Cavefish

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 and hypogean fish.

Being aquatic, they are a part of the troglobite group known as stygofauna. The more than 200 scientifically described species of obligate cavefish are found on all continents, except Antarctica. Although widespread as a group, many cavefish species have very small ranges and are seriously threatened. Cavefish are members of a wide range of families and do not form a monophyletic group. Typical adaptations found in cavefish are reduced eyes and pigmentation.

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 adaptions 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. Other examples of adaptations are larger fins for more energy-efficient swimming, and a loss of scales, swim bladder and behaviors such as certain types of display. The loss can be complete or only partial, for example resulting in small (but still existing) eyes. In some cases, "blind" cavefish may still be able to see; juvenile Mexican tetras of the cave form are able to sense light via certain cells in the pineal gland (pineal eye). In the most extreme cases, the lack of light has changed the circadian rhythm (24-hour internal body clock) of the cavefish. In the Mexican tetra of the cave form and in Phreatichthys andruzzii the circadian rhythm lasts 30 hours and 47 hours, respectively. This may help them to save energy. Without sight, other senses are used and these may be enhanced. Examples include the lateral line for sensing vibrations, chemoreception (via smell and taste buds), electroreception, and mouth suction to sense nearby obstacles (comparable to echolocation). The level of specialized adaptations in a cavefish is generally considered to be directly correlated to the amount of time it has been restricted to the underground habitat, with species that recently arrived showing few adaptations and species with the largest number of adaptations likely being the ones restricted to the habitat for the longest time.

...
Wikipedia