Mexican tetra

Mexican blind cavefish, the cave form of the Mexican tetra One species, two ways of living: a silvery, sighted river fish and pale, eyeless fish in 29 deep caves in Mexico. That makes it one of the clearest natural experiments in cave evolution. Cave fish are born with eyes that degenerate as they grow, and populations in separate, unconnected caves such as Pachón, Tinaja and Molino lost both sight and sleep independently, through different genetic changes. The difference in sleep is stark: surface fish sleep around 13 hours a day, the cave fish just 2 to 4.