Astyanax mexicana, the Mexican tetra, is a small fish in the family Characidae native to subtropical and temperate North America, mostly central and eastern Mexico, Texas and New Mexico. Mexican tetras can form large schools, up to several hundred individuals. These fish prefer sandy or rocky bottomed pools, but they are found in a diversity of habitats, including fast-moving water. Astyanax mexicana is carnivorous, eating insects and small fish, some populations also also eat plants and green algae. Previously considered a subspecies of Astyanax fasciatus, it is now classified as its own species, and it may represent a species complex. The Mexican tetra is found in two distinct forms depending on its environment: a silver, surface dwelling form, and about 30 distinct populations of a cave-dwelling form morphologically distinct from the normal form by pronounced loss of pigmentation and eyes. The cave form is thought to be derived from two distinct lineages, implying that these morphological features were lost convergently. Astyanax mexicana has been much studied as a genetic model for adaptations to darkness and eye development and evolution, especially powerful because this species is closely related to another well-known laboratory model, the zebrafish (Danio rerio). Both the regular and cave form of the Mexican tetra are common and popular aquarium fish, easily bred in captivity.

(Hassan-Williams and Bonner; Dowling, Martasian and Jeffrey 2002; Froese 2010; Nico 2012; Protas et al. 2007; Retaux, Pottin and Alunni 2008; Wikipedia 2012)