Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/701

683 MARINE FISHES.] ICHTHYOLOGY (583 The largest of marine fishes, Rhinodon, Selache, Car- charodoii, MyliobatidfB, Thynnus, Xiphiidce, Ortkayoriscus, belong to the pelagic fauna. Young fishes are frequently found in mid-ocean, which are the offspring of shore fishes normally depositing their spawn near the coast. The manner in which this fry passes into the open sea is unknown ; for it has not yet been ascertained whether it is carried by currents from the place where it was deposited originally, or whether shore fishes sometimes spawn at a distance from the coast. We may remember that shore fishes inhabit not only coasts but also submerged banks with some depth of water above, and that, by the action of the water, spawn deposited on these latter localities is very liable to be dispersed over wide areas of the ocean. Embryos of at least some shore fishes hatched under abnormal conditions seem to have an abnormal growth up to a certain period of their life, when they perish. The Leptocepkali must be regarded as such abnormally developed forms. Fishes of a similar condition are the so-called pelagic Plagusice, young Pleuronactoids, the origin of which is still unknown. As already mentioned, flat-fishes, like all the other Anacanths, are not otherwise represented in the pelagic fauna, Deep-Sea Fishes. The knowledge of the existence of deep-sea fishes is one of the recent discoveries of ichthyology. It was only about twenty years ago that, from the evidence afforded by the anatomical structure of a few singular fishes obtained in the North Atlantic, an opinion was expressed that these fishes inhabited great depths of the ocean, and that their organization was specially adapted for living under the physical abyssal conditions. These fishes agreed in the character of their connective tissue, which was so extremely weak as to yield to, and to break under, the slightest pressure, so that the greatest difficulty was experienced in preserving their body in its continuity. Another singular circumstance was that some of the examples were picked up floating on the surface of the water, having met their &amp;lt;leath whilst engaged in swallowing or digesting other iishes not much smaller in size if not actually larger than themselves. The first peculiarity was accounted for by the fact that, if those fishes really inhabited the great depths supposed, their removal from the enormous pressure under which they lived would be accompanied by such an expansion of gases within their tissues as to rupture them, and to cause a separation of the parts which had been held together by the pressure. The second circumstance was explained thus. A raptorial fish organized to live at a depth of between 500 and 800 fathoms seizes another usually inhabiting a depth of between 300 and 500 fathoms. In its struggles to escape, the fish seized, being nearly as large or strong as the attacking fish, carries the latter out of its depth into a higher stratum, where the diminished pressure causes such an expansion of gases as to make the destroyer with its victim rise with increasing rapidity towards the surface, which they reach dead or in a dying condition. Specimens in this state are not rarely picked up ; and as, of course, it is but comparatively few that can by accident fall into the hands of naturalists, occurrences of this kind must happen very often. The existence of fishes peculiarly adapted for the deep sea has thus been a fact maintained and admitted for some time in ichthyology ; and as the same genera and species were found at very distant parts of the ocean, it was further stated that those deep-sea fishes were not limited in their range, and that, consequently, the physical conditions of the depths of the ocean must be the same or nearly the same over the whole globe. That deep-sea fishes were not of a peculiar order, but chiefly modified forms of surface types, was another conclusion arrived at from the sporadic evidence collected during the period which preceded sys tematic deep-sea dredging. Nothing, however, was positively known as to the exact depths inhabited by those fishes until observations were made during the voyage of H.M.S. &quot;Challenger.&quot; The results obtained by this expedition afforded a surer and more extended basis for our knowledge of deep-sea fishes. The physical conditions of the deep sea, which must affect the organization and distribution of fishes, are the folkw- ing: 1. Absence of sunlight. Probably the rays of the sun do not penetrate to, and certainly do not extend beyond, a depth of 200 fathoms, therefore we may consider this to be the depth where the deep-sea fauna commences. Absence of light is, of necessity, accompanied by modifications of the organs of vision and by simplification of colours. 2. The absence of sunlight is in some measure compen sated by the presence of phosphorescent light, produced by many marine animals, and also by numerous deep-sea fishes. 3. Depression and equality of the temperature. At a depth of 500 fathoms the temperature of the water is already as low as 40 Fahr., and perfectly independent of the temperature of the surface-water ; and from the greatest depth to about 1000 fathoms beneath the surface the tem perature is uniformly but a few degrees above the freezing point. Temperature, therefore, ceases to offer an obstacle to the unlimited dispersal of the deep-sea fishes. 4. The increase of pressure by the water. The pressure of the atmosphere on the body of an animal at the level of the sea is 15 Bb per square inch of surface ; but under water 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, &quot; like a constant rain,&quot; 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 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 organization, so that every deep-sea fish may be recognized 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 cavern ous texture ; they are light, with scarcely any calcareous matter, so that the point of a needle will readily penetrate them without breaking. The bones, especially the vertebrae, appear to be very loosely connected with one another ; and it requires the most careful handling to avoid the break ing 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, and the connective tissue being extremely loose, feeble, or