IV. Migratory fishes. A. anadromous (ascending fresh waters to spawn);
B. catadromous (descending to the sea to spawn).
About two-thirds of the known recent fishes are marine. Such are nearly
all the Selachians, and, among the Teleosteans, all the _Heteromi_,
_Pediculati_ and the great majority of _Apodes_, _Thoracostei_,
_Percesoces_, _Anacanthini_, _Acanthopterygii_ and _Plectognathi_. All
the _Crossopterygii_, _Dipneusti_, _Opisthomi_, _Symbranchii_, and
nearly all the _Ganoidei_ and _Ostariophysi_ are confined to
fresh-water.
The three categories of marine fishes have thus been defined by
Günther:--
"1. _Shore Fishes_--that is, fishes which chiefly inhabit parts of the
sea in the immediate neighbourhood of land either actually raised
above, or at least but little submerged below, the surface of the
water. They do not descend to any great depth,--very few to 300
fathoms, and the majority live close to the surface. The distribution
of these fishes is determined, not only by the temperature of the
surface water, but also by the nature of the adjacent land and its
animal and vegetable products,--some being confined to flat coasts
with soft or sandy bottoms, others to rocky and fissured coasts,
others to living coral formations. If it were not for the frequent
mechanical and involuntary removals to which these fishes are exposed,
their distribution within certain limits, as it no doubt originally
existed, would resemble still more that of freshwater fishes than we
find it actually does at the present period.
2. _Pelagic Fishes_--that is, fishes which inhabit the surface and
uppermost strata of the open ocean, and approach the shores only
accidentally or occasionally (in search of prey), or periodically (for
the purpose of spawning). The majority spawn in the open sea, their
ova and young being always found at a great distance from the shore.
With regard to their distribution, they are still subject to the
influences of light and the temperature of the surface water; but they
are independent of the variable local conditions which tie the shore
fish to its original home, and therefore roam freely over a space
which would take a freshwater or shore fish thousands of years to
cover in its gradual dispersal. Such as are devoid of rapidity of
motion are dispersed over similarly large areas by the oceanic
currents, more slowly than the strong swimmers, but not less surely.
An accurate definition, therefore, of their distribution within
certain areas equivalent to the terrestrial regions is much less
feasible than in the case of shore fishes.
3. _Deep-Sea Fishes_--that is, fishes which inhabit such depths of the
ocean that they are but little or not at all influenced by light or
the surface temperature, and which, by their organization, are
prevented from reaching the surface stratum in a healthy condition.
Living almost under identical tellurian conditions, the same type, the
same species, may inhabit an abyssal depth under the equator as well
as one near the arctic or antarctic circle; and all that we know of
these fishes points to the conclusion that no separate horizontal
regions can be distinguished in the abyssal fauna, and that no
division into bathymetrical strata can be attempted on the base of
generic much less of family characters."
A division of the world into regions according to the distribution of
the shore-fishes is a much more difficult task than that of tracing
continental areas. It is possible perhaps to distinguish four great
divisions: the Arctic region, the Atlantic region, the Indo-Pacific
region and the Antarctic region. The second and third may be again
subdivided into three zones: Northern, Tropical and Southern. This
appears to be a more satisfactory arrangement than that which has been
proposed into three zones primarily, each again subdivided according to
the different oceans. Perhaps a better division is that adopted by D. S.
Jordan, who arranges the littoral fishes according to coast lines; we
then have an East Atlantic area, a West Atlantic, an East Pacific and a
West Pacific, the latter including the coasts of the Indian Ocean. The
tropical zone, whatever be the ocean, is that in which fishes flourish
in greatest abundance and where, especially about coral-reefs, they show
the greatest variety of bizarre forms and the most gorgeous coloration.
The fish-fauna of the Indo-Pacific is much richer than that of the
Atlantic, both as regards genera and species.
As regards the Arctic and Antarctic regions, the continuity or
circumpolar distribution of the shore fishes is well established. The
former is chiefly characterized by its Cottids, Cyclopterids, Zoarcids
and Gadids, the latter by its Nototheniids. The theory of bipolarity
receives no support from the study of the fishes.
Pelagic fishes, among which we find the largest Selachians and
Teleosteans, are far less limited in their distribution, which, for many
species, is nearly world-wide. Some are dependent upon currents, but the
great majority being rapid swimmers able to continue their course for
weeks, apparently without the necessity of rest (many sharks, scombrids,
sword-fishes), pass from one ocean into the other. Most numerous between
the tropics, many of these fishes occasionally wander far north and
south of their habitual range, and there are few genera that are at all
limited in their distribution.
Deep-sea fishes, of which between seven hundred and eight hundred
species are known, belong to the most diverse groups and quite a number
of families are exclusively bathybial (_Chlamydoselachidae_,
_Stomiatidae_, _Alepocephalidae_, _Nemichthyidae_, _Synaphobranchidae_,
_Saccopharyngidae_, _Cetomimidae_, _Halosauridae_, _Lipogenyidae_,
_Notacanthidae_, _Chiasmodontidae_, _Icosteidae_, _Muraenolepididae_,
_Macruridae_, _Anomalopidae_, _Podatelidae_, _Trachypteridae_,
_Lophotidae_, _Ceratiidae_, _Gigantactinidae_). But they are all
comparatively slight modifications of the forms living on the surface of
the sea or in the shallow parts, from which they may be regarded as
derived. In no instance do these types show a structure which may be
termed archaic when compared with their surface allies. That these
fishes are localized in their vertical distribution, between the
100-fathoms line, often taken as the arbitrary limit of the bathybial
fauna, and the depth of 2750 fathoms, the lowest point whence fishes
have been procured, there is little doubt. But our knowledge is still
too fragmentary to allow of any general conclusions, and the same
applies to the horizontal distribution. Yet the same species may occur
at most distant points; as these fishes dwell beyond the influence of
the sun's rays, they are not affected by temperature, and living in the
Arctic zone or under the equator makes little difference to them. A
great deal of evidence has been accumulated to show the gradual
transition of the surface into the bathybial forms; a large number of
surface fishes have been met with in deep water (from 100 to 500
fathoms), and these animals afford no support to Alexander Agassiz's
supposition of the existence of an azoic zone between the 200-fathoms
line and the bottom.
Brackish-water fishes occur also in salt and fresh water, in some
localities at least, and belong to various groups of Teleosteans.
Sticklebacks, gobies, grey mullets, blennies are among the best-known
examples. The facility with which they accommodate themselves to changes
in the medium in which they live has enabled them to spread readily over
very large areas. The three-spined stickleback, for instance, occurs
over nearly the whole of the cold and temperate parts of the northern
hemisphere, whilst a grey mullet (_Mugil capito_) ranges without any
appreciable difference in form from Scandinavia and the United States
along all the Atlantic coasts to the Cape of Good Hope and Brazil. It
would be hardly possible to base zoo-geographical divisions on the
distribution of such forms.
The fresh-water fishes, however, invite to such attempts. How greatly
their distribution differs from that of terrestrial animals has long ago
been emphasized. The key to their mode of dispersal is, with few
exceptions, to be found in the hydrography of the continents, latitude
and climate, excepting of course very great altitudes, being
inconsiderable factors, the fish-fauna of a country deriving its
character from the headwaters of the river-system which flows through
it. The lower Nile, for instance, is inhabited by fishes bearing a close
resemblance to, or even specifically identical with, those of tropical
Africa, thus strikingly contrasting with the land-fauna of its banks.
The knowledge of the river-systems is, however, not sufficient for
tracing areas of distribution, for we must bear in mind the movements
which have taken place on the surface of the earth, owing to which
present conditions may not have existed within comparatively recent
times, geologically speaking; and this is where the systematic study of
the aquatic animals affords scope for conclusions having a direct
bearing on the physical geography of the near past. It is not possible
here to enter into the discussion of the many problems which the
distribution of fresh-water fishes involves; we limit ourselves to an
indication of the principal regions into which the world may be divided
from this point of view. The main divisions proposed by Günther in the
9th edition of the _Encyclopædia Britannica_ still appear the most
satisfactory. They are as follows:--
I. THE NORTHERN ZONE OR HOLARCTIC REGION.--Characterized
by Acipenseridae. Few Siluridae. Numerous Cyprinidae, Salmonidae,
Esocidae, Percidae.
1. Europaeo-Asiatic or Palaearctic Region. Characterized by
absence of osseous Ganoidei; Cobitinae and Barbus numerous.
2. North American or Nearctic Region. Characterized by osseous
Ganoidei and abundance of Catostominae; but no Cobitinae or
Barbus.
II. THE EQUATORIAL ZONE.--Characterized by the development
of Siluridae.
A. Cyprinoid Division. Characterized by presence of Cyprinidae,
Mastacembelidae. Anabantidae, Ophiocephalidae.
1. Indian Region. Characterized by absence of Dipneusti,
Polypteridae, Mormyridae and Characinidae. Cobitinae
numerous.
2. African Region. Characterized by presence of Dipneusti,
Polypterid and Mormyrid; Cichlid and Characinid numerous.
B. Acyprinoid Division. Characterized by absence of Cyprinidae and the
other families mentioned above.
1. Tropical American or Neotropical Region. Characterized by
presence of Dipneusti; Cichlidae and Characinidae numerous;
Gymnotidae and Loricariidae.
2. Tropical Pacific Region. Includes the Australian as well as
the Polynesian Region. Characterized by presence of
Dipneusti. Cichlidae and Characinidae absent.
III. THE SOUTHERN ZONE.--Characterized by absence of Cyprinidae and
scarcity of Siluridae. Haplochitonidae and Galaxiidae represent the
Salmonids and Esoces of the northern zone. One region only.
1. Antarctic Region. Characterized by the small number of species; the
fishes of
(a) The Tasmanian subregion;
(b) The New Zealand subregion; and
(c) The Patagonian or Fuegian subregion being almost identical.
Although, as expressed in the above synopsis, the resemblance between
the Indian and African regions is far greater than exists between them
and the other regions of the equatorial zone, attention must be drawn to
the marked affinity which some of the fishes of tropical Africa show to
those of South America (_Lepidosirenidae_, _Characinidae_, _Cichlidae_,
_Nandidae_), an affinity which favours the supposition of a connexion
between these two parts of the world in early Tertiary times.
The boundaries of Günther's regions may thus be traced, beginning with
the equatorial zone, this being the richest.
EQUATORIAL ZONE.--Roughly speaking, the borders of this zoological zone
coincide with the geographical limits of the tropics of Cancer and
Capricorn; its characteristic forms, however, extend in undulating lines
several degrees both northwards and southwards. Commencing from the west
coast of Africa, the desert of the Sahara forms a boundary between the
equatorial and northern zones; as the boundary approaches the Nile, it
makes a sudden sweep towards the north as far as northern Syria, crosses
through Persia and Afghanistan to the southern ranges of the Himalayas,
and follows the course of the Yang-tse-Kiang, which receives its
contingent of equatorial fishes through its southern tributaries. Its
continuation through the North Pacific may be indicated by the tropic,
which strikes the coast of Mexico at the southern end of the Gulf of
California. Equatorial types of South America are known to extend so far
northwards; and, by following the same line, the West India Islands are
naturally included in this zone.
Towards the south the equatorial zone embraces the whole of Africa and
Madagascar, and seems to extend still farther south in Australia, its
boundary probably following the southern coast of that continent; the
detailed distribution of the freshwater fishes of south-western
Australia has been little studied, but the tropical fishes of that
region follow the principal watercourse, the Murray river, far towards
the south and probably to its mouth. The boundary-line then stretches to
the north of Tasmania and New Zealand, coinciding with the tropic until
it strikes the western slope of the Andes, on the South American
continent, where it again bends southward to embrace the system of the
Rio de la Plata.
The four regions into which the equatorial zone is divided arrange
themselves into two well-marked divisions, one of which is characterized
by the presence of Cyprinid fishes, combined with the development of
_Labyrinthic_ Percesoces (_Anabantidae_ and _Ophiocephalidae_) and
Mastacembelids, whilst in the other these types are absent. The boundary
between the Cyprinoid and Acyprinoid division seems to follow the now
exploded Wallace's line--a line drawn from the south of the Philippines
between Borneo and Celebes, and farther south between Bali and Lombok.
Borneo abounds in Cyprinids; from the Philippine Islands a few only are
known, and in Bali two species have been found; but none are known from
Celebes or Lombok, or from islands situated farther east.
The Indian region comprises Asia south of the Himalayas and the
Yang-tse-Kiang, and includes the islands to the west of Celebes and
Lombok. Towards the north-east the island of Formosa, which also by
other parts of its fauna shows the characters of the equatorial zone,
has received some characteristic Japanese freshwater fishes. Within the
geographical boundaries of China the freshwater fishes of the tropics
pass gradually into those of the northern zone, both being separated by
a broad, debateable ground. The affluents of the great river traversing
this district are more numerous from the south than from the north, and
carry the southern fishes far into the temperate zone. Scarcely better
defined is the boundary of this region towards the north-west, in which
fishes were very poorly represented by types common to India and Africa.
The African region comprises the whole of Africa south of the Sahara. It
might have been conjectured that the more temperate climate of its
southern extremity would have been accompanied by a conspicuous
difference in the fish fauna. But this is not the case; the difference
between the tropical and southern parts of Africa consists simply in the
gradual disappearance of specifically tropical forms, whilst Silurids,
Cyprinids and even _Anabas_ penetrate to its southern coast; no new
form, except a _Galaxias_ at the Cape of Good Hope, has entered to
impart to South Africa a character distinct from the central portion of
the continent. In the north-east the African fauna passes the isthmus of
Suez and penetrates into Syria; the system of the Jordan presents so
many African types that it has to be included in a description of the
African region as well as of the Europaeo-Asiatic.
The boundaries of the Neotropical or Tropical American region have been
sufficiently indicated in the definition of the equatorial zone. A broad
and most irregular band of country, in which the South and North
American forms are mixed, exists in the north.
The Tropical Pacific region includes all the islands east of Wallace's
line, New Guinea, Australia (with the exception of its south-eastern
portion), and all the islands of the tropical Pacific to the Sandwich
group.
NORTHERN ZONE.--The boundaries of the northern zone coincide in the main
with the northern limit of the equatorial zone; but they overlap the
latter at different points. This happens in Syria, as well as east of
it, where the mixed faunae of the Jordan and the rivers of Mesopotamia
demand the inclusion of this territory in the northern zone as well as
in the equatorial; in the island of Formosa, where a Salmonid and
several Japanese Cyprinids flourish; and in Central America, where a
_Lepidosteus_, a Cyprinid (_Sclerognathus meridionalis_), and an
_Amiurus_ (_A. meridionalis_) represent the North American fauna in the
midst of a host of tropical forms.
There is no separate arctic zone for freshwater fishes; ichthyic life
becomes extinct towards the pole wherever the fresh water remains frozen
throughout the year, or thaws for a few weeks only; and the few fishes
which extend into high latitudes belong to types in no wise differing
from those of the more temperate south. The highest latitude at which
fishes have been obtained is 82° N. lat., whence specimens of char
(_Salmo arcturus_ and _Salmo naresii_) have been brought back.
_The Palaearctic or Europaeo-Asiatic Region._--The western and southern
boundaries of this region coincide with those of the northern zone.
Bering Strait and the Kamchatka Sea have been conventionally taken as
the boundary in the north, but the fishes of both coasts, so far as they
are known, are not sufficiently distinct to be referred to two different
regions. The Japanese islands exhibit a decided Palaearctic fish fauna
with a slight influx of tropical forms in the south. In the east, as
well as in the west, the distinction between the Europaeo-Asiatic and
the North American regions disappears almost entirely as we advance
farther towards the north. Finally, the Europaeo-Asiatic fauna mingles
with African and Indian forms in Syria, Persia and Afghanistan.
The boundaries of the North American or Nearctic region have been
sufficiently indicated. The main features and the distribution of this
fauna are identical with those of the preceding region.
SOUTHERN ZONE.--The boundaries of this zone have been indicated in the
description of the equatorial zone; they overlap the southern
boundaries of the latter in South Australia and South America, but we
have not the means of defining the limits to which southern types extend
northwards. This zone includes Tasmania, with at least a portion of
south-eastern Australia (Tasmanian sub-region), New Zealand and the
Auckland Islands (New Zealand sub-region), and Chile, Patagonia, Tierra
del Fuego and the Falkland Islands (Fuegian sub-region). No freshwater
fishes are known from Kerguelen's Land, or from islands beyond 55° S.
lat.
The Tropical American region is the richest (about 1300 species); next
follow the African region (about 1000), the Indian region (about 800),
the Europaeo-Asiatic region (about 500), the North American region
(about 400), the Tropical Pacific region (about 60); whilst the
Antarctic region is quite insignificant.
Of the migratory fishes, or fishes travelling regularly from the sea to
fresh waters, most, if not all, were derived from marine forms. The
anadromous forms, annually or periodically ascending rivers for the
purpose of spawning, such as several species of _Acipenser_, _Salmo_,
_Coregonus_, _Clupea_ (shads), and _Petromyzon_, are only known from the
northern hemisphere, whilst the catadromous forms, spending most of
their life in fresh water but resorting to the sea to breed, such as
_Anguilla_, some species of _Mugil_, _Galaxias_ and _Pleuronectes_, have
representatives in both hemispheres. (G. A. B.)