ANTARCTIC OCEAN. - To this fauna we refer the shore fishes of the southernmost extremity of South America, from 50° S. lat., with Tierra del Fuego and the Falkland Islands, and those of Kerguelen's Land, with Prince Edward's Island. No fishes are known from the other oceanic islands of these latitudes.
In the southern hemisphere surface fishes do not extend so far towards the pole as in the northern ; none are known beyond 60° S. lat., and the Antarctic fauna which is analogous to the Arctic inhabits coasts more than ten -degrees nearer to the equator. It is very probable that the shores between 60° and the Antarctic Circle are inhabited by fishes sufficiently numerous to supply part of the means of snbsistence for the large seals which there pass at least some portion of the year, hut hitherto none have been obtained by naturalists ; all that the present state of our knowledge justifies us in saying is, that the general character of the fauna of Magellan's Straits and Kerguelen's Land is extremely similar to that of Iceland and Greenland.
As in the Arctic fauna, Chondropterygians are rare, and are represented by Acanthias vulgaris and species of Raia. Holocephali have not yet been found so far south, but Callorhynchus, which is pot uncommon near the northern boundary of this fauna, may prove to extend into it.
As to Acanthopterygians, Cataphracti and Scorpcenicke are represented as in the Arctic fauna, two of the genera -(Sebastes and Agonus) being identical. The Cottidce are replaced by six genera of Traainidce, remarkably similar in form to Arctic types; hut Discoboli and the characteristic Arctic Blennioids are absent.
Gadoid fishes reappear, hut are less developed ; as usual they are accompanied by Hyxine. The reappearance of so specialized a genus as Lycodes is most remarkable. Flatfishes are few as in the north, and belong to peculiar genera.
Physostomes are , probably not entirely absent, but hitherto none have been met with so far south. Lophobranchs are rare, as in the Arctic zone ; it is noteworthy, however, that a peculiar genus, with persistent embryonic -characters (Protocampus), is rather common on the shores •of the Falkland Islands.
Pelagic fishes, - that is, fishes inhabiting the surface of mid-ocean, - belong to various orders, viz., Chondropterygians, Acanthopterygians, Physostomes, Lophobranchs, and Plectognaths. Neither Anacanths nor Pharyngognaths contribute to this series of the marine fauna. The following genera and families are included in it : - Chendropterystii. - Carcharias, Galeocerdo, Thalassorhinus, Zygna, Trieenodon, Lamnid, Rhinodon Notida- niche, Lmmargus, Euprotomicrus, Echinorhinus, Isistius ; Myliobatidse.
Acculthopterygii. - Dactylopterus, Micropteryx, Scorn-brine, Ga.strochisma, Nomeus, Centrolophns, Coryphmnina, Seriola, Temnodon, Naucrates, Psenes, Xiphiida, Antennarius.
Physostomi. - Sternoptychidm, Scopelus, Astronesthes, Scombresocidm (majority).
Loph,obranchii. - Hippocampus. Plectognathi. - Orthagoriscus, and some other Gymnodon ts.
Pelagic fishes differ much from one another in their mode of life. The majority are excellent swimmers, which not only can move with great rapidity, but are also possessed of great powers of endurance, and are thus enabled to con. tinue their course for weeks, apparently without the necessity of rest ; such are many sharks, scombroids, dolphins, pilot-fish, sword-fishes. In some, as in Dactylopterus and Exoccetus, the ability to take flying leaps out of the water is superadded to the power of swimming (flying-fishes). But in others the power of swimming is greatly reduced, as in Antennarius, Hippocampus, and Gymnodonts ; they frequent places in the ocean covered with floating seaweed, or drift on the surface without resistance, at the mercy of wind and current. The Echeneis or sucking-fishes attach themselves to other large fishes, ships, or floating objects, and allow themselves to be carried about, unless change of climate or want of food obliges them to abandon their temporary carrier. Finally, another class of pelagic fishes come to the surface of the ocean during the night only ; in the day time they descend to some- depth, where they are undisturbed by the rays of the sun or the agitation of the surface-water ; such are Brama, the Sternoptychidce, Scopelus, Astronesthes, - fishes the majority of which are provided with those extraordinary visual organs that we find so much developed in the true deep-sea fishes. Indeed, this last kind of pelagic fishes constitutes a connecting link with the deep-sea forms.
Pelagic fishes, like shore fishes, are most numerous in the tropical zone ; and, with few exceptions (Echinorhinus, Psenes, Sternoptychidce, Astronesthes), the same genera are represented in the tropical Atlantic as well as in the Indo-Pacific. The number of identical species occurring in both these oceans is great, and probably still greater than would appear from systematic lists, in which there are retained many specific names that were given at a time when species were believed to have a very limited range. The pelagic fauna of the tropics gradually passes into that of the temperate zones, only a few genera, like Cybium, Psenes, Antennarius, being almost entirely confined to the tropics. All the other tropical genera range into the temperate zones, but their representatives become fewer with the increasing distance from the equator. North of 40° N. lat. many genera have disappeared, or are met with in isolated examples only, as Carcharias, Zygcena, Notidanus, Myliobaticke, Dactylopterns, Echeneis, .?'omens, Coryphuena, Schedophilus, Seriola, 2'eninoclon, Antennarius, Sternoptychidce, Astronesthes, Exoccetus, Tetrodon, Diodon ; and only one genus of sharks, Caleocerdo, approaches the Arctic Circle. Some few species, like Antennarius, Scopelus, are carried by currents near to the farther confines of the temperate zones ; but such occurrences are accidental, and these fishes must he regarded as entirely foreign to the fauna of those latitudes. On the other hand, some pelagic fishes inhabit the temperate zones, whilst their occurrence within the tropics is very problematical ; thus, in the Atlantic, Thalassorhinus, Selache, Lcemargus, Centrolophus, Diana, Ausonia, Lampris (all genera composed of one or two species only). Besides the shark mentioned, no other pelagic fishes are known from the Arctic Ocean.
We possess very little information about the pelagic fish-fauna of the southern oceans. This much only is certain, that the tropical forms gradually disappear ; but it would be hazardous, in the present state of our knowledge, to state even approximately the limits of the southward range of a single genus. Scarcely more is known about the appearance of types peculiar to the southern temperate zone, - for instance, the gigantic shark Rhinodon representing the northern Selache, near the coasts of South Africa, and the Scombroid genus Gastrochisma, in tire South Pacific.
The largest of marine fishes, Rhimodon, Selache, Ca•.charodon, Jfyliobatidw, Thynitus, iphiidcv, Orthagoriscns, -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 Leptocephali must be regarded as such abnormally developed forms. Fishes of a similar condition are the so-called pelagic Plagusice, young Pleuronectoids, 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.
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 draanization 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 death whilst engaged in swallowing or digesting other fishes 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 'tot 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 ever 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.
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. "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 organization and distribution of fishes, are the folk w• ing :- 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.
The absence of sunlight is in some measure compensated by the presence of phosphorescent light, produced by many marine animals, and also by numerous deep-sea fishes.
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 temperature 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.
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 lb per square inch of surface ; but under water the pressure amounts to a ton weight for every 1000 fathoms of depth.
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.
The perfect quiet of the water at great depths. The agitation of tire 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 cavernous texture ; they are light, with scarcely any calcareots matter, so that the point of a needle will readily penetrate them without breaking. The bones, especially the vertebra, appear to be very loosely connected with one another ; and it requires the most careful handling to avoid 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 une another or torn, and the connective tissue being extremely loose, feeble, or apparently absent. This peculiarity has been observed in the Trackyptericia?, Plagyoci us, Chiasmodus, Melanocetus, Saccopkarynx. 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. It is evident, therefore, 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 penetrate 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 fathoms or more 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 suffice 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 in fishes which are comparatively little removed from the surface (that is, to depths of from 100 to 200 fathoms) the lateral line much wider than in their congeners or nearest allies which live on the surface, as in Trackichthys, Iloplostetkus, many Scorpcenidce. But in fishes inhabiting depths of 1000 fathoms and more the whole muciferous system is dilated ; it is especially the surface of the skull which is occupied by large cavities (Macruridce, deep-sea Opkidiidce), and the whole body seems to he covered with a layer of mucus. These cavities collapse and shrink in specimens which have been preserved in spirit for some time, but a brief re-immersion in water 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 rare.
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 organs being necessary to collect as many rays of light as possible. Beyond that depth small-eyed as well as large-eyed fishes occur, the former having their want of vision compensated 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 badies, 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 kind of organs possess in the interior a lenticular body, like the lens of an eye, and are considered by some naturalists true organs of vision (accessory eyes), the function of the latter, which have a glandular structure, being left unexplained by them.
There is no doubt that the functions of these organs 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. And it is highly probable that all produce and emit phosphorescent light, enabling the fishes to see in the darkness of the night or of the depths of the sea.
Whenever we find in a fish long delicate filaments de-. veloped in connexion with the fins or the extremity of the tail, we may conclude thtit it is an inhabitant of still water and of quiet habits. Many deep-sea fishes (7'rackypteridce, Macruridce, Ophichidce, 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 fi 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; they rather draw themselves over their victim, after 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, six times as many new species and genera having been discovered. Modifications of certain organs, perfectly novel, and of the greatest inter est, were found ; 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.
The fish fauna of the deep sea is chiefly composed of forms or modifications of forms which we find represented at the surface in the cold and temperate zones, or which belong to the class of nocturnal pelagic fishes. The Chondropterygians are few in number, not descending to a greater depth than GOO 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 depth as the Chondropterygians, whilst those Aeanthopterygians which are so much specialized for a 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,, Trachypteridce, Lophotidce, and Nolacanikithe • they consist of three, one, and two genera respectively.
Gadidce, Ophidi2Wce, and Macruridce arc very numerous, ranging through all depths ; • they constitute about one- fourth of the whole deep-sea fauna, Of Physostomi, the families of Steryto-ptychidw, Scopelidce,, Stamiatidce, Salmanithe, Bath yekrissidce, Alepoceph dace, Halosavrick, and Nurcenidce are represented. Of these. the Scopeloids are the most numerous, constituting nearly another fourth of the fauna. Salmonidce are only represented by three small genera. Batkythrissiche include one species only, which is probably confined in its vertical as well as its horizontal range ; it occurs at a depth of about 3b0 fathoms in the sea of Japan. The Atepocepha/idm and lici/osaurida?, known before the "Challenger" expedition from isolated examples only, prove to be true, widely-- spread, deep-sea types. Eels are well represented, and scent to descend to the greatest depths. Myxine has been ob tanned from a depth of 345 fathoms.
The greatest depth hitherto reached 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 do undoubtedly 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 class of fishes is divided into four subclasses :- I. Pala,iclahyes. - Ileart with a contractile conus arteriosus; intestine with a spiral valve ; optic nerves non-decussating, or only partially decussating.
Teleostei. - Heart with a non-contractile bulbus arteriosus ; intestine without spiral valve ; optic nerves decussating. Skeleton ossified, with completely separated vertebrve.
Cyclostomata. - Heart without bulbus arteriosus ; intestine simple. Skeleton cartilaginous and notoehordal. One nasal aperture only. No jaws ; mouth surrounded by a circular lip.
Leptocardii. - Heart replaced by pulsating sinuses ; intestine simple. Skeleton mernbrano-cartilaginous and notochordal. No brain ; no skull.
This subclass comprises the sharks and rays and the Ganoid fishes. Though it is based upon a singular concurrence of most important characters, its members exhibit as great a diversity of form, and as manifold modifications in the remainder of their organization, as the Teleostei. The PuLeiellthyes stand to the Teleostei in the same relation as the Marsupials to the Placentalia. Geologically, as a subclass, they were the predecessors of Teleosteous fishes; and it is a remarkable fact that all those modifications which show an approach of the ichthyic type to the Batrachians are found in this subclass. It is divided into two orders, - the Chondropterygii and the Ganoidei.