xxiv THE VOYAGE OF H.M.S. CHALLENGER. 



CHARACTERISTICS OF DEEP-SEA FISHES/ 



The physical conditions under which fishes live at a depth of 100 and more fathoms 

 affect certain parts of their organisation. We know now, chiefly from the Norwegian 

 and North America explorations, that many littoral fishes descend within the limits of 

 the truly abyssal fauna. These descents, however, are not permanent removals from 

 the littoral zone, but, at the most, periodical ; and therefore no conspicuous change in 

 any of the organs of these fishes has taken place. But already, in many fishes which 

 permanently reside at from 80 to 120 fathoms, we find indications of their habitat in the 

 black coloration of their pharynx and in the size of their eye, which is proportionally 

 laro-er than in their representatives at the surface. In the true deep-sea fishes 

 certain organs are so conspicuously modified that every one of these fishes may be 

 recognised as a deep-sea fish, without accompanying positive evidence of its capture at a 

 great depth ; and vice versa, fishes reputed to have been obtained at a great depth, and 

 not having any of the characteristics of the dwellers of the deep sea, must be regarded as 

 surface fishes. The question whether the amount of modification is proportioned to the 

 depth, must be negatived from the evidence at present available, inasmuch as deep-sea 

 fishes caught at depths of between 300 and 400 fathoms may show a much more con- 

 spicuous development of abyssal peculiarities than those from 2000 and more fathoms. 



The tremendous pressure under which deep-sea fishes live must be one of the primary 

 causes affecting their organisation. The pressure of the atmosphere at the level of the 

 sea amounts to fifteen pounds per square inch of the surface of the body of an animal ; 

 but below the surface of the ocean the pressure is increased to a ton weight for every 

 1000 fathoms of depth. In many deep-sea fishes we find, then, that the osseous or 

 muscular systems, or both, are, as compared with the same parts of surface fishes, very 

 feebly developed, as for instance in the Trachypteridse, Melanocetus, Chiasmodus, 

 Plagyodus, Omosudis, Saccopharynx. The bones have a fibrous, fissured and cavernous 

 texture, are light, with scarcely any calcareous matter, so that the point of a fine needle will 

 readily penetrate them without breaking. In some the primordial cartilage is persistent 

 in a degree rarely met with in surface fishes, and the membrane bones remain more or 

 less membranous or are reduced in extent, like the operculum, which frequently is too 

 small to cover the gills. When the fish is brought to the surface, all the bones, and 

 more particularly the vertebrse, are most loosely connected with one another. Likewise 

 the muscles, especially the great lateral muscles of the trunk and tail, are ihin ; the 

 fascicles can be readily separated or torn, the connective tissue being extremely loose and 



1 The contents of this chapter formed the subject of a lecture delivered at Cambridge in 1874, and of the notes 

 published in Introduction to a Study of Fishes, 1880, p. 296. 



