4. The increased pressure by the water. The pressure of the atmosphere on the level of the sea amounts to fifteen pounds per square inch of the surface of the body of an animal; but the pressure amounts to a ton weight for every 1000 fathoms of depth.
5. With the sunlight, vegetable life ceases in the depths of the sea. All Deep-sea fishes are therefore carnivorous; the most voracious feeding frequently on their own offspring, and the toothless kinds being nourished by the animalcules which live on the bottom, or which, “like a constant rain,” settle down from the upper strata towards the bottom of the sea.
6. The perfect quiet of the water at great depths. The agitation of the water, caused by the disturbances of the air, does not extend beyond the depth of a few fathoms; below this surface-stratum there is no other movement except the quiet flow of ocean-currents, and near the bottom of the deep sea the water is probably in a state of almost entire quiescence.
The effect upon fishes of the physical conditions described is clearly testified by the modification of one or more parts of their organisation, so that every Deep-sea fish may be recognised as such, without the accompanying positive evidence that it has been caught 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 most striking characteristic, found in many Deep-sea fishes, is in relation to the tremendous pressure under which they live. Their osseous and muscular systems are, as compared with the same parts of surface-fishes, very feebly developed. The bones have a fibrous, fissured, and cavernous texture; are light, with scarcely any calcareous matter, so that the point of a needle will readily penetrate them without breaking. The bones, especially the vertebræ, appear to be most loosely connected with one another; and it requires the most careful handling to prevent the breaking of the connective ligaments. The muscles, especially the great lateral muscles of the trunk and tail, are thin, the fascicles being readily separated from one another or torn, the connective tissue being extremely loose, feeble, or apparently absent. This peculiarity has been observed in the Trachypteridæ, Plagyodus, Chiasmodus, Melanocetus, Saccopharynx. But we cannot assume that it actually obtains whilst those fishes exist under their natural conditions. Some of them are most rapacious creatures which must be able to execute rapid and powerful movements to catch and overpower their prey; and for that object their muscular system, thin as its layers may be, must be as firm, and the chain of the segments of their vertebral column as firmly linked together as in surface-fishes. Therefore, it is evident that the change which the body of those fishes has undergone on their withdrawal from the pressure under which they live is a much aggravated form of the affection that is experienced by persons reaching great altitudes in their ascent of a mountain or in a balloon. In every living organism with an intestinal tract there are accumulations of free gases; and, moreover, the blood and other fluids, which permeate every part of the body, contain gases in solution. Under greatly diminished pressure these gases expand, so that, if the withdrawal from a depth is not an extremely slow and gradual process, the various tissues must be distended, loosened, ruptured; and what is a vigorous fish at a depth of 500 or more fathoms, appears at the surface as a loosely-jointed body which, if the skin is not of sufficient toughness, can only be kept together with difficulty. At great depths a fibrous osseous structure and a thin layer of muscles suffices to obtain the same results for which, at the surface, thickness of muscle and firm osseous or cartilaginous tissue are necessary.
The muciferous system of many Deep-sea fishes is developed in an extraordinary degree. We find already in fishes which are comparatively little removed from the surface (that is to depths of 100–200 fathoms), the lateral line much wider than in their congeners or nearest allies which live on the surface, as in Trachichthys, Hoplostethus, many Scorpænidæ. But in fishes inhabiting depths of 1000 and more fathoms, the whole muciferous system is dilated; it is especially the surface of the skull which is occupied by large cavities (Macruridæ, deep-sea Ophidiidæ), and the whole body seems to be covered with a layer of mucus. These cavities collapse and shrink in specimens which have been preserved in spirit for some time, but a re-immersion in water for a short time generally suffices to show the immense quantity of mucus secreted by them. The physiological use of this secretion is unknown; it has been observed to have phosphorescent properties in perfectly fresh specimens.
The colours of Deep-sea fishes are extremely simple, their bodies being either black or silvery; in a few only are some filaments or the fin-rays of a bright scarlet colour. Among the black forms albinoes are not scarce.
The organ of sight is the first to be affected by a sojourn in deep water. Even in fishes which habitually live at a depth of only 80 fathoms, we find the eye of a proportionally larger size than in their representatives at the surface. In such fishes the eyes increase in size with the depth inhabited by them, down to the depth of 200 fathoms, the large eyes being necessary to collect as many rays of light as possible. Beyond that depth small-eyed fishes as well as large-eyed occur, the former having their want of vision compensated for by tentacular organs of touch, whilst the latter have no such accessory organs, and evidently see only by the aid of phosphorescence. In the greatest depths blind fishes occur with rudimentary eyes and without special organs of touch.
Many fishes of the deep sea are provided with more or less numerous, round, shining, mother-of-pearl-coloured bodies, imbedded in the skin. These so-called phosphorescent or luminous organs are either larger bodies of an oval or irregularly elliptical shape placed on the head, in the vicinity of the eye, or smaller round globular bodies arranged symmetrically in series along the side of the body and tail, especially near the abdominal profile, less frequently along the back. The former have not yet been anatomically examined. The number of pairs of the latter is in direct relation to that of the segments of the vertebral column, the muscular system, etc. (meta*-meres); and two kinds may be distinguished differing from each other in their anatomical structure. The organs of one kind consist of an anterior, biconvex, lens-like body, which is transparent during life, simple or composed of rods (Chauliodus); and of a posterior chamber which is filled with a transparent fluid, and coated with a dark membrane composed of hexagonal cells, or of rods arranged as in a retina. This structure is found in Astronesthes, Stomias, Chauliodus, etc. In the other kind the organ shows throughout a simply glandular structure, but apparently without an efferent duct (Gonostoma, Scopelus, Maurolicus, Argyropelecus). Branches of the spinal nerves run to each organ, and are distributed over the retina-like membrane or the glandular follicles. The former kind of organs are considered by some naturalists true organs of vision (accessory eyes), the function of the latter being left unexplained by them.
Although, thus, these organs morphologically differ from each other, there is no doubt that the functions of all have some relation to the peculiar conditions of light under which the fishes provided with them live; these fishes being either deep-sea forms or nocturnal pelagic kinds. There are three possible hypotheses as to the function of these organs:—