The most remarkable form of free-swimming Tunicate that has come to light is Octacnemus bythius, a form that is probably allied to Salpa. It was found twice, once in the dredge that came from a depth of 1,070 fathoms, and once from 2,160 fathoms. The tunic of the animal is gelatinous and hyaline, but the most important feature it possesses is an imperforate membrane separating the branchial sac from the peribranchial cavity. Octacnemus, in other words, possesses no true stigmata, these structures being represented only by little pits in the walls of the branchial sac. This curious and extremely interesting modification of the respiratory organs points very strongly to the conclusion that Octacnemus is truly a deep-sea animal.
CHAPTER VIII
THE FISH OF THE DEEP SEA
Of all the groups of animals that constitute the deep-sea fauna, the fish show the greatest number of peculiarly abysmal characters. Being much more highly differentiated than the invertebrates, they possess more organs liable to undergo modifications of colour, size, and structure, and consequently we are able to point to a great many more features characteristic of deep-sea fish than we can do in any other group of animals.
The first point that calls for remark in the consideration of the fish fauna of the deep sea is the almost entire absence of ancient and primitive types. The Elasmobranchii, including the Sharks, Rays, and Chimæra, constituting the order that from anatomical embryological grounds is always regarded by naturalists as the most primitive order of this class, is represented in very deep water by only one species. Raia hyperborea and Chimæra monstrosa, it is true, just enter into the abysmal zone, but Chimæra affinis is the only Elasmobranch that extends to depths of over 1,000 fathoms.
The Ganoidei too, the order that in palæozoic and mesozoic times was so rich in genera and species, is entirely absent from the abysmal zone, not a single representative having been found at any time by any of the deep-sea expeditions.
The Dipnoi, that remarkable order including the three fresh-water genera, Ceratodus from Australia, Lepidosiren from Brazil, and Protopterus from West Africa, has no representative and no ally in the deep waters of the ocean.
The fishes of the deep sea, in fact, with only one or two exceptions, all belong to the Order Teleostei, the most modern and most highly differentiated order of the class, the families that are most fully represented being the Macruridæ and then the Ophidiidæ and Gadidæ, and the Berycidæ.
At the limits of the katantic and abysmal zones, a large number of families of Teleosteans entirely disappear, and as we approach the deepest parts of the ocean, the number of fish that are found is considerably reduced. As Dr. Günther very wisely remarks, ‘this diminution in the number may be due to the difficulty of capturing fishes at great depths, a difficulty which increases in proportion to the depths at which the dredge is worked. But it must also be regarded as evidence of the actually diminished variety of fishes.’
It may be interesting to the reader to give Dr. Günther’s table of the number of species found at different depths, as it shows, among other things, the marked change that occurs in the character of the fauna in passing from the katantic to the abysmal zone.
| Between | 100–300 | fathoms, | 232 species |
| Between | 300–500 | fathoms, | 142 species |
| Between | 500–700 | fathoms, | 76 species |
| Between | 700–1,500 | fathoms, | 56 species |
| Between | 1,500–2,000 | fathoms, | 24 species |
| Between | 2,000–2,900 | fathoms, | 23 species |