Astyanax Mexicanus: Mexican Tetra and Blind Cavefish

Astyanax mexicanus, the Mexican tetra, is a single species that lives as two forms: a silvery, sighted fish in rivers and streams, and pale, eyeless fish in caves. A 2020 review in the Journal of Experimental Biology counts at least 29 blind cave populations in the Sierra de El Abra and Sierra de Guatemala regions of northeastern Mexico; Wikipedia gives about 30. Each cave population is named after its cave, such as Pachón or Molino.

The two forms can interbreed and produce fertile young (Wikipedia), and that is what makes the species so useful to biologists: crossing a cave fish with a river fish shows which genes are behind eye loss, pale skin or short sleep. Wikipedia describes the cave populations as young in evolutionary terms, under 20,000 years old, with at least five separate invasions of caves.

Small. Wikipedia gives about 12 cm (4.7 inches) at maturity for the species, and LiveAquaria lists a maximum of 3.5 inches for the blind cave tetras it sells. Cave fish look larger-bodied than you might expect because they store fat: Wikipedia reports they can hold about four times as much fat as the river form, which helps them survive long gaps between meals in caves.

Cave fish embryos start to grow eyes, then the eyes break down: BrainFacts, the Society for Neuroscience's public site, reports that this happens by about day five, after which skin grows over the small remaining eye. It describes the mechanism as methylation that switches off dozens of eye-development genes, rather than broken genes.

The leading explanation is energy. A 2015 study led by Damian Moran in Science Advances, reported by Science News, found that the eyes and the brain tissue used for vision take about 15 percent of a 1-gram young fish's resting energy budget, falling to about 5 percent in an 8.5-gram fish. In a cave with no light and little food, that is expensive equipment. BrainFacts adds that cave fish gained stronger taste, smell and motion senses in place of sight.

Cave populations have independently evolved to sleep far less. A 2025 study in eLife reports that cave fish sleep as little as 1 to 2 hours a day, while the river form sleeps 6 to 10 hours. Despite elevated signs of DNA damage in the brain, the researchers found no obvious decline in health or lifespan, and both forms are reported to live more than 20 years. The 2020 Journal of Experimental Biology review adds other changes: cave fish are less aggressive, have lost schooling, are attracted to vibrations in the water and find food in the dark better than river fish.

It depends on the form. The Journal of Experimental Biology review states that surface A. mexicanus are highly aggressive, while cave fish from several populations have evolved reduced aggression. Wikipedia says the species schools and becomes semi-aggressive with age, and LiveAquaria rates its blind cave tetras as peaceful.

Blind cave tetras are hardy and find food by taste and their lateral line, so blindness does not stop them feeding (Wikipedia; LiveAquaria). Care numbers from the sources:

• Group: they prefer to live with their own kind; LiveAquaria suggests a 20-gallon tank for a group of 4 to 5 and lists 10 gallons as the minimum
• Water: 68 to 77F and pH 6.5 to 8.0 (LiveAquaria); Wikipedia gives pH 6.5 to 8, hardness up to 30 dGH and 20 to 25C
• Setup: subdued light and rocky substrate (Wikipedia)
• Food: omnivore; live and frozen foods such as bloodworms, brine shrimp, daphnia and mysis are taken (Wikipedia)

All aquarium stock traces back to a 1936 collection from Cueva Chica, and the fish are now bred commercially, especially in Asia (Wikipedia). LiveAquaria listed blind cave tetras at $24.99 when we checked on 25 September 2026. Never release aquarium fish into the wild.

People who search for the Mexican blind cavefish often ask this, and the answer is another blind cave fish: the Alabama cavefish, Speoplatyrhinus poulsoni. Outdoor Alabama, the state wildlife agency, calls it one of the rarest cave-dwelling fish in North America; it is known only from Key Cave in Lauderdale County, is 1.2 to 2.3 inches long, has no eyes or pigment, and is federally listed as endangered. Wikipedia estimates fewer than 100 individuals. It is not related to Astyanax: it belongs to the North American cavefish family, Amblyopsidae. See our cave fish page for more.

Research and drafting for this page were assisted by AI and checked line by line against the sources listed below. Every number names the organisation it comes from, and where sources disagree we say so instead of picking one silently. Prices and rules are given with the date we checked them; they change, so confirm before you rely on them. If you find an error, tell us and we will correct it.