The Mexican tetra (Astyanax mexicanus), also known as the blind cave fish, is a freshwater fish belonging to the Characidae family. Native to the lower Rio Grande, Nueces, and Pecos Rivers in Texas and parts of Mexico, this fish can reach about 12 cm (4.7 in) in length.
The cave-dwelling variant of the Mexican tetra has evolved a pale, pinkish-white color and lacks functional eyes, having instead developed enhanced sensory organs to navigate its dark environment. These adaptations are similar to echolocation, helping them detect objects through changes in water pressure.
The Mexican tetra is a sociable schooling fish that prefers mid-level waters and a subtropical climate with a pH of 6.5–8. It feeds on crustaceans, annelids, and aquatic insects, and in captivity, it thrives on a varied diet, including live and frozen foods.
While sometimes classified as a subspecies of A. fasciatus, this classification is not widely accepted. The blind cave form is occasionally recognized as a separate species, A. jordani, though this contradicts phylogenetic evidence.
Astyanax mexicanus is known for its blind cave form, commonly referred to as the blind cave tetra. Depending on the population, these cave forms may exhibit complete or partial loss of sight due to the down-regulation of the protein αA-crystallin, leading to lens cell death. Despite being blind, cavefish have cells that respond to light and maintain a circadian rhythm. They navigate using their lateral lines, which detect changes in water pressure.
Currently, around 30 cave populations are found in the karst region of northeastern Mexico, with varying degrees of eye development. Genetic studies indicate that blindness in these fish evolved independently across different caves within the last 20,000 years. The eyed and eyeless forms belong to the same species and can interbreed, making them valuable for research on evolution and genetic traits.
Some scientists propose recognizing the blind and colorless cave form as a separate species, Astyanax jordani, which complicates the classification of A. mexicanus. Additionally, another cave population with varying traits exists in Granadas Cave but is classified under A. aeneus.
