The fauna of the deep sea

by Sydney J. Hickson

In many groups of animals it has been shown that some of the deep-sea species are relatively much larger in size than the shallow-water species, and that others, more rarely, are much smaller, the abysmal fauna reminding us in this respect of the characters of the alpine flora.

The Isopoda show many examples of this largeness in size, thus Bathynomus giganteus, dredged by Professor Agassiz off the Tortugas at a depth of over 900 fathoms, reaches the enormous size, for an Isopod, of 9 inches (fig. [15]). Stenetrium haswelli, again, is larger than any of the shallow-water species of the genus, and the same remark applies to the deep-sea species of the genus Ichnosoma, while Iolanthe acanthonotus, from a depth of nearly 2,000 fathoms, is considerably larger than most of the shallow-water Asellidæ.

There is another very common character of deep-sea Crustacea that is also well exemplified in the group of the Isopods, and that is the extraordinary length and number of the spines covering the body.

I have already referred to this character in the supposed deep-sea Copepod Pontostratiotes abyssicola, and I shall have again to refer to it in treating of the Decapoda and other groups of the Crustacea.

Besides its enormous size Bathynomus possesses some other characters that may be correlated with its deep-sea environment. The respiratory organs are quite different from those of other Isopods; instead of being borne by the abdominal appendages, they are in the form of branched outgrowths from the body-wall containing numerous blood-lacunæ, and the appendages simply act as opercula to cover and protect them. The eyes of the Bathynomus too are remarkably well developed, each one bearing 4,000 facets, and they are directed not dorsally as in the Cymothoadæ, but ventrally. The cause of these curious modifications of structure in Bathynomus is by no means clear, but it is quite probable that they are connected with the conditions of pressure and light in the deep sea. It is a remarkable fact that the other deep-sea Isopods do not exhibit precisely these modifications, and it might be supposed that the same causes would produce the same or similar effects on the structure of animals belonging to the same order. That is perfectly true, but we cannot yet determine how long ago any one species has taken to a deep-sea life, or what length of time, in other words, these conditions have been at work in modifying the structure of the organism. A recent immigrant into the abyss will naturally exhibit closer affinities with its shallow-water allies than those that have dwelt in the region since secondary or tertiary times. If we take this into consideration we should expect to find considerable differences occurring between deep-sea species of the same order, which is precisely what we do find.

Fig. 15.—Bathynomus giganteus. From a depth of 1,740 metres. (From Filhol.)

Concerning the Cirripedia, that curious group of profoundly modified Crustacea that includes the barnacles and acorn shells, Dr. Hoek writes in the ‘Challenger’ monograph:—

‘Though unquestionably by far the greater part of the known Cirripedia are shallow-water species, and though some of the species are capable of living at a considerable variety of depths, as, for instance, Scalpellum stroemii, yet it must be granted that the number of true deep-sea species of Cirripedia is very considerable.’ Only two genera, however, occur in depths of over 1,000 fathoms, and these—Scalpellum and Verruca—occur also as fossils in secondary and tertiary deposits. The oldest of all fossil cirripedes, however, namely, Pollicipes, never occurs, at the present day, in deep water, but is purely littoral or neritic in habit. But what is perhaps more interesting still is the fact, that, when we come to compare the living and the fossil species, we find that in the one genus (Scalpellum) the deep-sea forms have preserved the more archaic characters, and in the other (Pollicipes) the shallow-water forms.

Here then we are presented with a veritable puzzle for which we can at present frame no manner of answer. Pollicipes on the one hand—like Lingula among the brachiopods—has been able to maintain itself almost unchanged amid the tremendous struggle for life of the shallow water of the tropics ever since the Lower Oolite epoch; while Scalpellum, on the other hand, has either become profoundly modified, or been driven into the abysmal depths of the ocean.