CHAPTER VI

CRUSTACEA OF THE DEEP SEA

It has already been mentioned that the animals living on the sea-bottom in shallow water do not differ greatly in character from those that may be found between tide-marks. As we go farther out from land, however, into the deeper water, the character of the fauna gradually changes. One by one the species found near the shore become rare and disappear, and their places are taken by others characteristic of the intermediate depths. These in their turn give way to others, till in the abysses of the great oceans we find an assemblage of strange animals adapted to the conditions of life in the great depths, and differing widely in many respects from the more familiar inhabitants of the coastal waters. In this "fauna of the deep sea," which extends to the greatest depths reached by the dredge or trawl, the Crustacea occupy a prominent place. Before proceeding to discuss some of these peculiar forms, however, it is necessary to attempt to form some idea of the conditions under which they live.

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, 14-1/2 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 2-1/2 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.

In animals that live in perpetual darkness we should expect to find, in accordance with the principle of adaptation which runs through the whole of organic nature, that the eyes are wanting or imperfectly developed. In a great many deep-sea animals this is indeed the case. The deep-sea Lobsters of the genus _Nephropsis_ (Fig. 42), which are very closely allied to the Norway Lobster (_Nephrops_) of shallow water, have very short and slender eye-stalks hidden under the rostrum, and showing at the tip only the merest traces of what was once an eye. In the lobster-like Eryonidea (see Fig. 46, p. 133), the reduced eye-stalks are firmly fixed in notches in the front edge of the carapace. Some of the deep-sea Crabs and Prawns seem also to be totally blind. In a great many cases degeneration has not quite gone so far, and the eyes are present, although much reduced and modified. Thus, the very numerous deep-sea species of Galatheidæ, belonging to the genus _Munidopsis_ (Fig. 43) and its allies have, as Alcock says, "pallid, milky-yellow, lack-lustre eyes which, though they may perhaps serve to distinguish between light and darkness, can never form a definite visual image." It is probable, indeed, that these pale-coloured eyes are specially adapted for vision in a dim light, for it has been shown that in certain deep-sea Euphausiacea the pigment-sheaths between the separate elements of the compound eyes are greatly reduced, and are fixed in the position temporarily assumed by those in the eyes of normal Crustacea when kept in the dark. Be this as it may, there are many deep-sea Crustacea which have well-developed and darkly-pigmented eyes. Some of these are swimming forms, which may at times migrate into the upper strata of water to which some rays of light penetrate; but there are some cases of Crabs and other bottom-living species that have well-developed eyes, although they live at great depths. This would seem to suggest that, although shut off from the light of day, they are not condemned to grope in perpetual darkness. Many deep-sea animals are known to be phosphorescent, and it seems probable that the large-eyed species may profit by the light emitted by the glow-worms and fireflies of the abyss. Thus, Alcock points out that the deep-sea Hermit Crab _Parapagurus pilosimanus_ (Plate XVI.), which lives in partnership with a colony of Sea-anemones which it carries about with it, has large eyes, although it descends to depths of at least 2,000 fathoms; and he suggests that the Crab may be able to see its way by the light emitted by the zoophytes.

Some of the Crustacea, however, are themselves luminous. Thus, Alcock records how specimens of a deep-sea Prawn, _Heterocarpus alphonsi_, "poured out, apparently from the orifices of the 'green glands' at the base of the antennæ, copious clouds of a ghostly blue light of sufficient intensity to illuminate a bucket of sea-water so that all its contents were visible in the clearest detail." Certain other Prawns are known to possess special light-producing organs on various parts of the body and limbs. It is in the Euphausiacea, however, that these organs have been most fully examined, and although the members of this group (see Fig. 24, p. 56) are by no means all deep-sea animals, some of them occurring at the surface of the sea, the structure of their luminous organs, or "photophores," may appropriately be described here. They are situated on the under-surface of the abdomen, in the basal segments of some of the thoracic legs, and on the upper surface of the eye-stalks. Each consists of a globular capsule covered by a layer of pigment, except on the outer side, where there is a transparent biconvex lens. In the centre of the capsule is a peculiar "striated body" which seems to be the actual seat of luminescence, and behind it is a concave reflector composed of concentric lamellæ, and having a silvery lustre. Before their luminosity was observed, these organs were described as "accessory eyes," but there can be little doubt that they serve rather as searchlights, although, from the positions that some of them occupy on the body, it is not easy to see how they can illuminate objects within range of the eyes. That the function of phosphorescent organs is not always that of giving light for their possessor to see by is shown by the fact that many luminous animals are blind. It is important to notice, however, that these blind animals never have complex "photophores" like those just described, but only exhibit a diffuse luminosity or give off luminous secretions; as an example among Crustacea, the blind Eryonidea (see Fig. 46, p. 133) may be mentioned, one species of which was observed by Alcock to be "luminous at two points between the last pair of thoracic legs, where there is a triangular glandular patch." In a recent discussion of the whole question of phosphorescence in marine organisms, Dr. Doflein concludes that the part it plays in the life of the animal probably differs in the different cases. In some it may serve to attract prey, as moths are attracted to a candle; in others it may help individuals of the same species to keep together in a swarm or to find their mates, the varying arrangement of the photophores producing characteristic light-patterns that serve as "recognition marks" like the colour-patterns of animals that live in the light of day. The clouds of luminous secretion thrown out by _Heterocarpus_ and other Prawns, and by certain Mysidacea and Ostracods, may serve to baffle pursuers, like the cloud of ink thrown out by a Cuttlefish, and in some cases the more complex organs may illuminate objects within the range of vision. That this does not exhaust the possibilities of speculation on the subject, however, is shown by the case of certain deep-sea Prawns which have been recently discovered to possess photophores placed so as to illuminate the interior of the gill cavities. What function they can discharge in this position seems beyond conjecture.

The colours of deep-sea Crustacea are very curious. Few of them have the blanched appearance common, for instance, in animals that live in the darkness of caves; on the contrary, their colours are often very vivid, but they are nearly always uniform, without spots or markings, and in a large proportion of cases are in some shade of red or orange. This red colour seems to be associated, in some way that we do not understand, with the darkness of their habitat. The general absence of markings is very striking. Dr. Alcock remarks that in deep-sea Crustacea we never see "those freaks of colour, or those labyrinthine mottlings and dapplings, that excite our curiosity when handling the Crabs and Shrimps of the reefs." Possibly the explanation of this may be that in these dwellers in darkness colour is merely, as it were, an accident, a by-product of physiological processes directed to other ends, not a character of protective or warning value, as in animals that hunt and are hunted in the light of day. It is a curious fact, which may have some bearing on this problem, that in many cases, while the adults are coloured in some shade of red, the eggs carried by the female are bright blue or green.

Some of the peculiarities of structure observed in deep-sea Crustacea seem to be correlated with the difficulties of resting or moving about with security on the soft ooze of the sea-floor. Among the Crabs we find a preponderance of long-legged species, not only among the true Spider Crabs (Oxyrhyncha), but also in other groups (Dromiacea like _Latreillia_, figured on Plate XIX., and Oxystomata), the members of which assume the same spider-like form. In some cases the legs are fringed with long stiff hairs, which may help to prevent the animal from sinking in the ooze, and the spines on the body and legs of many species may have the same effect. Among the deep-sea Prawns, the species of the family Nematocarcinidæ (Plate XVII.) have extremely long and slender legs, which we may assume to be used like stilts for walking over the soft ooze.

Not much is known regarding the food of deep-sea animals. In the absence of plant-life they must of necessity be all carnivorous, and all ultimately dependent on the food-supply falling from above. Some species have been found to have the food-canal filled with Globigerina ooze, which they no doubt swallow, as earth-worms do the soil in which they burrow, for the purpose of extracting the nutriment that it contains. In one species of deep-sea Cumacea (_Platycuma holti_), which appears to feed in this manner, the food-canal is coiled, a condition very rare in Crustacea; in all probability this is due to the necessity for an increase of the absorptive surface, since it is common to find such an increase, either by lengthening and consequent coiling of the gut, or by infolding of its walls, in animals that have to swallow large quantities of relatively innutritious food material. Many species, however, no doubt have more selective habits of feeding. The lobster-like _Thaumastocheles_ (Fig. 44), which was dredged by the _Challenger_ expedition in the West Indies at a depth of 450 fathoms, and has since been got from deep water off the Japanese coast, has one of the chelæ enormously enlarged, with long and slender fingers set with spines like the teeth of a rake. It has been suggested that this remarkable claw may be used for raking or sifting the ooze for small animals on which the _Thaumastocheles_ feeds. A similar function may be suggested for the long and spiny first pair of walking legs in the Spider Crab _Platymaia_ (Fig. 45).

In many deep-sea Crustacea the eggs are of very large size, indicating that the young are hatched in an advanced stage of development. For example, in the numerous species of the genus _Munidopsis_ the eggs are always large and correspondingly few in number, in striking contrast to the closely allied genus _Galathea_, from shallow water, in which the eggs are small and very numerous. Alcock mentions that a deep-sea Prawn of the genus _Psathyrocaris_, although only about 3-1/2 inches long, has eggs nearly a quarter of an inch in length. It would seem that, in some way or other, the conditions are unfavourable for a free-swimming larval life; but they cannot be altogether prohibitive, for there are a good many characteristically deep-sea Crustacea, such as the Eryonidea, that have small eggs and presumably a larval metamorphosis.

The uniformity of the physical conditions over vast areas in the deep sea is no doubt the cause of the enormously wide geographical range of many species of deep-sea animals. There are many examples of this among Crustacea, and they are added to by every deep-sea dredging expedition. For example, the giant Isopod _Bathynomus_ (Plate XVIII.) was first discovered in West Indian seas, and the same species has since been dredged near Ceylon, while a second species has been found off the Japanese coast. Of the strange lobster-like _Thaumastocheles_ (Fig. 44), mentioned above, only four specimens are known--one dredged by the _Challenger_ in the West Indies, and three others more recently brought from Japan.

The low temperatures prevailing in deep water, even in tropical seas, render it possible for many Crustacea to live there which are closely allied to, or identical with, species occurring in shallow water in the colder seas of the North and South. Many examples of this are mentioned by Dr. Alcock in his discussion of the deep-sea fauna of Indian seas; for example, the Lobster _Nephrops andamanicus_, found at depths of 150 to 400 fathoms in the Indian seas, is very closely allied to the Norway Lobster (_Nephrops norvegicus_) of our own coasts. To some extent this fact affords an explanation of the phenomenon that has been called "bipolarity" in the distribution of marine animals. It has been observed that certain families, genera, and even species, are found in the Arctic and Antarctic seas, although they seem to be entirely absent from the intervening tropical zones. In some cases, however, it has been found that these forms occur in the deep sea in the warmer regions where the cold water offers them a connection between North and South without any great difference of temperature.

In the early days of deep-sea exploration, when naturalists were becoming aware of the rich fauna inhabiting the abysses of the ocean, which till then had been supposed to be barren of all life, it was confidently expected that representatives would be discovered of some of the animals known as fossils from the earlier geological periods. It was believed that the great ocean basins had remained unchanged for vast periods of geological time, and that numerous "living fossils" would be found surviving in the depths. These hopes have not been fully realized, for the deep-sea fauna as a whole has proved to be of a comparatively modern type; nevertheless, it does include a considerable number of primitive and old-fashioned forms of life, some of which belong to groups elsewhere extinct. This is conspicuously the case among the Crustacea. The lobster-like Eryonidea, which at the present day are only found in the deep sea, were long known as fossils before they were discovered to survive as living animals. The existing species (Fig. 46) are all blind, with only vestiges of eye-stalks, and they may be readily distinguished by the fact that the first four, and sometimes all five, pairs of legs end in chelæ, no other Decapods having more than three pairs of chelate legs. The fossils occur in rocks of the Secondary Period, from the Trias to the early Cretaceous. Some of them, at least, had well-developed eyes, and probably lived in shallow water. This was almost certainly the habitat of those (Fig. 47) that are found preserved in a marvellously perfect state in the lithographic limestone of Solenhofen (famous for the discovery of _Archæopteryx_ and many other remarkable fossils), which is believed to have been deposited in a lagoon. After the early part of the Cretaceous epoch, the Eryonidea are no longer found as fossils, and it is, at all events, a probable conjecture that about that period they forsook the shallow waters for the deeper recesses of the ocean, where their descendants have held their own till the present day.

Another group of deep-sea Crustacea which has affinities with certain fossil forms is the little family Homolodromiidæ among the Crabs. It has already been mentioned that the Dromiacea are the most primitive tribe of the Brachyura, and Professor Bouvier has shown that among these the Homolodromiidæ approach most nearly to the lobster-like forms from which the Crabs have been derived. He has further shown that the members of this family closely resemble in the arrangement of the grooves upon the carapace the extinct Prosoponidæ, which are known as fossils from Jurassic and Cretaceous rocks.

It is in the deep sea also that we find the curious Hermit Crabs of the family _Pylochelidæ_ (Fig. 37, p. 94), which are perfectly symmetrical and show no trace of having ever adopted the habit of living in Gastropod shells; so primitive, indeed, are these forms that it is not easy to find characters by which to define them from the lobster-like Thalassinidea or from the true Lobsters themselves, and, although no fossil representatives are yet known, there seems no reason to doubt that the Pylochelidæ are nearly related to the primitive stock from which the other Hermit Crabs have been evolved. Among the deep-sea Prawns there are many forms, both of Penæidea and of Caridea, which are more primitive than most of their relatives from shallow water; and although in these cases also the geological records are faulty, we may assume, if we cannot prove in detail, a general similarity to the fossil Prawns of Mesozoic rocks.

When all has been said, however, perhaps the most surprising thing about the deep-sea fauna is, not that the animals are unlike those living in shallow water, but that they differ from them so little. When we consider the physical conditions of the oceanic abysses--the absolute darkness, the freezing cold, the pressure measured in tons on the square inch--it would seem inevitable that the physiological processes of deep-sea animals must differ greatly from those of animals living in shallow water; yet in very many cases these differences of function are accompanied only by the most trivial differences in structure. To take one example, the "Pink Shrimp" (_Pandalus montagui_), which we may find commonly between tide-marks on our own coasts, differs only in inconspicuous details from species of the same genus living at a depth of 600 fathoms; while other genera of the family Pandalidæ range downwards to 2,000 fathoms or more, without any important divergences in structure.