Tétra aveugle

According to some specialists, a strain of Astyanax mexicanus found itself, 500,000 years ago, enclosed in a cave following a landslide or collapse. Despite the absence of light, the fish survived, multiplied and gradually adapted to their new living conditions. Thus, over time, their useless eyes atrophied and then completely disappeared without affecting the survival of the species. The date when the strain that would become blind was isolated from the rest of the population is subject to discussion among specialists. Some mention more than a million years. A recent study suggests it was rather less than 30,000 years.

The Mexican tetra is a gregarious species that spends most of its time mid-water above rocky and sandy bottoms of rivers and streams in its native habitat. Cave populations have very different behavior from surface populations. For example, a mutation in an enzyme makes cave fish much less aggressive than surface fish. Cave fish do not swim in schools contrary to surface fish.

Reaching up to 12 cm in length, the Mexican tetra is of tetra characin type, without particularity and with dull coloration. However, its cave form is remarkable for the absence of eyes and pigmentation, not being albino, its livery is just pink, but some individuals display violet or green reflections. Its lateral line is well marked by small black dots.

Coming from a subtropical climate, the surface form prefers water at 6.0-7.8 pH, with hardness up to 30 DGH, and a temperature of 20 to 25°C. In winter, it migrates to warmer waters. The blind form lives in rather cool and calm waters (18-23°C), in an environment devoid of light, therefore vegetation.

Its natural diet consists of crustaceans, insects and annelids, but in captivity, it is omnivorous. The blind form has well-developed dentition, probably used to devour small crustaceans and insects with resistant exoskeletons, very frequent in caves. It also feeds on bat droppings.