The fauna of the deep sea

by Sydney J. Hickson

Most of the errant Polychætes found at great depths are said to be most brilliantly coloured, and some of these, such as Eunice amphiheliæ, have the power of emitting a bright phosphorescent light; but there seem to be no very definite and constant characters separating these forms from the Polychætes of shallow waters.

As is the case with many other orders of animals, the species of Annelida living in deep water are either blind or possess eyes of a remarkably large size. Genityllis oculata may be taken as an example of a deep-sea annelid with large eyes. This annelid, belonging to the family Phyllodocidæ, was found at a depth of 500 fathoms in the Celebes sea. It possesses two enormous eyes which cover almost the whole of the head, and there can be no doubt, from the investigations of Dr. Gunn on their minute anatomy, that they are perfectly functional.

Before leaving the Annelida a brief notice must be made of the very extraordinary form Syllis ramosa, found parasitic on a hexactinellid sponge at depths of about 100 fathoms. It is chiefly remarkable for the very complicated manner it has of producing buds which do not immediately become detached from the parent, but form a compound network which ramifies through the interstices of the sponge like the colony of a Hydromedusan.

Passing now to the sub-kingdom Mollusca, we shall find that all the classes are represented in the abysmal fauna.

The Lamellibranchiata, or bivalves, occur in almost all depths of the ocean, Callocardia pacifica and Callocardia atlantica having been found at the enormous depth of 2,900 fathoms. Some species, such as Venus mesodesma, have a very wide bathymetrical distribution, but others are only known to occur in deep water.

Concerning the characters of the deep-sea Lamellibranchiates, Mr. Smith, in his report on the Lamellibranchia of the ‘Challenger’ expedition, says ‘very deep-water benthal species certainly have a tendency to be without colour, and of thin structure, no doubt resulting from the absence of light the difficulty of secreting lime, the scarcity of food and other unfavourable conditions of existence.’ But notwithstanding this, the same author continues: ‘The species are apparently few in number in comparison with those of shallow water; and new and peculiar generic forms which we naturally expected would have been discovered are of even still rarer occurrence.’

As in the group of the Lamellibranchia, so in the Gasteropoda, no very remarkable new genera or species have been found in deep sea. Some shallow-water genera, such as Fusus for example, have representative species in the abyss; but, with the exception of a want of brilliant coloration and marking and a thinness of the shell, the deep-sea forms do not exhibit any characteristic features. One of the most brightly coloured shells found at great depths is that of Scalaria mirifica, which is tinted rose and white, but this seems to be quite an exceptional character among the deep-sea Gasteropods. Several new genera were found in the deep water, but their general characters do not call for any special remark.

Among the Cephalopoda there seems to be little doubt that the genera Cirroteuthis, Bathyteuthis, and Mastigoteuthis are entirely abysmal, and the same applies probably to one or two species of octopus; but as Hoyle remarks, ‘apart from the single fact that Bathyteuthis and Mastigoteuthis both have slender filiform tentacles with minute suckers, no structural features have been discovered which will serve to diagnose a deep-sea form from a shallow-water one.’

The exact habitat of the interesting genus Spirula is unfortunately still unknown. In some parts of the tropics the shores are covered with spirula shells, and yet the animals that secrete them are still to be reckoned amongst the greatest rarities of our museums. The numerous dredgings of the ‘Challenger’ only brought to light one specimen of this animal, and that from a depth of 360 fathoms, and the ‘Blake’ caught one at a depth of 950 fathoms, so that there can be little doubt that Spirula lives in deep water.