosphere. Pelagic fish are able to live both at the surface of the sea and in the depths — say of ten to twenty fathoms; thus the herring must go down to the sea-bottom in order to spawn, and they are often caught by means of the trawl-net in the same manner as the proper demersal fishes are captured. But we may be sure that these fishes are able by means of some control over the blood vessels of the swim bladder to alter their specific gravity.

In the case of the fishes of moderately shallow water — say down to twenty fathoms — there is little, if any essential difference in their form: a herring, i]

for instance, does not differ greatly in structure from a whiting, though the one is a pelagic, surface-living animal and the other is one which nearly always lives at the sea-bottom, and both have swim bladders. But in the case of the oceanic fishes the structure may be strikingly different according to the depth of water which the animal usually inhabits. A flying fish ;differs very notably from any of the abyssal forms which we have already considered ; and those species which live at intermediate depths are also very remarkable in form. They are, as a rule, small, very delicate in structure, with semi-transparent bodies, often compressed from side to side. It is quite easy to read ordinary print through the body of the peculiar Leptocephalus larva of the eel. Many of them are tinted in shades of violet or silver-blue, and have a peculiar shimmering appearance. Often their eyes telescope into sockets. As a rule these are the characters of the pelagic fishes found in the oceans, in the intermediate layers down to a depth of about 200 fathoms. Below that level the colour is usually black or dark red, and the form of the fish is that which we have already recognised as charac¬ teristic of the abyssal category.