CHAPTER II

THE SEA A VAST AQUARIUM

No results of investigation in natural history have been more amazing than those that show the marvelous richness of the sea in plant and animal life--not merely at its warm margin, but far out in the centre of what the ancients used to call "the desert of waters"; not only at its surface, but in its profoundest depths, and under the polar ice as well as amid the tropics. Sea populations differ somewhat according to situation, those of the shallow shore lines, which are of the "littoral" fauna, differ largely from those living in the open sea and belonging to the "pelagic" fauna, and there are surface swimmers, and others confined to the abysses; but virtually every class and subdivision in the animal kingdom is represented in greater or less variety in the zoölogy of the ocean. The list stretches from the merest monads to the huge sharks and still bigger whales.

This multitude and diversity of animal life is possible in the sea because of an even greater plenitude of plants there, which furnish a never-failing food resource. Bacteria and blue-green algæ are at the base of this. Bacteria exist in all seas, as in all soils, and the fertility of nature above ground and under water depends on these microscopic organisms, whose numbers in the ocean are as incalculable as the grains of sand on its brink. In equal multitude are the diatoms, unicellular algæ with flinty cases, by which the waves are sometimes discolored over broad areas; and millions of other green plants, living alone, or in chains, minute in size, but each a chemical laboratory converting the salt water they absorb into meals for the animals that swallow them--animals in most cases almost as small and simple as the things they eat, and themselves destined to be sucked into the mouth of something a little bigger, to be in turn a tidbit for a third hungry mouth, and so on to the broiled mackerel for our own breakfast.

THE PLANKTON AND ITS PHOSPHORESCENCE

The assemblage of plants and animals that together float or swim at or near the surface of the ocean (or other water), say within a layer of water one hundred fathoms thick, is scientifically called _plankton_ of the sea. In the open ocean, the pelagic plankton is much alike all round the world of waters, although it varies a little in composition, and still more in relative abundance, being denser in temperate than in either tropical or polar latitudes; but nowhere is it absent. The "waste of waters" teems with life. The plankton of the shallow waters near continental shores, however, presents a decidedly different assemblage from the pelagic plankton.

In the pelagic plankton, single-celled animals of the groups called foraminifers and radiolarians are exceedingly prominent, and play an enormous part in the economy of the sea, although almost or quite microscopic in size. They are incased in chambered shells of lime or flint; and over vast areas in warm latitudes the ocean floor is so thickly covered with the dead shells of one kind that the mud is called globigerina ooze. They are the eaters of the microscopic plants, and themselves are food for a wide variety of hydroids and jellyfish, large and small, whose silvery forms are often visible to the voyager, and which are mostly responsible for the pale stars of phosphorescence that shine about his prow and glorify his wake in dark nights. The queen of these far swimmers is the radiant Portuguese man-of-war. In the night a dragging fine-meshed net will capture more than by day of the plankton, because many little creatures that in daylight sink to considerable depths come to the surface at night.

Rising a step to the worms, we find them comparatively rare, but one kind of marine flatworm that abounds in midocean is rose-red and several inches long. Much more numerous is another flatworm, Sagitta, "which along with copepoda, salpæ, pteropoda and radiolaria, everywhere constitute the bulk of the small pelagic organisms" captured by towing nets. Like almost all of these usually defenseless creatures they are perfectly transparent, but some of them depart from the rule of pale blue in tint and shine in bright red. A longer step takes us to the Crustacea, represented in the pelagic plankton by queer little shrimplike forms that in countless hosts of individuals play a part in the ocean comparable to that of insects on land. The copepods are the most numerous probably--little things only a fraction of an inch in length, but amazingly abundant, and the principal users of plant food. Their relatives, the little ostracods, have similar habits, and are noted for their intense phosphorescence. Haeckel relates that on his way to Ceylon he saw the entire sea like a twinkling ocean of light, and his microscope showed him that it was made by throngs of ostracods, with some jellyfishes, salpæ and worms. Crustaceans of higher rank abound also. In northern waters species of Schizopoda, small, transparent prawns with red spots around the mouth and big, black eyes, swarm in enormous numbers, and are known to the fishermen as "kril."

An important part of the pelagic plankton consists of certain small mollusks; and "as regards abundance of individuals few groups of pelagic animals can compare with the winged snails, or Pteropoda." These are minute, rapidly swimming creatures with thin, glassy shells, and in some parts of the warmer oceans these discarded shells are so numerous on the bottom that they give the name pteropod ooze to the mud. One kind (Limacina), with a coiled shell about the size of a pinhead, which abounds in the north Atlantic, is much feared by the Norwegian fishermen because they very often spoil the herring that feed on them. Another kind (Clione), looking somewhat like a reddish butterfly an inch or so long, swims in shoals in the icy seas of the far North, and is known as "whales' food." Some larger mollusks, of which the beautiful purple Ianthina is most conspicuous, live among the vast patches of floating seaweed in the Sargasso Sea.

Great numbers and variety of tunicates or ascidians and their larvæ are taken in the surface nets of the sea naturalists, among them the salpæ--free-swimming, barrel-shaped, transparent animals well known to all seafaring people, and often seen crowding the surface of the ocean. One genus of them is Pyrosoma, which has from the earliest days excited the interest of mankind, mainly on account of the strong phosphorescent light emitted, the name, indeed, meaning "fire animal." These salpæ aggregate into colonies often several yards in length which glow like fiery serpents as they move sinuously on their way.

This property of luminosity, so widely possessed by marine animals, is one of the unsolved mysteries. It is called "phosphorescence," because it resembles the cold light given by phosphorus when undergoing slow oxidation, but phosphorus has nothing to do with the manifestation here, or in such insects as the firefly; nor is it owing to bacteria, as in the case of shining wood or decaying fish. What it really is no one knows, but it has, at least, been learned that in animals the power of emitting light is always attributable to certain structures of a glandular nature that secrete a slimy, luminous substance, or, rather, two substances, one luciferin and the other luciferase. When both together are exposed to seawater phosphorescent light results.

As a rule, the light organ is surrounded by a layer of black pigment that acts as a reflector, and often the light is projected through a transparent lens; and there is reason to believe that in the case of the higher animals, such as deep-sea fishes and squids, the rays may be thrown when and where the creature desires, as a man handles an electric flashlight. But for what purpose? Is it to illuminate the surrounding water so as to perceive, or to attract prey, or is it to avoid foes? A learned oceanographer replies that no one certainly knows. "At all events," he concludes, "the answers would probably tend to show that the many different kinds of light organs serve different purposes."

PRISONERS IN THE DARK AND ICY DEPTHS

So much for the surface population of the ocean--the plankton layer is regarded as a hundred fathoms thick. We have considered only that over the mid-oceanic depths, but that of the shallow margins is different simply in the absence of some purely pelagic creatures, and in the presence of vast hordes of eggs and larvæ of the animals rooted in the sand or attached to the rocks and weeds from high-water mark down to a comparatively short distance below low-water mark. These I shall speak of more completely hereafter.

Before that, however, I want to say a few words in regard to the extraordinary inhabitants of the ocean's depths--depths which in some places exceed the elevation of the highest mountains on the land.

The conditions under which animal life exists there are vastly different from those at the surface, and it is not surprising to find these creatures of an extraordinary character. The pressure exerted by water on anything lowered into it increases at a rapid rate as the object sinks, so that at a depth of only 500 fathoms it equals about 100 times the pressure at the surface. This contributes to the density of underlying waters; the saltiness of the sea also adds to the water's density, but this decreases slightly from the surface downward. More important than density in its effect on living things is temperature. In the Sargasso Sea in summer the water at the surface will indicate about 52 degrees F., and at 100 fathoms of depth 48 degrees, below which it diminishes slowly to a little below the freezing point--32 degrees F. The water below a few hundred fathoms may therefore be regarded as a series of layers measured by degrees of density, temperature, etc., and this means a series of biological strata in each of which the denizens are more or less limited by unfavorable conditions above and below them.

A fourth factor conditioning deep-sea life is that of light. The sunlight penetrates to a much greater distance than was formerly believed; and experiments with photographic plates show that the blue rays may sink as far as 800 fathoms, but the red rays go much less down. Below that glimmer is absolute darkness, illuminated only by the phosphorescent glow of the lanterns carried by the animals moving about in that Stygian and icy abode--which would seem to us the most dreadful fate to which any creature on the globe is born.

It has been said that the ocean depths seem to be divided into horizontal zones, certain groups of animals being confined, when adults, within limits of depth determined by conditions suitable to them, one zone above the other. Practically, however, these intermediate life-zones can hardly be defined, and vary in different seas, and under changing conditions, as of season, and so forth. Animals taken only by deep hauls of the nets within the tropics, for instance, may be captured in cooler latitudes near the surface; furthermore, the vertical distribution of fishes, as a class, may differ from that of crustaceans as a class. Nevertheless it is true in general that many sorts of pelagic animals dwell at intermediate depths, from which, when they have become mature, they cannot either rise or descend any great distance. Among them are representatives of all the classes of marine life.

Let us now consider the creatures of the lowest level--those abysmal depths where eternal cold, stillness, darkness, and equability unite to make an environment so forbidding that human imagination would refuse to people it with living beings; yet where life and strife do actually exist, although by no means uniformly distributed. We know most about it as it exists in the bed of the north Atlantic.

The real bottom animals are mainly fixed--sponges, hydroids, sea anemones, bryozoans, brittle-stars, crinoids, brachiopods, holothurians, worms and mollusks. They are nowhere numerous remote from a shore, and below 2,500 fathoms are very scarce, to judge by the results of dredging. Their food comes wholly from the surface, apparently, some catching it as it falls and others sucking it out of the ooze. Moving about among these, and feeding on them, is a scanty population of snails, squids, crabs, and fishes, making their living upon or close to the bottom; and a larger and more varied company of relatives swim in the water above them up to, say, the 2,000-fathoms line. All these are of forms different in many respects from kindred species at or near the surface; and some brought up by the deep-sea 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 fifty millions of years.

Through these dark abysses swim fishes with extraordinary and grotesque adaptations to their conditions. All are small, rarely six inches long, often less than an inch, yet armed to the teeth. This is especially true of the families Stomiatidæ and Sternoptychidæ, in which one finds fishes of the queerest shape, with big heads and a savage array of long sharp teeth. All are voracious, for food is scant and must be fought for; and some, as Chiasmodus, have mouths so capacious that they often swallow fishes larger than themselves, when their stretched stomachs hang beneath their slender bodies like the yolk sacs of 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 gloom of their habitat, but it is now known that light-giving organs are especially characteristic of pelagic fishes of the region between the surface and 250 fathoms of depth. It must be remembered, however, that the sedentary invertebrates of the bottom glow with phosphorescence.

This outline of a vast body of information shows that the waters of the oceans are everywhere inhabited, to their uttermost deeps, by living beings; that these are adapted to various circumstances, and so form faunas of local extent and character; and that probably the sea derived its wealth of population--at least all that part superior to the monads--from the land, beginning with the earliest dawn of life on the globe.