DISTRIBUTION OF PELAGIC FISHES.
Pelagic Fishes,--that is, fishes inhabiting the surface of mid-ocean
(see p. 255), belong to various orders, viz. Chondropterygians,
Acanthopterygians, Physostomes, Lophobranchs, and Plectognaths. But
neither Anacanths nor Pharyngognaths contribute to this series of the
Marine Fauna. The following genera and families are included in it:--
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.
[Illustration: Figs. 109 and 110.--_Antennarius
candimaculatus_, a pelagic fish, from the Indian Ocean.]