CHAPTER VI.. Deep-Sea Fauna.

DEEP-SEA FAUNA.

Some of the most important conditions under which life at the bottom of the deep-sea occurs have been mentioned in the first chapter. We have pointed out that the pressure is enormous, that the temperature is only a few degrees above the freezing point, and that, except in those places where phosphorescent animals emit a faint light, it is absolutely dark, no rays of direct sunlight being able to penetrate such a mass of water as lies between the bottom of the ocean and its surface.

With such conditions to contend with it is not surprising that the naturalists at the beginning of the century believed that no animals could possibly live on the floor of the great oceans. Their beliefs, however, merely afford us an example of the danger of prophesying without knowing, for the great expeditions which have investigated the ocean-bed during the last thirty years have proved the existence of a rich and peculiar Fauna in all the great depths that have been dredged. The general results of these investigations have been recently summed up by Dr John Murray in the last volume of the Challenger Reports. He points out the extraordinary variety of life in the deep sea as shown by the contents of the dredge. “At Station 146 in the Southern Ocean, at a depth of 1375 fathoms, the 200 specimens captured belonged to 59 genera and 78 species.” He can find no record of species equal to this in depths of under 50 fathoms, and concludes that the evidence at present before us is sufficient to warrant the belief that the great depths of the ocean are as a general rule extremely rich in species.

From what has been already said, it may be gathered that nearly all the most important groups of marine animals have representatives in the deep-sea. There are Fish, Tunicates, Crustaceans, Molluscs, Echinoderms, Worms, Cœlenterates and Protozoa. Nearly all of these are so modified, either in form or colour, or the structure of their organs of sense, or in other particulars, that they could be recognised at once in a collection as deep-sea animals; but there is a small minority which seem to have undergone but little change in adapting themselves to their strange environment.

We may commence our study of this remarkable Fauna by a few remarks on their colour. The first and most striking feature is that the animals are almost invariably uniform in colour. If they are dark-brown they are dark-brown all over, if they are red they rarely exhibit bands of white or spots of blue. Moreover, they are not always in harmony with the colour of their surroundings.

In the shallow waters the animals that live among the green Sea-weeds are green, those that live on the sand are coloured like the sand, and many of those that live among the rocks are darkly pigmented with black and blue. In the abyss of the ocean, where there is any light at all, the colour is, in all probability, fairly uniform over wide tracts, and yet we may find in one haul of the dredge, black Fish, red Crustaceans, and purple Trepangs.

There seems to be no particularly predominant colour among the deep-sea animals. Most of the Fish are black or dark-brown, but many are light violet, some are pale rose and others bright red. Among the Crustaceans bright red seems to be the prevailing shade, just as the darker tints of black and brown are among the Fish.

Among the Echinoderms we find white, purple, yellow, red, and pink forms, and among the Jelly-fish and Corals, red, violet, and green. In fact it would be necessary to describe every class of animals in turn, and then almost every genus in each class, to give an adequate idea of the variety of colour met with in the Fauna of the deep-sea.

It is inconceivable that each of these animals can live amid surroundings of a colour similar to its own, and therefore we may without much hesitation believe, that the colour of deep-sea animals is not, as a general rule, of use as a protection.

Next to the peculiarities of colour, the most striking features of the more highly organised inhabitants of the bottom of the sea are the modifications of the organs of special sense. The Fish, the Crustaceans and the Molluscs almost invariably exhibit some remarkable modifications of the eyes. In their natural haunts there must be either absolute darkness, or the faint and usually intermittent light emitted by phosphorescent animals. How intense this light may be it is impossible to judge. The light that is emitted by animals on the deck of a ship can afford no criterion of the light they emit under a pressure of two tons to the square inch. However, the fact that the deep-sea animals have either very large eyes or no eyes at all, suggests forcibly that this light is not sufficient to cause a general illumination.

Some of the Fish are quite blind, and although most of these have a very small and rudimentary eye, in at least one Fish, Ipnops, which is peculiar to deep water, no trace of an eye is to be found.

In some genera with a very wide distribution, a very interesting series of stages may be found, indicating the changes that may have taken place in the history of the blind Fish of the abyss. In the genus of deep-sea Cods (Macrurus) for example, those species which live in water of less than a thousand fathoms depth have very large eyes, and those that are found in greater depths have much smaller ones.

The same in general is true of the Crustaceans. The deep-sea Cray-fish have lost not only their eyes, but also the stalks which supported them. In Bathynomus (a Crustacean belonging to the group Isopoda), however, there is a pair of enormous eyes. But as a rule the eyes of Crustaceans degenerate and disappear in shallower water than the eyes of Fishes. At depths of 500 fathoms or greater, the eyes of the Crustaceans usually show signs of reduction in size or other retrogressive changes, and in the greatest depths they are nearly always wanting altogether.

Accompanying the loss of eyesight in deep-sea animals we often find a very remarkable development of organs, which may be regarded as especially tactile in function.

Many of the deep-sea Fish, for example, with rudimentary eyes possess long barbels, and in some cases the paired fins are enormously elongated to form delicate pointed organs like the tentacles of a Polyp. Among the blind Crustaceans, too, we often find enormously long antennæ, and even the claws and legs are so long and delicate that they bring to mind the appendages of a Daddy-long-legs or a Harvest spider.

Just as a blind man acquires a remarkably acute sense of touch, so, it seems, in the course of generations, these blind animals of the abyss have acquired extremely delicate tactile organs.

The deep-sea Fauna is also remarkable for the great number of animals which are phosphorescent. As in the surface-swimming creatures the phosphorescence is not confined to a few classes, but probably occurs to a greater or less extent in all the more important groups. The word “probably” must be used in the previous sentence, because it is not yet scientifically proved that many forms which are supposed to be phosphorescent are actually so; but the evidence is conclusive that phosphorescence is a common and widespread character of most of the deep-sea Fauna.

The Fish exhibit, perhaps more than any other group, peculiar organs which are supposed, and in many cases proved to be, used for the purpose of generating light. In the Stomiatidæ, a family of Fish related to the Salmons, there are often numerous little organs, like minute bull’s-eye lanterns, arranged in rows on the sides of the body from the head to the tail, and in addition to these in some species one or more pairs of larger organs are seen on the upper lip just in front of or below the large eyes. It is not certain what the colour of the light is that is emitted by these organs, but it is very probable that if the Fish could be seen in their natural haunts they would have an extremely beautiful effect.

In describing the general characters of the shallow water Fauna in