The Life of Crustacea

by W. T. Calman

In the first place, the character of the sea-bottom changes very greatly as we pass away from the coast. Near the shore it is extremely diversified, consisting in one place of rocks swept bare by the tides or overgrown with jungles of sea-weed, in another of banks of gravel or shingle, of sand or of mud, but in all cases derived from the "waste" of the land, as it is eaten away by the waves or washed down by the rivers. As the distance from land increases, the deposits become finer and finer, till they shade off into a soft oozy mud, composed of the finest particles brought down by the rivers. In the neighbourhood of large rivers this mud may sometimes extend for hundreds of miles from the land, but there is a limit to the distance to which even the finest particles can drift before they settle to the bottom, and beyond this limit the floor of the ocean is covered by sediments which owe their origin, not to the land, but to the ocean itself. The surface waters of the ocean everywhere teem with a vast variety of floating animals and plants, and, as these die, their remains sink to the bottom "like a perpetual shower of rain."

Among the most abundant floating organisms—in the warmer seas, at any rate—are certain minute animals known as Foraminifera, which belong to the lowest class of the animal kingdom, and have shells composed, in most cases, of carbonate of lime. Over vast areas the bottom of the ocean is covered with a soft grey ooze, made up almost entirely of the dead shells of Foraminifera rained down from above. Since the commonest species of Foraminifera found under these circumstances belong to the genus Globigerina, the deposit is known as "Globigerina ooze."

In certain regions of the ocean the shells of other floating organisms largely replace those of the Foraminifera in covering the ocean floor, and in the deepest abysses—so deep that the shells of surface animals are dissolved before they can sink to the bottom—there is found a deposit known as the "red clay," which appears to be derived largely from the impalpable volcanic and cosmic dust that floats in the atmosphere. It is not necessary for our present purpose to enter more fully into the interesting questions connected with these deep-sea deposits, but it is important to remember that, generally speaking, the floor of the deep sea is everywhere soft ooze, without rocks or stones, except for an occasional water-logged lump of pumice or a stone dropped by a melting iceberg. This fact is probably of great importance in the life of deep-sea animals.

One of the most peculiar and characteristic of the physical conditions in the deep sea is the enormous pressure under which life has to be carried on. At the surface of the sea the pressure of the atmosphere is, roughly speaking, 14½ pounds per square inch. At a depth of only 33 feet of water this pressure is doubled, and at greater depths the pressure increases in proportion, till at 2,000 fathoms it is more than 2½ tons on the square inch. As a matter of fact, however, the animals at the bottom of the sea are probably but little affected by this enormous pressure. Only, when they are brought up by the dredge the sudden release of pressure causes the fluids of the body to expand and destroys the tissues, so that the animals are generally dead or dying when they reach the surface.

More important than the pressure in its influence on life is the darkness of the depths. The light of the sun only penetrates the water of the sea to a comparatively small depth. At 200 fathoms there is not enough light to produce any effect on a photographic plate. Even at a considerably less depth the absence of light puts an end to all plant-life, except for the ubiquitous bacteria, and it follows that all the animals of the deep sea ultimately depend for their food-supply on the rain of dead bodies of surface animals which, as already mentioned, is constantly falling on the sea-bottom.

The temperature at the bottom of the deep sea is always very low. Dr. Alcock states that "in the open part of the Bay of Bengal, where the mean surface temperature is about 80° F., the temperature at a depth of 100 fathoms is only about 60° F., at a depth of 300 fathoms not quite 50° F.; while at a depth of 2,000 fathoms the temperature all the year round is only 3° above freezing-point."

Finally, it is important to notice the uniformity of the conditions at the bottom of the sea; not only are the alternation of night and day and the progress of the seasons unfelt in the abysses, but the conditions are practically the same over vast areas in all the oceans.

In the case of deep-sea Crustacea, we are frequently confronted with a difficulty which does not occur in the case of some other groups of animals—Corals or Echinoderms, for example—the difficulty, namely, of deciding whether the animals really lived on or near the bottom, or were captured by the open mouth of the trawl on its way to the surface. When the animals are plainly not well adapted for swimming—as, for instance, most of the Crabs—it may be assumed that they did actually live on the bottom; but, with the prawn-like forms, the possibility that they may really be inhabitants of the intermediate depths must always be taken into consideration.