Losing Sight

Living in three dark caves in San Luis Potosi, Mexican blind cave fish have lost their eyes.

In one of the world's more recent - and evident - examples of evolution, fish from the species Astyanax fasciatus (a type of tetra) migrated into a system of dark caves in Mexico and lost all need for their eyes, so the useless organs gradually disappeared. Now a separate subspecies named Astyanax fasciatus mexicanus, these fish are born with functional eyes, but after about two weeks, the eyes become distorted and partially enclosed in tissue. As the young fish grow, their rudimentary eyes shrink and become completely covered, leaving them eyeless and blind.

Although they have lost their sight, blind cave fish have gained other senses that help them to live in the dark. An especially keen sense of smell guides them toward their favorite food sources, including animal or plant remains that wash into the caves and nutrient-rich bat droppings that fall from the craggy ceilings. Another sense, which is still being studied by researchers, prevents the fish from bumping into both each other and their surroundings. Although a number of physiological changes have taken place since these fish moved into their blackened home, they split off from their seeing relatives recently enough that the two subspecies can still interbreed.

Blind cave fish, Astyanax fasciatus mexicanus, live in freshwater pools like this one within the dark caverns of three caves in San Luis Potosi, Mexico
Blind cave fish, Astyanax fasciatus mexicanus.
Photo De Filippi, 1853 (c) Virtual Museum of Natural History
Map by Colleen Sudekum