Cavefish

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Cavefish or cave fish is a generic term for fresh and brackish water fish adapted to life in caves and other underground habitats. Related terms are subterranean fish, troglomorphic fish, troglobitic fish, stygobitic fish, phreatic fish, and hypogean fish. There are more than 200 scientifically described species of obligate cavefish found on all continents, except Antarctica. Although widespread as a group, many species have very small ranges and are threatened. Cavefish are members of a wide range of families and do not form a monophyletic group. Typical adaptations include reduced eyes and depigmentation. Many aboveground fish may enter caves on occasion, but obligate cavefish (fish that require underground habitats) are extremophiles with a number of unusual adaptations known as troglomorphism. In some species, notably the Mexican tetra, shortfin molly, Oman garra, Indoreonectes evezardi, and a few catfish, both "normal" aboveground and cavefish forms exist. Many adaptations seen in cavefish are aimed at surviving in a habitat with little food. Living in darkness, pigmentation and eyes are useless, or an actual disadvantage because of their energy requirements, and therefore typically reduced in cavefish.