III.—The Equatorial Zone.

As we approach the Tropic from the north, the tribes characteristic of the Arctic and Temperate zones become scarcer, and disappear altogether: to be replaced by the greater variety of Tropical types. Of Chondropterygians, the Chimœridœ, Spinacidœ, Mustelus, and Raja, do not pass the Tropic, or appear in single species only; and of Teleosteans, the Berycidæ, Pagrus, the Heterolepidina, Cottus and allied genera, Lophius, Anarrhichas, Stichæus, Lepadogaster, Psychrolutes, Centriscus, Notacanthus, the Labridæ and Embiotocidæ, the Lycodidæ, Gadidæ, and marine Salmonidæ disappear either entirely, or retire from the shores and surface into the depths of the ocean.

With regard to variety of forms, as well as to number of individuals, this zone far surpasses either of the temperate zones; in this respect, the life in the sea is as that on the land. Coast fishes are not confined to the actual coast-line, but abound on the coral reefs, with which some parts of the Atlantic and Pacific are studded, and many of which are submerged below the water. The abundance of animal and vegetable life which flourishes on them renders them the favourite pasture-grounds for the endless variety of coral-fishes (Squamipinnes, Acronuridæ, Pomacentridæ, Julidæ, Plectognathi, etc.), and for the larger predatory kinds. The colours and grotesque forms of the Fishes of the Tropics have justly excited the admiration of the earliest observers. Scarlet, black, blue, pink, red, yellow, etc., are arranged in patterns of the most bizarre fashion, mingling in spots, lines, bands; and reminding us of the words of Captain Cook when describing the coral-reefs of Palmerston Island: “The glowing appearance of the Mollusks was still inferior to that of the multitude of fishes that glided gently along, seemingly with the most perfect security. The colours of the different sorts were the most beautiful that can be imagined—the yellow, blue, red, black, etc., far exceeding anything that art can produce. Their various forms, also, contributed to increase the richness of this sub-marine grotto, which could not be surveyed without a pleasing transport.”

Of Chondropterygians the Scylliidæ, Pristis (Saw-fishes), Rhinobatidæ, and Trygonidæ attain to the greatest development. Of Acanthopterygians Centropristis, Serranus, Plectropoma, Mesoprion, Priacanthus, Apogon, Pristipoma, Hæmulon, Diagramma, Gerres, Scolopsis, Synagris, Cæsio, Mullidæ, Lethrinus, Squamipinnes, Cirrhites, some genera of Scorpænidæ, Platycephalus, Sciænidæ, Sphyræna, Caranx Equula, Callionymus, Teuthis, Acanthurus, Naseus, are represented by numerous species; and the majority of these genera and families are limited to this zone. Of Pharyngognaths the Pomacentridæ, Julidina, and Scarina, are met with near every coral formation in a living condition. Of Gadoids, a singular minute form, Bregmaceros, is almost the only representative, the other forms belonging to deep water, and rarely ascending to the surface. Flat-fishes (Pleuronectidæ) are common on sandy coasts, and the majority of the genera are peculiar to the Tropics. Of Physostomi only the Saurina, Clupeidæ, and Murænidæ are represented, the Clupeidæ being exceedingly numerous in individuals, whilst the Murænidæ live more isolated, but show a still greater variety of species. Lophobranchii and Sclerodermi are generally distributed. Branchiostoma has been found on several coasts.

Geographically it is convenient to describe the Coast fauna of the tropical Atlantic separately from that of the Indo-Pacific ocean. The differences between them, however, are far less numerous and important than between the freshwater or terrestrial faunæ of continental regions. The majority of the principal types are found in both, many of the species being even identical; but the species are far more abundant in the Indo-Pacific than in the Atlantic, owing to the greater extent of the archipelagoes in the former. But for the broken and varied character of the coasts of the West Indies, the shores of the tropical Atlantic would, by their general uniformity, afford but a limited variety of conditions to the development of specific and generic forms, whilst the deep inlets of the Indian ocean, with the varying configuration of their coasts, and the different nature of their bottom, its long peninsulas, and its archipelagoes, and the scattered islands of the tropical Pacific, render this part of the globe the most perfect for the development of fish-life. The fishes of the Indian and Pacific oceans (between the Tropics) are almost identical, and the number of species ranging from the Red Sea and east coast of Africa to Polynesia, even to its westernmost islands, is very great indeed. However, this Indo-Pacific fauna does not reach the Pacific coast of South America. The wide space devoid of islands, east of the Sandwich Islands and the Marquesas group, together with the current of cold water which sweeps northwards along the South American coast, has proved to be a very effectual barrier to the eastward extension of the Indo-Pacific fauna of coast fishes; and, consequently, we find an assemblage of fishes on the American coast and at the Galapagoes Islands, sufficiently distinct to constitute a distinct zoological division.

The following list, which contains only the principal genera and groups of coast fishes, will give an idea of the affinity of the tropical Atlantic and Indo-Pacific:—[28]

Trop.-Atl.Indo-Pac.
Scylliidæ—13
Pristis34
Rhinobatidæ48
Torpedinidæ18
Trygonidæ1424
Etelis11
Aprion—1
Apsilus1—
Centropristis15—
Anthias45
Serranus3085
Plectropoma115
Grammistes—2
Rhypticus3—
Diploprion—1
Myriodon—1
Mesoprion1550
Priacanthus412
Apogon and Chilodipterus275
Pristipoma1214
Hæmulon15—
Diagramma—30
Gerres1216
Scolopsis—20
Dentex and Symphorus—7
Synagris and Pentapus—24
Cæsio—12
Mullidæ522
Sargus72
Lethrinus118
Chrysophrys17
Pimelepterus15
Squamipinnes13110
Toxotes—2
Cirrhites—20
Scorpænidæ265
Myripristis315
Holocentrum625
Platycephalus—25
Prionotus1—
Trigla—4
Peristethus26
Uranoscopina28
Champsodon—1
Percis—10
Sillago—5
Latilus12
Opisthognathus25
Pseudochromis—8
Cichlops and Pseudoplesiops—2
Sciænidæ4443
Sphyræna110
Trichiuridæ65
Caranx2060
Chorinemus47
Trachynotus64
Psettus12
Platax—7
Zanclus—1
Equula and Gazza—20
Teuthis—30
Acanthurus342
Naseus—12
Kurtidæ16
Gobiodon—7
Callionymus—17
Batrachidæ54
Tetrabrachium—1
Malthe1—
Petroscirtes—30
Clinus6—
Dactyloscopus1—
Malacanthus12
Cepola—1
Gobiesocidæ51
Amphisile—3
Fistulariidæ33
Pomacentridæ17120
Lachnolæmus1—
Julidina36190
Pseudodax—1
Scarina2165
Pseudophycis—1
Bregmaceros—1
Ophidiidæ37
Fierasfer—6
Pleuronectidæ2156
Saurina59
Clupeidæ.3384
Chirocentrus—1
Murænidæ47130
Pegasus—3
Solenostoma—2
Syngnathidæ741
Sclerodermi1667
Gymnodontes2340

A. Shore Fishes of the Tropical Atlantic.

The boundaries of the tropical Atlantic extend zoologically a few degrees beyond the Northern and Southern Tropics, but as the mixture with the types of the temperate zone is very gradual, no distinct boundary line can be drawn between the tropical and temperate faunæ.

Types, almost exclusively limited to it, and not found in the Indo-Pacific, are few in number, as Centropristis, Rhypticus, Hæmulon, Malthe. A few others preponderate with regard to the number of species, as Plectropoma, Sargus, Trachynotus, Batrachidæ, and Gobiesocidæ. The Sciænoids are equally represented in both oceans. All the remainder are found in both; but in the minority in the Atlantic, where they are sometimes represented by one or two species only (for instance, Lethrinus).

B. Shore Fishes of the Tropical Indo-Pacific Ocean.

The ichthyological boundaries of this part of the tropical zone may be approximately given as 30° of lat. N. and S.; on the Australian coasts it should probably be placed still farther south, viz., to 34°; it includes, as mentioned above, the Sandwich Islands, and all the islands of the South Sea, but not the American coasts.

Some eighty genera of Shore fishes are peculiar to the Indo-Pacific, but the majority consists of one or a few species only; comparatively few have a plurality of species, as Diagramma, Lethrinus, Equula, Teuthis, Amphiprion, Dascyllus, Choerops, Chilinus, Anampses, Stethojulis, Coris, Coilia.

The Sea-perches, large and small, which feed on Crustaceans and other small fishes, and the coral-feeding Pharyngognaths are the types which show the greatest generic and specific variety in the Indo-Pacific. Then follow the Squamipinnes and Murænidæ, the Clupeidæ and Carangidæ families in which the variety is more that of species than of genus. The Scorpænidæ, Pleuronectidæ, Acronuridæ, Sciænidæ, Syngnathidæ, and Teuthyes, are those which contribute the next largest contingents. Of shore-loving Chondropterygians the Scylliidæ and Trygonidæ only are represented in moderate numbers, though they are more numerous in this ocean than in any other.

C. Shore Fishes of the Pacific Coasts of Tropical America.

As boundaries within which this fauna is comprised, may be indicated 30° lat. N. and S., as in the Indo-Pacific. Its distinction from the Indo-Pacific lies in the almost entire absence of coral-feeding fishes. There are scarcely any Squamipinnes, Pharyngognaths or Acronuridæ, and the Teuthyes are entirely absent. The genera that remain are such as are found in the tropical zone generally, but the species are entirely different from those of the Indo-Pacific. They are mixed with a sprinkling of peculiar genera, consisting of one or two species, like Discopyge, Hoplopagrus, Doydixodon, but they are too few in number to give a strikingly peculiar character to this fauna.

The Three districts are distinguishable:—

a. Central American district, in which we include, for the present, Lower California, shows so near an affinity to the tropical Atlantic that, if it were not separated from it by the neck of land uniting the two American Continents, it would most assuredly be regarded as a portion of the Fauna of the tropical Atlantic. With scarcely any exceptions the genera are identical, and of the species found on the Pacific side nearly one-half have proved to be the same as those of the Atlantic. The explanation of this fact has been found in the existence of communications between the two oceans by channels and straits which must have been open till within a recent period. The isthmus of Central America was then partially submerged, and appeared as a chain of islands similar to that of the Antilles; but as the reef-building corals flourished chiefly north and east of those islands, and were absent south and west of them, reef-fishes were excluded from the Pacific shores when the communications were destroyed by the upheaval of the land.

b. The Galapagoes district received its coast fauna principally from the Central American district, a part of the species being absolutely the same as on the coast of the Isthmus of Panama, or as in the West Indies. Yet the isolation of this group has continued a sufficiently long period to allow of the development of a number of distinct species of either peculiarly Atlantic genera (such as Centropristis, Rhypticus, Gobiesox, Prionotus), or at least tropical genera (such as Chrysophrys, Pristipoma, Holacanthus, Caranx, Balistes). A few other types from the Peruvian coast (Doydixodon), or even from Japan (Prionurus), have established themselves in this group of islands. A species of Cestracion has also reached the Galapagoes, but whether from the south, north, or west, cannot be determined.

The presence of the Atlantic fauna on the Pacific side is felt still farther west than the Galapagoes, some Atlantic species having reached the Sandwich Islands, as Chætodon humeralis and Blennius brevipinnis.

c. The Peruvian district possesses a very limited variety of shore fishes, which belong, with few exceptions, like Discopyge, Hoplognathus, Doydixodon, to genera distributed throughout the tropical zone, or even beyond it. But the species, so far as they are known at present, are distinct from those of the Indo-Pacific, as well as of the tropical Atlantic; and therefore this district cannot be joined either to the Central American or the Galapagoes.