Fishes are characterized as aquatic vertebrates that belong to the three classes Agnatha, Chondrichthyes and Osteichthyes of the Phylum Vertebrates in the Animal Kingdom. In India, about 2200 species of fishes have been identified so far, out of which 73 (3.32%) inhabit the cold freshwater, 544 (24.73%) inhabit warm fresh water, 143 (6.50%) inhabit the brackish water and 1440 (65.45%) inhabit the sea. Out of the 2200 species from about 22000 found in the world, 223 are only found in India (endemic species). Some families (eg: Cyprinidae) are fresh water fishes, some inhabit only the salt water, some are anadromous (which live in the sea and migrate to freshwater for spawning), some are catedromous (which live in the fresh water and migrate to sea for spawning) and yet some are euryhaline (which can adapt to a wide range of salinities).
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Most species of truly fresh water fishes (which do not migrate to sea) in Indian subcontinent are unique to the region due to partial isolation from rest of the world. They are isolated from the rest of Asia due to the great range of Himalaya on the North-East and Indian ocean on the other sides. Only a few of the true fresh water fishes of India are found in countries outside the sub-continent.
Many of the fresh water fishes are ornamental varieties used in aquariums, having high potential in domestic and international markets. Most of the freshwater fishes are well adapted to survive in the differing climates and environments. Many fishes have got additional respiratory organs to enable breathing air while crawling through land, that enable them to migrate from one habitat to another or from one pond to another. Some of the fresh water fishes inhabiting the paddy fields bury themselves in mud during drought, and remain there in dormant stage, till the next monsoon comes.
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