The fauna of the deep sea

by Sydney J. Hickson

As regards the general character presented by the deep-sea fishes, I have already pointed out in the chapter dealing with the general characters of the deep-sea fauna, the peculiarities in the size of the eyes, the colours and markings of the body, and the texture of the bones and muscles. There are, however, a few more characters of which mention must be made.

Notwithstanding the fact that all the abysmal fishes are carnivorous and must consequently be capable—in the great number of cases—of rapid and vigorous movement, the muscles of the trunk and tail are usually thin, and the fascicles loosely connected with one another.

Deep-sea fish are not characterised by an absence of the swimming bladder. This organ occurs just as frequently and in the same families as in the shallow-water fauna, but we do not know whether it possesses any special peculiarities or not, as it is usually so ruptured and destroyed by the change of pressure it undergoes in being brought to the surface, that it is impossible to make any thoroughly accurate investigation of its anatomy and relations.

The extraordinary development of glands in the skin which secrete mucus, and the presence in many forms of very highly specialised organs for emitting phosphorescent light, are characters of the deep-sea fish fauna, to which I have referred in a previous chapter.

As with the Tunicates, some of the Crustacea and other groups, the fish of the abysmal zone show curious modifications of the respiratory system. The gill laminæ of these animals are not only reduced in number, but appear to be short and shrunken. It is possible, of course, that during life they may end in fine delicate points which are broken off or ruptured during their capture, but still the horny rods that support them are shorter than they are in shallow-water forms, and the general evidence of their structure tends to show that they have undergone profound modifications in the change to the conditions of deep-sea life.

An extremely common and almost general character of deep-sea fishes is the black coloration of some of the body cavities; this is limited to the pharynx in many of the fishes that live about the hundred fathoms limit, but the colour is more intense and spread all over the oral, branchial, and peritoneal cavities in typical deep-sea forms. It may seem very difficult at first to account for this remarkable development of black pigment in parts of the body that are not usually, and, in some cases, cannot at any time be exposed to view. It is obvious that it cannot be functional as a hiding colour, either in offence or defence. But it is quite possible that it is due to some modification of the function of excretion. It is well known that in many cases of disease or injury to the kidneys in vertebrates, the colour of the skin is affected, and every one recognises now the fact that in many invertebrates the colour of the skin is greatly dependent upon the function of the secretion of the urates.

It would at least be interesting to know if this dark coloration of the mucous membranes is in any way correlated with any modification of the structure or function of the kidneys. At present we have no recorded observations on this point, but it is to be hoped that, when we have a sufficient number of specimens brought home from the deep water, a systematic investigation of this subject will be made.

Lastly, it should be pointed out that our knowledge of the abysmal fauna has not, at present, brought to light any evidence that the fish are of an extraordinarily large size. In many groups of animals, as I have frequently pointed out in the last few chapters, the large and gigantic species or specimens are only found in the abyss. This may also be the case with fishes, but we have no evidence that it is so. The only methods that have been used at present for the investigation of the fauna living on or near the floor of the deep oceans, are not of a kind to lead to the capture of really large fish. That they may exist is highly probable, but all that we know at present is, that the fish with which we are acquainted living at great depths are not in any way remarkable for their great size.

Of the only two Elasmobranchs, one, namely Raia hyperborea, has been found in water extending from 400 to 608 fathoms in depth. Only four specimens have yet been taken, one by the Norwegian expedition off Spitzbergen and three by the ‘Knight Errant’ off the northern coasts of Scotland. It is interesting to find that this, the only deep-sea species of the Rays, shows some striking peculiarities. ‘The teeth are remarkably slender,’ says Günther, ‘small, irregularly and widely set, different from those of other British Rays. In young specimens at any rate those of the male do not differ from those of the female. The mucous membrane behind the upper jaw forms a pad with a lobulated surface. The mucous cavities of the head are extremely wide, and finally the accessory copulatory organs have a spongy appearance, and are flexible, the cartilage by which they are supported being a simple slender rod.’

The other Elasmobranch, that extends into very deep water, is Chimæra affinis, a species which can hardly be distinguished from the better known Chimæra monstrosa, a fish that itself very frequently wanders within the limits of the abysmal zone.