Cave fish
Fish that live their whole lives in underground water and have often lost their eyes and colour.
Also: troglobite, subterranean fish
More than 250 fish species live only in caves, sinkholes, aquifers and wells. Many are blind and pale, because eyes and pigment cost energy and give no benefit in the dark, and they find food with the lateral line, taste and smell. The Mexican tetra has blind cave populations and sighted river populations of the same species, which makes it a favourite of researchers. India has several, including the blind catfish of wells in Kerala and the cave fish of Meghalaya's limestone caves, one of which, described in 2019, is the largest known cave fish.