The fauna of the deep sea

by Sydney J. Hickson

During the voyage of the ‘Talisman,’ for example, the French naturalists caught in one haul of the dredge off the coast of Morocco in 500 fathoms of water no fewer than 134 fish, of which number 95 belonged to the family Macruridæ.

They are usually small fish, measuring from a few inches to two feet in length, with a body terminating in a long compressed tapering tail and covered with spiny, keeled, or striated scales.

The Pleuronectidæ or flat fish are not, as a rule, found in the abysmal zone; one species, however, Pleuronectes cynoglossus, was found by the American ship ‘Blake’ to extend into 732 fathoms of water.

The families Sternoptychidæ and Scopelidæ are of particular interest to us, as almost all the genera they contain belong either to the pelagic or abysmal zones, and lend support to the view enunciated by Moseley, that the deep-sea fauna has, partly at any rate, been derived from the fauna of the pelagic zone. They are nearly all small slender fish with delicate and frequently semi-transparent bodies, large gaping mouths armed with numerous long irregular teeth, and frequently provided upon the head and sides of the trunk with rows of eye-like phosphorescent organs.

These families, and others that have still to be referred to, belong to the group of Teleostei that is called Physostomi, the name referring to the open communication that usually exists in all these families between the swimming bladder and the alimentary canal. It is a remarkable fact that in none of the deep-sea representatives has this open communication been discovered. It is true that many specimens are, when examined, so lacerated by the diminution in pressure as to render anatomical study a matter of difficulty, but still a fair number of uninjured well-preserved specimens have now been examined and the duct has not been found.

Of the family Sternoptychidæ, Gonostoma microdon has a most remarkable distribution. It has been found at numerous stations in both the Pacific and Atlantic Oceans at depths ranging from 500 to nearly 3,000 fathoms of water.

The Scopelidæ are represented by some very extraordinary types. The genus Bathypterois, for example, occurring in depths ranging from 500 to 2,500 fathoms, is characterised by the development of enormously long pectoral fins to serve probably as organs of touch. ‘The rays of the pectoral fin,’ says Dr. Günther, ‘are much elongated. The ventral fins abdominal, with the outer rays prolonged, eight-rayed.... Gill rakers long.’ They are further characterised by the absence of any true phosphorescent organs and the smallness of their eyes.

There can be little doubt, I think, that in these fishes the sense of touch or taste to a great extent takes the place of the sense of sight in other Scopelids. Not being provided with well-developed eyes or phosphorescent organs to attract their prey, the pectoral fins and the outer rays of the pelvic fins have become elongated and provided with special sense organs for searching for their food in the fine mud of the floor of the ocean.

These long pectoral rays must have a very curious appearance in the living fish. Mr. Murray observes: ‘When taken from the trawl they were always dead, and the long pectoral rays were erected like an arch over the head, requiring considerable pressure to make them lie along the side of the body; when erected they resembled the Pennatulids like Umbellula.’ Filhol considers that when the fish is progressing through the obscurity of the abyss it probably carries these organs directed forward, seeking with them in the mud for any worms or other animals upon which it preys, or receiving through them warning of the approach of an enemy from whom it is necessary to make an immediate escape. One of the most remarkable of the deep-sea fish is closely related to Bathypterois, namely Ipnops Murrayi, living in depths of over 1,000 fathoms. It is about five inches long, of a yellowish brown colour, with an elongated subcylindrical body covered with large thin deciduous scales. There are no phosphorescent organs of the ordinary type met with in the Scopelidæ, but upon the upper surface of the head there is found a pair of organs somewhat resembling the ordinary eyes of fishes but devoid of retina and optic nerve, that, from the researches of Moseley, seem to be undoubtedly organs for emitting light. ‘The organs are paired expanses, completely symmetrical in outline, placed on either side of the median line of the upper flattened surface of the head of the fish, extending from a line a little posterior to the nasal capsules nearly to a point above the posterior extremity of the cranial cavity.’ They are covered by the upper walls of the skull, which is extremely thin and completely transparent in the region lying over them. ‘They are membranous structures 0·4 mm. in thickness marked by hexagonal areas about 0·04 mm. in diameter. When their surface is viewed by reflected light the appearance is that of a number of glistening white isolated short columns standing up in relief from its basal membrane.’ Each hexagonal column is composed of a number of transparent rods disposed side by side at right angles to the outer surface of the organ, with their bases applied against the concave surface of a large hexagonal pigment cell, one of which forms the basis of each hexagonal column. It is still very doubtful what are the true homologies of this extraordinary phosphorescent organ, but Moseley was of opinion that, ‘on the whole, it seems not unlikely that the remarkable head organs of Ipnops may be regarded as highly specialised and enormously enlarged representatives of the phosphorescent organs on the heads of such allied Scopelidæ as Scopelus rafinesquii and Scopelus metopoclampus. It may be conceived that in Ipnops the supra-nasal and sub-ocular phosphorescent organs of these species on either side have united and become one with the result of the total obliteration of the eye.’

Most of the species of the genus Scopelus are undoubtedly pelagic in habit, descending during the day to depths of semi-darkness but rising at night to the surface waters. It is not certain how many of the known species occasionally or habitually dwell in very deep water, but there seems to be no doubt that two species at least—S. macrolepidotus and S. glacialis—belong to the abysmal zone. Both of these species were found in dredges that had been at work in depths of over 1,000 fathoms and showed signs when examined of having been brought from the abyss.