Chondropterygii: Carcharias, Galeocerdo, Thalassorhinus, Zygæna, Triænodon, Lamnidæ, Rhinodon, Notidanidæ, Læmargus, Euprotomicrus, Echinorhinus, Isistius; Myliobatidæ.
Acanthopterygii: Dactylopterus, Micropteryx, Scombrina, Gastrochisma, Nomeus, Centrolophus, Coryphænina, Seriola, Temnodon, Naucrates, Psenes, Xiphiidæ, Antennarius.
Physostomi: Sternoptychidæ, Scopelus, Astronesthes, Scombresocidæ (majority).
Lophobranchii: Hippocampus.
Plectognathi: Orthagoriscus, and some other Gymnodonts.
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 also are possessed of great powers of endurance, and are thus enabled to continue 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 Exocoetus, the ability of taking 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 fish, 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 Sternoptychidæ, Scopelus, Astronesthes; fishes, the majority of which are provided with those extraordinary luminary organs that we find so much developed in the true Deep-sea fishes. Indeed, this last kind of Pelagic fishes forms a passage to the Deep-sea forms.
Pelagic fishes, like shore fishes, are most numerous in the Tropical Zone; and, with few exceptions (Echinorhinus, Psenes, Sternoptychidæ, 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 scarcer with the increasing distance from the equator. North of 40° lat. N. many genera have disappeared, or are met with in isolated examples only, as Carcharias, Zygæna, Notidanus, Myliobatidæ, Dactylopterus, Echeneis, Nomeus, Coryphæna, Schedophilus, Seriola, Temnodon, Antennarius, Sternoptychidæ, Astronesthes, Exocoetus, Tetrodon, Diodon; and only one genus of Sharks, Galeocerdo, approaches the Arctic circle. Some few species, like Antennarius, Scopelus, are carried by currents near to the northern confines of the temperate zones; but such occurrences are accidental, and these fishes must be 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, Læmargus, Centrolophus, Diana, Ausonia, Lampris (all genera composed of one or two species only). Beside 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. So 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 the South Pacific.
The largest of marine fishes, Rhinodon, Selache, Carcharodon, Myliobatidæ, Thynnus, Xiphiidæ, Orthagoriscus, 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. Embryoes 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 fish (see p. 179). Fishes of a similar condition are the so-called Pelagic Plagusiæ, young Pleuronectoids, the origin of which is still unknown. As mentioned before, Flat-fishes, like all the other Anacanths, are otherwise not represented in the Pelagic fauna.
