The fauna of the deep sea

by Sydney J. Hickson

The species of the group Perennichordata, which includes all those Tunicates that possess a notochord persistent through life, are chiefly pelagic in habit, the little creatures, rarely more than two or three millimetres in length, swimming or drifting about with the sagittas, copepods, ctenophores, and medusæ that compose the pelagic plankton. Fol has recently described a gigantic form belonging to this group, reaching a size of thirty millimetres in length, called Megalocercus abyssorum, which he dredged from a depth of 492 fathoms; and other species have been recorded down to a depth of 710 fathoms in the Mediterranean Sea.

Among the simple Ascidians we find no family that is peculiar to deep water; but the Cynthiidæ and Ascidiidæ both contain genera that are abysmal, and the Molgulidæ have one species, Molgula pyriformis, that extends into the abysmal zone to a depth of 600 fathoms.

In the genus Culeolus and in Fungulus cinereus and Bathyoncus, all deep-water Ascidians, there is a very curious modification of the branchial sac, the stigmata being apparently not formed, in consequence of the suppression of the fine inter-stigmatic vessels. This peculiar feature is only found in the deep-sea simple Ascidians and, as we shall see presently, in one species of the deep sea compound Ascidians, but it is not apparently an essential character of those living in the abysmal zone, notwithstanding the fact that it is found in such widely separated genera; for Corynascidia, Abyssascidia, and Hypobythius, living in depths lying between 2,000 and 3,000 fathoms below the surface, have branchial sacs of the ordinary type. Professor Herdman is of opinion that this simple form of branchial sac is not a primitive form, but most probably a modification of a more complicated type.

In Culeolus Murrayi there is a remarkably abundant supply of blood-vessels to the tunic, and these send special branches to a number of small papilliform processes on its outer surface. This system of highly vascular processes probably constitutes, as Professor Herdman suggests, an additional or complementary respiratory apparatus. All these modifications of the branchial system are of particular interest, for we find so many instances of a similar kind among the inhabitants of very deep water. I need only refer here to the modifications of this system in the Isopod Bathynomus already referred to (p. [129]), and to the reduction in the number of the gills of many of the deep-sea fishes. Why there should be such modifications is a question upon which the physical and natural history investigations of the conditions of life in the great depths of the ocean at present throw no light.

In a previous chapter I have referred to the fact that many of the bathybial animals are characterised by being stalked. Among the simple Ascidia we find many examples of stalked kinds living in deep water, such as Culeolus and Fungulus, but also several exceptions, such as Bathyoncus, Styela bythii, and Abyssascidia, that are sessile. It is a noteworthy fact, however, that the genus that has the most deep-sea species—namely, Culeolus—is a genus that is provided with a very long stalk. Furthermore, the only known stalked forms of the very large family Ascidiidæ are the abysmal genera Corynascidia and Hypobythius.

Fig. 20.—Hypobythius calycodes. G, nerve ganglion; H, heart; M, the position of the atriopore. The large opening on the upper side is the mouth. (From a drawing by Professor Moseley in Herdman’s ‘Tunicata of the “Challenger” Expedition.’)

The most remarkable character of the genus Hypobythius is the simple condition of its branchial sac, reminding one of the structure of this organ in the shallow-water genus Clavelina. ‘There are no folds and there are no internal bars,’ to quote the description given by Professor Herdman; ‘only a single system of vessels can be recognised, branching and anastomosing so as to form a close network, the small rounded meshes of which are the stigmata. The tentacles and dorsal lamina cannot be made out.’

Among the compound Ascidians only four families extend into the abysmal zone, namely, the Botryllidæ, Polyclinidæ, Didemnidæ, and Cœlocormidæ, and of these only one species, Pharyngodictyon mirabile, of the family Polyclinidæ, extends into water of greater depth than 1,000 fathoms. In Pharyngodictyon we find the same curious simplification of the branchial sac that we have just referred to in the genera of simple Ascidians, Culeolus, Fungulus, and Bathyoncus. Cœlocormus Huxleyi from a depth of 600 fathoms is a very peculiar form and the type of a separate family, the Cœlocormidæ.

The free-swimming Tunicata included in the group Ascidiæ salpiformes, which contains the genus Pyrosoma, and the order Thaliacea containing the salps, are in all probability mainly confined to the surface waters. A few specimens of Pyrosoma were captured by the ‘Challenger’ dredges which came up from very deep water, but it is doubtful at what point in the journey to the surface the specimens entered the net.