Astyanax fasciatus (Cuvier, 1819) (or Astyanax mexicanus (De Filippi, 1853), depending on one's taxonomic preferences) - Mexican blind cavefish from southern Mexico (captive, Newport Aquarium, Newport, Kentucky, USA).

Many caves are known to have blind fish in subterranean bodies of water. Members of several unrelated families have become blind cavefish. This is the Mexican blind cavefish, Astyanax fasciatus (sensu lato), which has been recorded from throughout Central America. It completely lacks eyes (eyes are useless in a permanently dark environment) and also has no pigmentation (hence the pinkish color from blood). However, this species does still possess the genes for eyes. Why? It’s ancestors did have eyes - they’ve been lost through evolution. This is the ultimate fate for all vestigial organs. However, the genes for lost structures are often retained for long geologic intervals of time (for example, chickens still have genes that code for teeth, despite the lack of teeth in chicken beaks).

New info. - recent published research has determined that young cavefish of this species do have functioning eyes and have a light-sensitive area in their brains. The eyes degenerate with ontogeny, as does the light-sensitive area in the brain. The adults have zero light sensing ability.