Zoogeography of the Bathypelagic Fish, Chauliodus

Abstract

THE MID-DEPTHS of the oceans comprise a significant part of the volume of the biosphere, yet the conditions for life here are so different from those encountered by man that it is difficult to appreciate the factors limiting the distribution of the bathypelagic organisms living there. Catch records, however, indicate that many species are restricted in their geographical distribution. An attempt is made here to study the conditions of life in these mid-depths by centering attention on a group of fish that has a worldwide distribution. There are a number of fish groups which could serve for such a study, among them the gonostomiatids and the myetophids. Although these two groups have a wide distribution and occur in large numbers, their taxonomy has not been sufficiently clarified to make a zoogeographic study possible. The genus Chauliodus was chosen because it fulfilled the requirements. It has a world-wide distribution; large numbers of body measurements are available for the comparison of different populations; physical and chemical data are available for the stations at which the specimens were caught. The Stomiatoidei, the suborder to which Chauliodus belongs, can be considered herring-like fish with photophores and extremely large teeth. The genus Chauliodus has been placed in the family Chauliodontidae by Berg (1940), removing it from the Stomiatidae because the inner ear of Chauliodus lacks a lagena, whereas all the other members of that family have one. These two families make up the superfamily Stomiatoidae in the suborder Stomiatoidel. This suborder of the Clupeiformes is characterized by Berg as being the Clupeoidei, especially near the Alepocephalidae. (p. 431). Although Chauliodus (Plate 1, p. 120) lacks scales, the pattern of the pigment is such that it forms hexagonal areas in which there is a thin deposit of an opalescent substance, which appears under magnification like guanine crystals. The body is covered by a thick transparent coating containing microscopic spheres of unknown composition. This gelatinous layer forms, in part, the base of the adipose fin, and does not seem to be merely a slime secretion. Brauer (1908) was of the opinion that the gelatinous layer is a part of the corneum of the skin, although his earlier observations had led him to believe that it was a slime secretion. The fang-like teeth (Plate 2) appear admirably suited to the capture of the animals found in the mid-depths, and, indeed, stomach analyses show that Chauliodus feeds on chaetognaths (Clemens and Wilby, 1946) and on macroplanktonic crustacea, probably deep-sea prawns (personal observation). It is tempting to speculate that the prey is lured to the mouth by the many lights of the photophores. These light organs are of three types: microscopic spheres without a pigment layer; large spheres with a pigment coat, reflector and lens; and larger, bell-shaped organs with a pigment coat, lens and reflector. The microscopic photophores are scattered over the dorsal surface of the fish, whereas the larger, pigment-covered spherical organs are found in groups within each hexagonal scale area, around the eye socket, and in rows along the dorsal surface of the base of the pectoral and pelvic fins, and the base of the dorsal and anal fins. There is also a lateral row between the rows of bell-shaped organs. The two suborbital photophores (Plate 2) are special types, the more an-