III. 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 at present the means of exactly 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 Chili, Patagonia, Terra del Fuego, and the Falkland Islands (Fuegian sub-region). No freshwater fishes are known from Kerguelen’s Land, or from islands beyond 55° lat. S. The southern extremity of Africa has to be excluded from this zone so far as Freshwater fishes are concerned.

This zone is, with regard to its extent as well as to the number of species, the smallest of the three; yet its ichthyological features are well marked; they consist in the presence of two peculiar families, each of which is analogous to a northern type, viz. the Haplochitonidæ, which represent the Salmonidæ, Haplochiton being the analogue of Salmo, and Prototroctes that of Coregonus; and the Galaxiidæ, which are the Pikes of the Southern Hemisphere.

Although geographically widely separate from each other, the Freshwater fishes of the three divisions are nevertheless so closely allied that conclusions drawn from this group of animals alone would hardly justify us in regarding these divisions as sub-regions. One species of Galaxias (G. attenuatus) and the three Lampreys are found in all three, or at least two, sub-regions.

Freshwater Fishes of the Southern Zone.

Tasmanian.N. Zealand.Fuegian.
Percichthys......3
Siluridæ—
Diplomystax......1
Nematogenys......1
Trichomycterina [Neotrop.]......5
Gadopsidæ1......
(Retropinna...1...)
Haplochitonidæ111
Galaxiidæ654
Petromyzontidæ313
11818

But little remains to be added in explanation of this list; Percichthys is in Chili the autochthont form of the cosmopolitan group of Percina. Diplomystax, an Arioid fish of Chili, and Nematogenys seem to have crossed the Andes from Tropical America at a comparatively early period, as these genera are not represented on the eastern side of South America; the Trichomycterina occur on both sides of the Andes, which they ascend to a considerable height. Retropinna is a true Salmonoid, allied to, and representing in the Southern Hemisphere the Northern Smelt, Osmerus. In both these genera a part of the specimens live in the sea, and ascend rivers periodically to spawn; another part remain in rivers and lakes, where they propagate, never descending to the sea, this freshwater race being constantly smaller than their marine brethren. That this small Teleostean of the Northern Hemisphere should reappear, though in a generically modified form, in New Zealand, without having spread over other parts of the Southern Zone, is one of the most remarkable, and at present inexplicable facts of the geographical distribution of freshwater fishes.

Fig. 104.—Haplochiton zebra, Straits of Magelhæn.

CHAPTER XVIII.
THE FISHES OF THE BRACKISH WATER.

On such parts of a coast at which there is a mixture of fresh and salt water, either in consequence of some river emptying its water into the sea or from an accumulation of land surface water forming lagunes, which are in uninterrupted or temporary communication with the sea, there flourishes a peculiar brackish water fauna which is characterised by the presence of fishes found sometimes in sea-, sometimes in pure freshwater.

This fauna can be rather sharply defined if a limited district only is taken into consideration; thus, the species of the brackish water fauna of Great Britain, the Pacific coast of Central America, of the larger East India Islands, etc., can be enumerated without much hesitation. But difficulties arise when we attempt to generalise in the enumeration of the forms referable to the brackish water fauna; because the genera and families enumerated include certain species and genera which have habituated themselves exclusively either to a freshwater or marine existence; and, besides, because a species of fish may be at one locality an inhabitant of brackish water, at another of the sea, and at a third of fresh water. The circumstance that these fishes can live in sea and fresh water has enabled them to spread readily over the globe, a few only being limited to particular regions; therefore, for the purposes of dividing the earth’s surface into natural zoological regions the brackish water forms are useless. The following fishes may be referred to this Fauna:—

1. Species of Rajidæ (Raja, Trygon) prefer the mouths of rivers, probably because the muddy or sandy bottom offers the most suitable conditions for fishes which can feed on the bottom only; such brackish water species belong chiefly to the Equatorial Zone, some having taken up their abode entirely in fresh water (South American Trygons).

2. Ambassis, a Percoid genus, consisting of numerous small species, inhabiting the shores of the tropical parts of the Indian Ocean and the coasts of Tropical Australia. Many species enter, and all seek the neighbourhood of, fresh water; hence they disappear in the islands of the Pacific, and are scarce in the Red Sea.

3. Therapon, with the same distribution as the former.

4. Numerous Sciænidæ of the Equatorial Zone.

5. The Polynemidæ, chiefly inhabitants of brackish water of the Equatorial Zone, most developed in the Indian region, and scarce in the Tropical Pacific.

6. Numerous species of Caranx (or Horse Mackerels) of the Equatorial Zone.

7. Nearly all species of Gastrosteus enter brackish water, G. spinachia being almost exclusively confined to it: Northern Zone.

8. The most important genera of the Gobies (Gobiina): Gobius (nearly cosmopolitan), Sicydium, Boleophthalmus, Periophthalmus, Eleotris (equatorial). Many of the species are entirely confined to fresh water.

9. The Amblyopina, similar to the Gobies, but with more elongated body: Tropical Indo-Pacific.

10. The Trypauchenina: Coasts of the Indian region.

11. Many species of Blennius, of which several are found far inland in fresh waters—for instance in North Italy, in the Lake of Galilee, in the eastern parts of Asia Minor.

12. The majority of Atherinidæ, and

13. The Mugilidæ: both families being most numerous and abundant in brackish water, and almost cosmopolitan.

14. Many Pleuronectidæ prefer the mouths of rivers for the same reason as the Rays; some ascend rivers, as the Flounder, Cynoglossus, etc.

15. Several Siluridæ, as especially the genera Plotosus, Cnidoglanis, Arius, which attain their greatest development in brackish water.

16. The Cyprinodontidæ are frequently found in brackish water.

17. Species of Clupea, some of which ascend rivers, and become acclimatized in fresh water, as Clupea finta, which has established itself in the lakes of northern Italy.

18. Chatoessus, a genus of Clupeoid fishes of the Equatorial Zone, of which some species have spread into the Northern Zone.

19. Megalops: Equatorial Zone.

20. Anguilla. The distribution, no less than the mode of propagation, and the habits generally, of the so-called Freshwater-eels still present us with many difficult problems. As far as we know at present their birthplace seems to be the coast in the immediate neighbourhood of the mouths of rivers. They are much more frequently found in fresh water than in brackish water, but the distribution of some species proves that they at times migrate by sea as well as by land and river. Thus Anguilla mauritiana is found in almost all the fresh and brackish waters of the islands of the Tropical Indian Ocean and Western Pacific, from the Comoros to the South Sea; Anguilla vulgaris is spread over temperate Europe (exclusive of the system of the Danube, the Black and Caspian Seas), in the Mediterranean district (including the Nile and rivers of Syria), and on the Atlantic coast of North America; Anguilla bostoniensis, in Eastern North America, China, and Japan; Anguilla latirostris, in Temperate Europe, the whole Mediterranean district, the West Indies, China, and New Zealand. The other more local species are found, in addition to localities already mentioned, on the East Coast of Africa, South Africa, on the continent of India, various East Indian Islands, Australia, Tasmania, Auckland Islands; but none have ever been found in South America, the West Coast of North America, and the West Coast of Africa: surely one of the most striking instances of irregular geographical distribution.

21. Numerous Syngnathidæ have established themselves in the Northern Zone as well as in the Equatorial, in the vegetation which flourishes in brackish water.

This list could be considerably increased if an enumeration of species, especially of certain localities, were attempted; but this is more a subject of local interest, and would carry us beyond the scope of a general account of the distribution of Fishes.

Fig. 105.—Mugil octo-radiatus.

Fig. 106.—Mugil auratus.

Fig. 107.—Mugil septentrionalis.

Heads of Grey Mullets, fishes of Brackish water.

CHAPTER XIX.
THE DISTRIBUTION OF MARINE FISHES.

Marine fishes fall, with regard to their mode of life and distribution, into three distinct categories:—

1. Shore Fishes—That is, fishes which inhabit chiefly 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 of these fishes 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, which 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 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, but as surely as, the strong swimmers. Therefore, an accurate definition 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 as to be but little or not influenced by light or the surface temperature; and which, by their organisation 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 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.

It must not be imagined that these three categories are more sharply defined than Freshwater and Marine Fishes. They gradually pass into each other, and there are numerous fishes about which uncertainty exists whether they should be placed in the Shore or Pelagic series, or in the Pelagic or Deep-sea series; nay, many facts favour the view that changes in the mode of life and distribution of fishes are still in progress.

The change in habitat of numerous fishes is regulated by the distribution of their favourite food. At certain seasons the surface of the sea in the vicinity of land swarms with mollusks, larval Crustaceans, Medusæ, attracting shoals of fishes from the open ocean to the shores; and these are again pursued by fishes of larger size and predacious habits, so that all these fishes might be included, with equal propriety, in the littoral or pelagic series. However, species which are known to normally spawn in the open ocean must be always referred to the latter division.

Chondropterygii, Acanthopterygii, Anacanths, Myxinoids, and Pharyngobranchii furnish the principal contingents to the Marine Fauna; whilst the majority of Malacopterygians, the Ganoids, and Cyclostomes are Freshwater Fishes.