MEXICAN BLIND CAVEFISH
Many caves around the world are known for having blind fish occupying subterranean bodies of water. Members of several unrelated families have become blind cavefish. Here's 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).
Above & below: Astyanax fasciatus (Cuvier, 1819) (or Astyanax mexicanus (De Filippi, 1853), depending on one's taxonomic preferences) (captives) from southern Mexico.
Classification: Animalia, Chordata, Vertebrata, Actinopterygii, Cypriniformes, Characoidei, Characidae