DEEP-SEA LIFE. In this account of midoceanic, or “pelagic,” life the writer purposes to restrict himself to the open ocean, that is the spaces of deep water covering the real oceanic basins. The great land-masses are bordered by a submerged rim of varying width, where the water does not exceed an average depth of 100 fathoms, beyond which the bottom falls or slopes to the plain of the original ocean-basin. The sum of these continental borders and similar shallows elsewhere, as in the archipelagic region of the western Pacific, forms only about 15 per cent of the 140,000,000 square miles of salt water, leaving more than four-fifths of it deeper than 100 fathoms, while nearly 10,000,000 square miles exceeds 3,000 fathoms in depth.
We speak of “oceans,” but in reality it is all one body of water, which by a constant interchange through currents both horizontal and vertical, maintains a virtual uniformity, yet not a complete one, since distinct variations are observable in temperature, density (salinity), pressure and other physical characteristics that influence animal being. Therefore some parts of the ocean are more populous than others; some are more populous in summer and less so in winter, and there is a definite (although as yet undetermined) geographical distribution of marine life, both horizontal and vertical.
Ocean Floors.— The character of the bottom must be considered. The rivers discharge into the ocean daily an enormous amount of land-material, which is sorted out by weight, readiness to dissolve, etc., and by the action of waves and currents, and is spread out on the bottom to a greater or less distance according to circumstances, but nowhere goes far from the coast. This outer border of deposits from rivers shows a bottom of bluish mud, or, in places, of coral sand or of volcanic dust. It is rich in food-material, and supports in the water above it a far larger assemblage of plants and animals than does the sea outside of it. We have no further concern with this littoral rim further than to point out that, by means of currents, it contributes considerable food to pelagic animals. The floor of the vast spaces of ocean-bottom shows none of this land mud, but is covered with deposits of three kinds. The most extensive of these is a red clay, which covers the bottom in the great bight of the Atlantic between North and South America west of the 50th meridian, a large part of the mid-Atlantic, and most of the floor of the Pacific and Indian Oceans; it is believed to have been derived from the decomposition of submarine rocks, and of pumice and other volcanic débris showered into the sea, mainly in past ages.
Scarcely less widespread is a loose deposit called “ooze,” composed almost exclusively of the remains of minute animals and plants that have silicious or lime-carbonate skeletons which have sunk down from the surface. All over the bed of the Atlantic, except where the red clay appears, the ooze shows more tests of the protozoan Globigerina than of anything else, and it is therefore known as globogerina ooze. Pteropod ooze, patches of which occur here and there, has a predominance of pterapod shells; and so on. Bottom of this kind is far more thickly inhabited by benthonic (deep-sea) animals than is the clay bottom; and it lies under the paths of the great oceanic currents, whereas the red day underlies chiefly the areas of comparatively stationary water — the great eddies in the oceans.
These oozes imply an abundant life in the open sea. What is its character and how does it exist in a world of only sea-water?
Factors of Deep-Sea Life.— First we must glance at certain conditions. The pressure exerted by water on anything lowered into it increases at a rapid rate as you go down, so that at a depth of only 500 fathoms it equals about 100 times that at the surface. This contributes to the density of the underlying water. The saltiness of the sea also contributes to its density, but varies somewhat according to local conditions, and decreases slightly from the surface downward.
More important than either in its effect on life is temperature, the factor determining to a great extent both the amount and the distribution of marine life. In the middle of the Atlantic, near the equator (Sargasso Sea), the water at the surface in summer, will he about 22.30° C., and at 100 fathoms of depth about 16.70° C., below which it diminishes more slowly to about 2.25° C. at 2,000 fathoms, and to below the zero of centigrade (freezing-point of Fahrenheit) in the deeps. The temperature, density, etc., below a few hundred fathoms are probably fairly constant for each measure of descent, so that the water may be regarded as consisting of a series of layers bounded by thermometric limits, which are substantially permanent, and are biological strata as well. At the surface, however, temperature varies with latitude, with seasons, and with circumstances, especially under the influence of winds and currents — those rivers that flow “on the face of the deep,” here warm, there cold, each one fostering and carrying far the kinds of life agreeable to it
A fourth factor conditioning deep-sea life is that of light, which penetrates to a distance only recently understood; but the rays at the blue end of the spectrum go much further than do the red rays — a fact of considerable biological significance. The result of late trials with the best photographic plates show that the blue rays at least penetrate to a depth of about 800 fathoms. Below that is absolute darkness, illumined only by the phosphorescent glow of the lanterns carried by animals living in those Stygian depths.
Such in broad outline are the physical conditions of plant and animal life in midocean; and there is good reason to believe that they have changed little since the beginning of earthly time.
Oceanic Plants the Basis of Subsistence'.— Now how do organic beings maintain life in the world of sea water? What do they get to eat? Well, here, as elsewhere, the larger and more advanced feed on the weaker; and at the base of the list, as on land, are plants. Leaving aside the vexed question of origin, investigation has shown that not only near shore, but all over the ocean, plants exist in myriads. Most of them are lowly algæ invisible to the unaided eye, yet of vast variety in form, degree of organization and nutritive value. They find their nourishment dissolved in the sea-water and distributed uniformly around them, hence they abound everywhere within the “photic zone,” or surface-layer penetrated by sunlight; and at certain times and places they may be so plentiful as to give the water a tinge of color.
The majority are single-celled, silicious-framed diatoms. As they are heavier than the water all would sink were it not that some have means of propulsion, others parachute-like suspension-organs. They may have bladder-like attachments, or be flattened into twisted ribbons, or drawn out like a hair, or be furnished with long projections (flagellæ). Still more remarkable suspension-organs and varieties of form belong to the Peridinæ, which are more complicated, mobile algæ often highly phosphorescent. A third series of pelagic algæ are the brown flagellates, which possess calcareous frames forming globules called cocoliths and other shapes. Microscopic “brown algæ” occupy the most important place in the economy of the sea, and their fields of lime may be met with in geological deposits as far back as the Cambrian, practically identical with those of the present day.
Masses of seaweed are found floating in various parts of the ocean. These have been uprooted from their fastenings on shore, and collect in quiet, eddy-like places. The greatest and most permanent collection is that of the Sargasso Sea, in the Atlantic just north of the equator, where the plant Sargassum predominates. These continue to live and grow, but do not propagate. Such floating rafts of seaweed are the home and hiding place of a great variety of marine animals.
These algæ and seaweeds form the basis of food-supply for the whole of the marine animal-life, since those creatures that feed upon them directly are themselves the prey of all the rest, from shore to shore, from the surface to the depths and from animalcules to whales.
Character of Plankton.— Associated with the almost invisible plants swarming in the surface-layers of the ocean, is an assemblage of animals, varying from single-celled globules of protoplasm (Protozoa) to all the higher classes of invertebrates. These together constitute the “plankton,” the swimming or floating sea-life, and its zone is regarded as about 100 fathoms thick. It is far more populous near most shores than in mid-sea, and in warm than in colder waters, and varies in density of life according to season, temperature, and other regional conditions, yet is everywhere present. Efforts have been made to estimate the amount of plankton, as that might help to determine the amount of fish-food present, but with little success.
In this plankton are multitudes of minute protozoans (q.v.), principally foraminifers and radiolarians in a profusion of species. They have shells consisting either of lime or of silica, and it is these shells, raining down upon the bottom, that constitute the globigerina and other oozes. Here, too, float a great variety of salpæ, jellyfish and other cœlenterates and their larvæ, and many swimming worms, notably the widespread sagittas. Outnumbering all these, except the Protozoa, are the oceanic crustaceans, which, in countless forms and multitudes, play there, as Hæckel remarks, a part corresponding to the insects on land. Most numerous of these are the copepods, all very small; and they are the chief consumers of the minute plants, and, in turn, are among the most important food-supplies for larger creatures. Almost equally numerous are the ostracods and other crustaceans, including some big crabs.
Mollusks are fewer, for these as a rule are bottom-dwellers; but one group, the delicate, winged pteropods, are so extraordinarily numerous, that their glossy shells characterize large patches of ooze in the tropics. One species is called “whale's food,” and actually is the principal subsistence of the northern whalebone whales. A large variety of small squids is also to be found at or near the surface of the open ocean.
The pelagic plankton, which is very largely different from that of coastal regions, is augmented in summer by an extensive variety and amount of larvæ and young animals born both there and in the deeper layers; and at night many animals rise to the surface that are never caught there in daytime. Finally, it is the abode of numerable fishes that seek their prey at or near the surface. Among these the family Scopelidæ, although represented also in the abyssal fauna, play a more important part at the surface than all other fishes combined; yet most of them are only a fraction of an inch long. Transparency and colorlessness (or a blue tint) are characteristic (for safety) of all the lesser creatures in this surface-fauna.
Intermediate Depth-Zones.— It has been said already that between the surface and the bottom the sea seems to be divided into layers or strata in respect to its inhabitants, certain kinds of animals when adult dwelling only within certain limits of depth. These intermediate zones, and their occupants are called “bathrpelagic” — a term including the facts of both their undersea and deep-water existence. Practically, however, these intermediate zones can hardly be defined, and seem to be determined by limits of temperature or of osmotic pressure, and thus vary in their distance below the surface according to the general warmth and density of various regions. Animals taken only by deep hauls of the nets within the tropics, for instance, may be found near the surface in cooler latitudes; furthermore the vertical distribution of fishes, as a class, may differ from that of crustaceans or other groups as a class. Nevertheless it it generally true that many sorts of pelagic animals dwell at intermediate depths, from which, when adult, they cannot either rise or descend to any great distance. Space cannot be given to much evidence of this, but the following extract from the report of the “Michael Sars Expedition” (1910) gives the following general results of its dredgings and net-haulings, as bearing on this point, as follows:
“These catches may be classified into three main regions: (1) A region extending downward from about 500 metres, characterized by the recurrence of Cyclothone and various black or dark-colored fishes, and of many peculiar invertebrates, red prawns being prominent; (2) a region ranging between 150 and 500 metres, characterized by a peculiar community of silvery or grayish fishes belonging to the families Sternoptychidæ and Stomtatidæ; and (3) the surface region, comprising the upper 150 metres. . . . The layer from 800 to 1,000 metres downward may require to be still further subdivided, for certain forms, like the larger Acanthophyra with red eggs, Notostomus, and several fishes and squids have been taken only in the deepest hauls at 1,500 or 2,000 metres.”
Abyssal Life.— Let us now consider the creatures of the abyssal depths, where eternal cold, stillness, darkness and equability unite to make an environment so forbidding that the imagination would refuse to people it with living beings, yet where life and strife do actually abound. This bottom fauna, however, is not equally distributed. It is far more scanty on the areas of red clay than on the ooze; and the faunæ of the various ocean-beds differ, because barrier ridges separate them. What is said here relates to that of the bed of the deep Atlantic, which is better known than any other.
The real bottom-animals are mainly fixed, and consist of sponges, hydroids, actinians, bryozoans, ophiurans, crinoids, brachiopods, holothurians, worms and mollusks. These are nowhere numerous, and below 2,500 fathoms are very scarce, to judge by the results of dredging. Their subsistence is wholly derived, apparently, from the surface, some catching it as it falls and others sucking it out of the top layer of the ooze. Moving about there also is a limited population of snails, crabs, squids and fishes, making thdr living upon or close to the bottom; and a larger and more varied company of their relatives swim in the water above them, up to, say, the 2,000-fathom line. All are of forms different in many respects from related species at or near the surface, and some brought up by the dredge can hardly be distinguished from fossils entombed in the oldest fossiliferous rocks, so unchangeable is the environment in which their race has been propagated for perhaps 50,000,000 years.
Through these black abysses swim fishes with extraordinary adaptations to their conditions. No attempt at even brief descriptions would be worth making. All are small, often less than an inch, rarely as much as six inches long, yet they are armed to the teeth. This is especially true of the families Stomiatidæ and Sternoptychidæ, in which fishes of grotesque shape have big heads with a savage array of long sharp teeth. All are voracious, and some, as Chiasmodus, have mouths so capacious and stomachs so distensible that they can often swallow fishes as large as themselves, when their stretched stomach with its load hangs beneath them like the yolk-sac of a newly-born trout. All are dark in color, brown, blue or violet marking the abyssal species. Some of them have light-giving organs; and this was formerly regarded as a peculiar possession of deep-sea fishes, enabling them to see their prey in the Stygian gloom of the sunless abyss; but it is now known that light-organs are especially characteristic of pelagic fishes of the region between the surface and 250 fathoms of depth; and ichthyologists are uncertain what service they are to the fishes that carry them.
The eyes of most fishes, such as Macrurus armatus, that inhabit the deepest bottom-layers are very large and have the quality of nocturnal eyes generally — are “day-blind”; yet many fishes, squids, etc., hauled from great depths, have no visible eyes at all, or small and nearly useless ones; and this puzzling contradiction applies to other fishes living at intermediate depths. Some of the blind abyssal fishes are provided with whiplike fin-rays or very long head-filaments that probably act as “feelers,” and so serve the purpose of eyes in seeking food or taking warning of danger.
The general deductions to be made from this outline are that the waters of the ocean are everywhere inhabited, even to their uttermost depths, by living beings; and these are adapted to various circumstances and so form faunas of local extent and character.
Bibliography.— Goode and Bean, ‘Oceanic Icthyology’ (Washington 1885); Mosely, ‘Notes of a Naturalist’ (London 1878); Murray and Hjort, ‘The Depths of the Ocean’ (London 1912); Thomson, ‘Voyage of the Challenger’ (London 1877), and publications of sea-exploring expeditions and scientific institutions.