Pearls & Parasites

by Sir A. E. Shipley

There is, however, an abundance of animal life. The dredge reveals a surprising variety and wealth of form. Sir John Murray records ‘at station 146 in the Southern Ocean, at a depth of 1,375 fathoms, that 200 specimens captured belonged to 59 genera and 78 species.’ He further states that this was ‘probably the most successful haul, as regards number, variety, novelty, size, and beauty of the specimens,’ up to the date of the dredging; but even this was surpassed by the captures from the depths at station 147. The Southern Ocean is particularly well populated. The same writer says: ‘The deep-sea fauna of the Antarctic has been shown by the Challenger to be exceptionally rich, a much larger number of species having been obtained than in any other region visited by the expedition; and the Valdivia’s dredgings, in 1898, confirm this.’ There seems to be no record of such a wealth of species in depths of less than 50 fathoms, and we are justified in the belief that the great depths are extremely rich in species.

The peculiar conditions under which the Benthos live have had a marked influence on their structure. Representatives of nearly all the great divisions of the animal kingdom which occur in the sea are found in the depths. Protozoa, sponges, cœlenterata, round-worms, annelids, crustacea, polyzoa, brachiopoda, molluscs, echinoderms, ascidians, fishes, crowd the sea-bottom. The Valdivia has brought home even deep-sea ctenophores and sagittas, forms hitherto associated only with life at the surface. The same expedition also secured adult examples of the wonderful free-swimming holothurian, Pelagothuria ludwigi, which so curiously mimics a jelly-fish. It was taken in a closing-net at 400 to 500 fathoms near the Seychelles. Most of these animals bear their origin stamped on their structure, so that a zoologist can readily pick out from a miscellaneous collection of forms those which have a deep-sea home. We have already referred to a certain ‘stalkiness,’ which lifts the fixed animals above the slowly deepening ooze. Possibly the long-knobbed tentacles of the deep-sea jelly-fish, Pectis, on the tips of which it is thought the creature moves about, may be connected with the same cause. The great calm of the depths and its effect upon the symmetry of the body have also been mentioned; but greater in its effect on the bodies of the dwellers in the ocean abysses is the absence of sunlight.

No external rays reach the bottom of the sea, and what light there is must be supplied by the phosphorescent organs of the animals themselves, and must be faint and intermittent. A large percentage of animals taken from the deep sea show phosphorescence when brought on deck; and it may be that this emission of light is much greater at a low temperature, and under a pressure of 1 to 2 tons on the square inch, than it is under the ordinary atmospheric conditions of the surface. The simplest form which these phosphorescent organs take is that of certain skin-glands which secrete a luminous slime. Such a slime is cast off, according to Filhol, by many of the annelids; and a similar light-giving fluid is exuded from certain glands at the base of the antenna and elsewhere in some of the deep-sea shrimps. But the most highly developed of the organs which produce light are the curious eye-like lanterns which form one or more rows along the bodies of certain fishes, notably of members of the Stomiadæ, a family allied to the salmons. From head to tail the miniature bull’s-eyes extend, like so many portholes lit up, with sometimes one or two larger organs in front of the eyes, like the port and starboard lanterns of a ship, so that when one of these fishes swims swiftly across the dim scene it must, to quote Kipling again, recall a liner going past ‘like a grand hotel.’ Sometimes the phosphorescent organ is at the tip of a barbel or tentacle, and it is interesting to note that the angler-fish of the deep sea has replaced its white lure, conspicuous in shallow water, but invisible in the dark, by a luminous process, the investigation of which leads many a creature into the enormous, toothed mouth of the fish.

A peculiar organ, known by the name ‘phæodaria,’ exists in the body of certain radiolarians found only in the deep seas. It has been suggested that this structure gives forth light; and, if this be the case, the floor of the ocean is strewn with minute glow-lamps, which perhaps give forth as much light as the surface of the sea on a calm summer’s night. There is, however, much indirect evidence that, except for these intermittent sources, the abysses of the ocean are sunk in an impenetrable gloom.

When physical conditions change, living organisms strive to adapt themselves to the changed conditions. Hence, when the inhabitants of the shallower waters made their way into the darker deeps, many of them, in the course of generations, increased the size of their eyes until they were out of all proportion to their other sense-organs. Others gave up the contest on these lines, and set about replacing their visual organs by long tactile tentacles or feelers, which are extraordinarily sensitive to external impressions. Like the blind, they endeavour to compensate for loss of sight by increased tactile perception; and in these forms the eyes are either dwindling or have quite disappeared. An instance in point is supplied by the crustacea, many of whom have not only lost their eyes, but have also lost the stalk which bore them; but amongst the crustacea some genera, such as Bathynomus, have enormous eyes with as many as four thousand facets. It is noticeable that this creature has its eyes directed downwards toward the ground and not upwards, as is the case with its nearest allies. On the whole the crustacea lose their eyes more readily, and at a less depth, than fishes. Many of the latter—e.g., Ipnops—are blind, and in others the eyes seem to be disappearing. Thus, amongst the deep-sea cod, Macrurus, those which frequent the waters down to about 1,000 fathoms, have unusually large eyes, whilst those which go down to the deeper abysses have very small ones. Many of the animals which have retained their eyes carry them at the end of processes. Chun, in his brilliant account of the voyage of the Valdivia, has figured a series of fishes whose eyes stand out from the head like a pair of binoculars; and similar ‘telescope’ eyes, as he calls them, occur on some of the eight-armed cuttle-fish. The larva of one of the fishes has eyes at the end of two stalks, each of which measures quite one-fourth of the total length of the body.

The colour of the deep-sea creatures also indicates the darkness of their habitat. Like cave-dwelling animals, or the lilac forced in Parisian cellars, many of them are blanched and pale; but this is by no means always the case. There is, in fact, no characteristic hue for the deep-sea fauna. Many of the fishes are black, and many show the most lovely metallic sheen. Burnished silver and black give a somewhat funereal, but very tasteful appearance to numbers of deep-sea fish. Others are ornamented with patches of shining copper, which, with their blue eyes, form an agreeable variety in their otherwise sombre appearance. Many of the fishes, however, present a gayer clothing. Some are violet, others pale rose or bright red. Others have a white almost translucent skin, through which the blood can be seen and its course traced even in its finer vessels. Purples and greens abound amongst the holothurians; other echinoderms are white, yellow, pink, or red. Red is, perhaps, the predominant colour of the crustacea, though it has been suggested that this colour is produced during the long passage to the surface, and that some of the bright reds which we see at the surface are unknown in the depths. Violet and orange, green and red, are the colours of the jelly-fishes and the corals.

It thus appears that there is a great variety and a great brilliancy amongst many of the bottom fauna. With the exception of blue, all colours are well represented; but the consideration of one or two facts seems to show that colour plays little part in their lives. Apart from the fact that to our eyes, at any rate, these gorgeous hues would be invisible in the depths, it is difficult to imagine that each of these gaily-coloured creatures can live amongst surroundings of its own hue. Again, it is characteristic that the colour is uniform. There is a marked absence of those stripes, bands, spots, or shading which play so large a part in the protective coloration of animals exposed to light. Although there is no protective coloration amongst the animals of the deep sea, the luminous organs, which make, for instance, some of the cuttlefishes as beautiful and as conspicuous as a firework, may, in some cases, act as warning signals. Having once established a reputation for nastiness, the more conspicuous an animal can make itself the less likely is it to be interfered with. One peculiarity connected with pigment, as yet inexplicable, is the fact that, in deep-sea animals, many of the cavities of the body are lined with a dark or, more usually, a black epithelium. The mouth, pharynx, and respiratory channels, and even the visceral cavity, of Bathysaurus and Ipnops, and indeed of all really deep-sea fishes, are black. It can be of no use to any animal to be black inside; and the only explanation hitherto given is that the deposit of pigment is the expression of some modification in the excretory processes of the abysmal fishes.

It was mentioned above that the absence of eyes is to some extent compensated by the great extension of feelers and antennæ. Many of the jelly-fishes have long free tentacles radiating in all directions; the rays of the ophiuroids are prolonged; the arms of the cuttle-fish are capable of enormous extension. The antennæ of the crustacea stretch widely through the water, and, in Aristoeopsis, cover a radius of about five times the body-length. In Nematocarcinus the walking-legs are elongated to almost the same extent; and this crustacean steps over the sea-bottom with all the delicacy of Agag. The curious arachnid-like pycnogonids have similarly elongated legs, and move about, like the ‘harvestmen’ or the ‘daddy-long-legs,’ with each foot stretched far from the body, acting as a kind of outpost. The fishes, too, show extraordinary outgrowths of this kind. The snout may be elongated till the jaws have the proportions of a pair of scissor-blades, each armed with rows of terrible teeth; or long barbels, growing out from around the mouth, sway to and fro in the surrounding water. In other cases the fins are drawn out into long streamers. All these eccentricities give the deep-sea fishes a bizarre appearance; their purpose is plainly to act as sensory outposts, warning their possessor of the presence of enemies or of the vicinity of food.

All deep-sea animals are of necessity carnivorous, and probably many of them suffer from an abiding hunger. Many of the fishes have enormous jaws, the angle of the mouth being situated at least one-third of the body-length from the anterior end. The gape is prodigious, and as the edge of the mouth is armed with recurved teeth, food once entering has little chance of escape. So large is the mouth that these creatures can swallow other fish bulkier than themselves; and certain eels have been brought to the surface which have performed this feat, the prey hanging from beneath them in a sac formed of the distended stomach and body-wall. It has been said of the desert fauna that ‘perhaps there never was a life so nurtured in violence, so tutored in attack and defence as this. The warfare is continuous from the birth to the death.’ The same words apply equally to the depths of the ocean. There, perhaps, more than anywhere else, is true the Frenchman’s description of life as the conjugation of the verb ‘I eat,’ with its terrible correlative, ‘I am eaten.’

Connected with the alimentary tract, though in some fishes shut off from it, is the air-bladder, an organ which contains air secreted from the blood, and which, amongst other functions, serves to keep the fish the right side up. The air can be reabsorbed, and is no doubt, to some extent, controlled by muscular effort; but there are times when this air-bladder is a source of danger to deep-sea fishes. When they leave the depths for shallower water, where the pressure is diminished, the air-bladder begins to expand; and, should this expansion pass beyond the control of the animal, the air-bladder will act as a balloon, and the fish will continue to rise with a rate of ascension which increases as the pressure lessens. Eventually the fish reaches the surface in a state of terrible distortion, with half its interior hanging out of its mouth. Many such victims of levitation have been picked up at sea, and from them we learnt something about deep-sea fishes before the self-closing dredge came into use.