Mexican Tetra, Astyanax mexicanus
Mexican Tetra, Astyanax mexicanus. Fish caught from the San Felipe Creek, Del Rio, Texas, February 2022. Length: 5.8 cm (2.3 inches). Catch, photograph, and identification courtesy of Chris Moore, Peoria, Arizona.
Mexican Tetra, Astyanax mexicanus. Fish caught from a freshwater stream in Brownsville, Texas, March 2021. Length: 6.5 cm (2.6 inches). Catch, photograph and identification courtesy of Eli (obsessiveangling.wordpress.com).
Mexican Tetra, Astyanax mexicanus. Fish caught from the Guadalupe River, Austin, Texas, June 2022. Length: 8.4 cm (3.3 inches). Catch, photograph and identification courtesy of Ben Cantrell, Sebastian, Florida.
Mexican Tetra, Astyanax mexicanus. Fish caught from a small feeder creek of the San Antonio River, San Antonio, Texas, June 2021. Length: 8.6 cm (3.4 inches). Catch, photograph and identification courtesy of Luke Ovgard, Klamath Falls, Oregon.
The Mexican Tetra, Astyanax mexicanus, is a member of the Tetra or Characidae Family, and is also known as the Blind Cavefish and Blind Cave Tetra and the Texan Tetra and in Mexico as sardinita Mexicana and sardinita texana. Globally, there are one hundred forty-seven species in the genus Astyanax, of which three are found in the freshwater systems of mainland Mexico. The Astyanax Genus was named after Astyanax, the son of Hector of Troy in Greek mythology (not easily confused with Helen).
The Mexican Tetra has two forms which includes the Blind Cave Form, found in dark caves, and the Eyed Form found in sunlight waters. The Blind Cave Form has no eyes and is a pinkish white albino color; the Eyed Form is pinkish red in color. They have an oblong strongly compressed small minnow-like body. They are silvery in color with a black lateral bar along the sides that expands near the caudal base and narrows on the caudal fin. Breeding males develop yellow to orange-red anal and dorsal fins with hooklets on the anal fin. They possess a small adipose fin; their anal fin has 19 to 23 rays and is long; their caudal fin is forked; their dorsal fin has 10 to 11 rays and is triangular in shape; their pectoral fins are pointed and reach the pelvic fin base; and, their pelvic fins are pointed. Their head is large, laterally compressed and blunt with a large terminal mouth equipped with incisor-like teeth on the jaws. The two types differ in skin color and the Blind Form has enhanced olfactory sense and the ability to store four times more fat making them capability of dealing with irregular food supplies. They are covered with scales.
The Mexican Tetra are found in schools of several hundred to thousands of individuals suspended above rock and sandy substrate within pools and backwaters of creeks and rivers in water temperatures between 20oC (68oF) and 25oC (77oF). They are known to make seasonal migrations during cold water episodes to warmer waters. They reach a maximum of 12 cm (4.7 inches) in length. They are an opportunistic omnivore that feed on small fish, plants including algae, detritus, aquatic and terrestrial insects and a variety of crustaceans. They have life spans in excess of two years.
The Mexican Tetra are native to the freshwater systems of northern and central Mexico in the States of Chihuahua, Coahuila, Durango, Nuevo León, San Luis Potosí, and Tamaulipas.
The Mexican Tetra is a straightforward identification that can be separated from the Carps or Cyprinids, via the presence of an adipose fin, the Catfishes or Ictalurids due to the absence of chin barbels, and the Salmonids which have more than 100 scales on their lateral line.
The Mexican Tetra from a conservation perspective is currently considered to be of Least Concern with stable, widely distributed populations. The Blind Form is a popular aquarium fish being hardy, peaceful and easygoing. They are utilized as a bait fish in some areas. Historically they have been of significant interest to the scientific community as they are easy to maintain in captivity, can interbreed and are an excellent model organism for the study of convergent and parallel evolution, regressive evolution in cave animals, and the genetic basis of regressive traits. They are the most studied global cave organism.