The fauna of the deep sea

by Sydney J. Hickson

The shells of the Gasteropods and Lamellibranchs living in the abyss are frequently so thin as to be almost transparent, and are, with very few exceptions, white or pale straw coloured. The colour of the only specimen of nudibranchiate Mollusca that has been found in the abysmal zone, namely, Bathydoris abyssorum, is described by Mr. Murray as follows: ‘The body of the living animal was gelatinous and transparent, the tentacles brown, the gills and protruding external generative organ orange, the foot dark purple.’

Among the Crustacea various shades of red are the prevailing colours. ‘The deep-sea types, like Gnathophausia, Notostomus, and Glyphocrangon,’ says Agassiz, ‘are of a brilliant scarlet; in some types, as in the Munidæ and Willemoesiæ, the coloration tends to pinkish or yellowish pink, while in Nephrops and Heterocarpus the scarlet passes more into greenish tints and patches.’[^[1]] But perhaps the most remarkable point in the colour of the crustacea is that which immediately follows the paragraph I have just quoted. ‘The large eggs of some of the deep-sea genera are of a brilliant light blue, and in one genus of Macrura we found a dark metallic blue patch on the dorsal part of the carapace in marked contrast to the brilliant crimson of the rest of the body.’

[1]. In the recent researches of the ‘Investigator’ a few crustacea of rather exceptional colour were found. Whilst the great majority of them are described to be pink or red in colour when alive, Gnathophausia bengalensis is deep purple lake, Haliporus neptunus lurid orange, and Aristaeus coruscans bright orange.

The occurrence of this blue colour in Crustaceans of the deep sea is very remarkable, for blue is a colour, as Moseley pointed out many years ago, that is rarely met with in the fauna of the abyss, and it is certainly very exceptional in the crustacea of that zone.

Among the deep-sea Echinoderma we find a wonderful variety of coloration. Moseley says that many deep-sea Holothurians, for example, are deep purple, and Agassiz reports that in one species the colour was of a delicate green tinge. ‘We obtained,’ he adds, ‘a white Cucumaria and some species of Benthodytes of the same colour,’ while others vary from transparent milky white to yellow and light yellowish brown and even pinkish colours. The Crinoids are described by the authorities to be white, purple, yellow and brownish-chestnut, and of the other groups of the echinoderms we read that the star-fishes are, as a rule, of duller colours than the crustacea, but all more or less pink or red. ‘The Hymenasteridæ, on the contrary, vary from light bluish violet to deep reddish chestnut colours.’ The brittle stars are red and orange or dullish grey, while the urchins may be deep violet, claret coloured, brownish, or of a delicate pink.

It is impossible to account for this extraordinary variety of colour in the deep-sea echinoderms. It is hardly probable that it can be protective or warning in function, and it is difficult to suppose that it is due to any peculiar excretory process. Whether it is due in any way to the influence of the environment, or, like the colour of autumn leaves, to the chemical degeneration of colours that in the shallow-water ancestry were functional, are problems that must be left for the future to decide.

The colour of the deep-sea Cœlenterates has unfortunately not been recorded in all cases, but still the few observations that we have show that in this group, as in the last, almost every tint and shade are represented.

The colouring of the deep-sea jelly-fishes is said to be usually deep violet or yellowish red. However ‘a species of Stomobrachium,’ says Agassiz, ‘is remarkable for its light carmine colour, a tint hitherto not observed among Acalephs.’

Moseley records most minutely the colour of some of the deep-sea anemones and corals, and calls attention to the very general presence of madder brown in the soft parts. Agassiz says: ‘Among deep-sea Actiniæ, a species of a new Cereanthus was of a dark brick-red, while other actinians allied to Bunodes were of a deep violet. Actinauge-like forms with tentacles of a pinkish-violet tinge frequently have the column of a yellow shade. The Zoanthidæ were greyish-green.’ And again, in his narrative of the voyage of the ‘Blake,’ he records that ‘some of the deep-sea corals are scarlet, deep flesh-coloured, pinkish orange, and of other colours,’ and in referring to the Gorgonian Iridogorgia he says: ‘The species are remarkable for their elegance of form and for the brilliant lustre and iridescent colours of the axis, in some of a bright emerald green, in others like burnished gold or mother-of-pearl.’

The fauna of the deep sea then, taken as a whole, is not characterised by the predominance of any one colour. The shades of red occur rather more frequently than they do in the fauna of any other zone or region, but whether this is in any way connected with the fact that red is the complementary colour to that of the phosphorescent light, in which many of these animals live, it is at present difficult to say; it is possible that, when we have further information concerning the colours of the animals living in the deeper parts of the Neritic zone, another explanation may be forthcoming.