CHAPTER XVIII.. The Fishes of the Brackish Water.

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.

Heads of Grey Mullets, fishes of Brackish water.]