The knowledge of the existence of deep-sea fishes is one of the recent discoveries of ichthyology. It is 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 organisation 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 is experienced to preserve their body in its continuity. Another singular circumstance was, that some of the specimens were picked up floating on the surface of the water, having met their deaths whilst engaged in swallowing or digesting another fish not much inferior or even superior in size to 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 raptatorial fish organised 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, 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 condition are not rarely picked up; and as, of course, comparatively few can by accident fall into the hands of naturalists, occurrences of the kind related must happen very often.

Thus, the existence of fishes peculiarly adapted for the deep sea has 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 systematic deep-sea dredging.

However, nothing was positively known as to the exact depths inhabited by those fishes until observations were made during the voyage of H.M.S. “Challenger.” 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 organisation and distribution of fishes, are the following:—

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 compensated for 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 depths upwards to about 1000 fathoms the temperature is uniformly but a few degrees above freezing-point. Temperature, therefore, ceases to offer an obstacle to the unlimited dispersal of Deep-sea fishes.

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:—

1. All the different kinds of organs are sensory, or, in other words, accessory eyes.

2. Only the organs with a lenticular body are sensory, and those with a glandular structure produce and emit phosphorescent light.

3. All are producers of light.

There are very serious objections to adopting the first view. Scopelus and Argyropelecus possess not only perfectly developed, but even large eyes, specially adapted for a nocturnal life; and therefore accessory organs of vision must appear to be quite superfluous to them. On the other hand, in Deep-sea fishes without external eyes, which would seem to especially require these metameric organs of sense, they are invariably absent. And, finally, it is quite inconceivable that the glandular structures should have the faculty of conveying impressions of light to the nervous centre. The second supposition seems therefore to be nearer the truth; and is supported by the fact that the glandular organs of Scopeli have actually been observed to gleam with phosphorescent light, and by the obvious morphological similarity of the organs with a lenticular body and retina-like membrane to an organ of vision. We are, moreover, justified, from an à priori consideration, in supposing that in depths to which no sunlight descends, and which are illuminated by phosphorescent light only, peculiar organs of vision would have been developed. On the other hand, this supposition is opposed by the fact that many fishes which dwell in those abyssal depths are provided with large ordinary eyes (as the Trachypteri, the majority of Macruridæ), and, therefore, that the ordinary organ of vision is quite sufficient for seeing by phosphorescent light. Thus, whilst we must admit that those compound organs may prove to be organs of sense, we maintain at the same time that their morphological nature is not opposed to the belief that they too, like the glandular organs, are producers of light. It may be produced at the bottom of the posterior chamber, and emitted through the lenticular body in particular directions, with the same effect as light is sent through the convex glass of a “bull’s eye.” This hypothesis seems to be less bold than the other, which would require us to believe that vertebrate animals, with a nervous centre specialised for the reception of the impressions of the higher senses, should receive them through the spinal chord.

Whenever we find in a fish long delicate filaments, developed in connection with the fins or the extremity of the tail, we may conclude that it is an inhabitant of still water and of quiet habits. Many deep-sea fishes (Trachypteridæ, Macruridæ, Ophidiidæ, Bathypterois) are provided with such filamentous prolongations, the development of which is perfectly in accordance with their sojourn in the absolutely quiet waters of abyssal depths.

Some of the raptatorial Deep-sea fishes have a stomach so distensible and capacious that it can receive a fish of twice or thrice the bulk of the destroyer (Melanocetus, Chiasmodus, Saccopharynx). Deglutition is performed in them not by means of the muscles of the pharynx, as in other fishes, but by the independent and alternate action of the jaws, as in Snakes. These fishes cannot be said to swallow their food, but rather draw themselves over their victim, in the fashion of an Actinia.

Before the voyage of H.M.S. “Challenger,” scarcely thirty Deep-sea fishes were known. This number is now much increased by the discovery of many new species and genera; but, singularly, no new types of families were discovered: nothing but what might have been expected from our previous knowledge of this group of fishes. Modifications of certain organs, perfectly novel, and of the greatest interest, were found, as we shall see in the “Systematic Part;” but the most important results of this voyage are that the general character of the abyssal fish-fauna, the abundance of fishes, and the exact depths to which fishes may descend, have been ascertained.

However, the statements of the depths at which the fishes collected by the “Challenger” were taken cannot be received without some critical examination of each individual species. No precaution was taken to keep the mouth of the dredge closed during its descent or ascent, and therefore it is quite within the limits of probability that sometimes fishes were accidentally enclosed within the dredge, whilst it was traversing the surface strata. And this has happened more than once; for it is quite certain that common surface fishes like Sternoptyx and Astronestles, never ranged to a depth of 2500 fathoms. On the other hand, the majority of the fishes obtained offer sufficient evidence from their own organisation that they live on the bottom, and are unable to support themselves in the water at a certain distance from the bottom or surface; and, consequently, that they actually were obtained at the depth to which the dredge descended.

As far as the observations go at present, no distinct bathymetrical regions, which would be characterised by peculiar forms, can be defined. The depths from 200 to 600 fathoms are inhabited by numerous forms, still strongly reminding us of surface types. To this fauna belong the few Chondropterygians of the deep sea, a Sebastes and Setarches, a Beryx and Polymixia, a Cottus, etc.; but they are associated with many others which descend to the greatest depths. And before anything like a division into bathymetrical zones can be attempted, the observations of the “Challenger” expedition must be confirmed and supplemented by other series of similar systematic observations. One of the most startling conclusions at which we would have to arrive from the “Challenger” observations is, that some of the species of Deep-sea fishes would range from a depth of some 300 fathoms down to one of 2000 fathoms; or, in other words, that a fish which has once attained in its organisation to that modification by which it is enabled to exist under the pressure of half a ton, can easily accommodate itself to one of two tons or more,—a conclusion which is not in accordance with anatomical facts, and which must be confirmed by other observations before we can adopt it. But if the vertical range of Deep-sea fishes is actually as it appears from the “Challenger” lists, then there is no more distinct vertical than horizontal distribution of Deep-sea fishes.

The greatest depth reached hitherto by a dredge in which fishes were enclosed is 2900 fathoms. But the specimens thus obtained belong to a species (Gonostoma microdon), which seems to be extremely abundant in upper strata of the Atlantic and Pacific, and were therefore most likely caught by the dredge in its ascent. The next greatest depth, viz., 2750 fathoms, must be accepted as one at which fishes undoubtedly do live; the fish obtained from this depth of the Atlantic, Bathyophis ferox, showing by its whole habit that it is a form living on the bottom of the ocean.

The fish-fauna of the deep sea is composed chiefly of forms or modifications of forms which we find represented at the surface in the cold and temperate zones, or which appear as nocturnal pelagic forms. The Chondropterygians are few in number, not descending to a depth of more than 600 fathoms. The Acanthopterygians, which form the majority of the coast and surface faunas, are also scantily represented; genera identical with surface types are confined to the same inconsiderable depths as the Chondropterygians, whilst those Acanthopterygians which are so much specialised for the life in the deep sea as to deserve generic separation, range from 200 to 2400 fathoms. Three distinct families of Acanthopterygians belong to the deep-sea fauna, viz. Trachypteridæ, Lophotidæ, and Notacanthidæ; they respectively consist of three, one, or two genera only.

Gadidæ, Ophidiidæ, and Macruridæ are very numerous, ranging through all depths; they constitute about one-fourth of the whole deep-sea fauna.

Of Physostomi, the families of Sternoptychidæ, Scopelidæ, Stomiatidæ, Salmonidæ, Bathythrissidæ, Alepocephalidæ, Halosauridæ, and Murænidæ are represented. Of these the Scopeloids are the most numerous, constituting nearly another fourth of the fauna. Salmonidæ are scarce, with three small genera only. Bathythrissidæ include one species only, which is probably confined in its vertical as well as horizontal range; it occurs at a depth of about 350 fathoms in the sea of Japan. The Alepocephalidæ and Halosauridæ, known before the “Challenger” expedition from isolated examples only, prove to be true, widely-spread, deep-sea types. Eels are well represented, and seem to descend to the greatest depths.

Myxine has been obtained from a depth of 345 fathoms.

It will be useful to append a complete list of Deep-sea fishes, with the depths as ascertained by the dredgings of the “Challenger:”—

List of Deep-sea Fishes.