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.