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)
Astyanax mexicanus :
ANSP 162587 , 2 ex., C&S, 39.4- 48.5 mm. USA , Texas , Victoria Co. , Rio Guadalupe .
- Juan Marcos Mirande, Gastón Aguilera, María de las Mercedes Azpelicueta (2007): A new species of Astyanax (Characiformes: Characidae) from the endorheic Río Salí basin, Tucumán, northwestern Argentina. Zootaxa 1646, 31-39: 32-32, URL:http://www.zoobank.org/urn:lsid:zoobank.org:pub:D83848ED-9E15-4C2C-A228-0288B2CAFBB9
occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: (200,000-2,500,000 square km (about 80,000-1,000,000 square miles)) This fish is native to eastern and central Mexico and the lower Rio Grande, Pecos, and Nueces river drainages in Texas and New Mexico; it may also occur in Guatemala and Belize, but records from these areas are disputed (Miller 2005, Page and Burr 2011). It is now established in stream on the Edwards Plateau in Texas (Page and Burr 2011) and has been collected elsewhere in the southwestern and south-central United States; the species' range has expanded through use of it as bait. Formerly, in the northern part of the range (e.g., New Mexico), it probably colonized streams during high water, then was decimated by cold winters; dams now limit or prevent this pattern (Sublette et al. 1990).
Length: 10 cm
ANSP 162587 , 2 ex., C&S, 39.4- 48.5 mm. USA , Texas , Victoria Co. , Rio Guadalupe .
Catalog Number: USNM 869
Collection: Smithsonian Institution, National Museum of Natural History, Department of Vertebrate Zoology, Division of Fishes
Collector(s): J. Graham
Locality: Rio Nueces, Texas., Zavala County, Texas, United States, North America
- Syntype: Baird, S. F. & Girard, C. F. 1854. Proceedings of the Academy of Natural Sciences of Philadelphia. 7: 27.
Habitat and Ecology
Habitat Type: Freshwater
Comments: This fish occurs in various stream and river habitats; it is most abundant in rock- and sand-bottomed pools and backwaters; also in caves in Mexico (if those populations are conspecific). In Mexico, this fish occurs in river pools and fairly shallow backwaters in moderate current (Miller 2005). Habitats in Arizona include swift rapids, eddies, and pools. In New Mexico, it occurs primarily in habitats with stenothermal flows (springs) (Sublette et al. 1990); young have been observed in shallow water near overhanging bank vegetation.
Non-Migrant: No. All populations of this species make significant seasonal migrations.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Makes seasonal migrations to escape low winter water temperatures in Texas.
Comments: Usually highly piscivorous. In northeastern Mexico, reported as omnivorous, eating higher plant remains, filamentous algae, and aquatic insects. (Sublette et al. 1990).
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: 81 - 300
Comments: This species is represented by a large number of occurrences (subpopulations).
Comments: Total adult population size is unknown but likely exceeds 1,000,000. Locally, this species is very abundant (Miller 2005).
Travels singly or in moderate size schools.
Life History and Behavior
Spawns in late spring and early summer (Sublette et al. 1990).
Molecular Biology and Genetics
Barcode data: Astyanax mexicanus
Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.
See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
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Download FASTA File
Statistics of barcoding coverage: Astyanax mexicanus
Public Records: 43
Specimens with Barcodes: 62
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
National NatureServe Conservation Status
Rounded National Status Rank: N5 - Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Reasons: Large range in streams in Mexico, Texas, and New Mexico; abundant in many areas; no major threats.
Total adult population size is unknown but likely exceeds 1,000,000. Locally, this species is very abundant (Miller 2005).
Trend over the past 10 years or three generation is uncertain but probably relatively stable.
Global Short Term Trend: Relatively stable (=10% change)
Comments: Trend over the past 10 years or three generation is uncertain but probably relatively stable.
Comments: No major threats are known in most of the range. In New Mexico (where rare), this fish is apparently extirpated from the Rio Grande drainage; probably it was negatively affected by habitat degradation caused by overgrazing, siltation, channelization, and water diversions; possibly also affected by severely cold winters of the 1960s (Sublette et al. 1990).
Relevance to Humans and Ecosystems
The Mexican tetra or blind cave fish (Astyanax mexicanus) is a freshwater fish of the family Characidae of the order Characiformes.  The type species of its genus, it is native to the Nearctic ecozone, originating in the lower Rio Grande and the Neueces and Pecos Rivers in Texas, as well as the central and eastern parts of Mexico.
Growing to a maximum overall length of 12 cm (4.7 in), the Mexican tetra is of typical characin shape, with unremarkable, drab coloration. Its blind cave form, however, is notable for having no eyes and being albino, that is, completely devoid of pigmentation; it has a pinkish-white color to its body.
This fish, especially the blind variant, is reasonably popular among aquarists.
A. mexicanus is a peaceful species that spends most of its time in midlevel water above the rocky and sandy bottoms of pools and backwaters of creeks and rivers of its native environment. Coming from a subtropical climate, it prefers water with 6.0–7.8 pH, a hardness of up to 30 dGH, and a temperature range of 20 to 25°C (68 to 77°F). In the winter, it migrates to warmer waters. Its natural diet consists of crustaceans, insects, and annelids, although in captivity it is omnivorous.
Blind cave form
A. mexicanus is famous for its blind cave form, which is known by such names as blind cave tetra, blind tetra, and blind cavefish. These forms have lost their sight and even their eyes. These fish can still, however, find their way around by means of their lateral lines, which are highly sensitive to fluctuating water pressure. Currently, 29 cave populations are known, dispersed over three geographically distinct areas in a karst region of northeastern Mexico. Recent studies suggest at least two distinct genetic lineages occur among the blind populations, and the current distribution of populations arose by at least five independent invasions.
The eyed and eyeless forms of A. mexicanus, being members of the same species, are closely related and can interbreed making this species an excellent model organism for examining convergent and parallel evolution, regressive evolution in cave animals, and the genetic basis of regressive traits.
Astyanax jordani, another blind cave fish, is sometimes confused with the cave form of A. mexicanus.
The surface and cave forms of the Mexican tetra have proven powerful subjects for scientists studying evolution. When the surface-dwelling ancestors of current cave populations entered the subterranean environment, the change in ecological conditions rendered their phenotype—which included many biological functions dependent on the presence of light—subject to natural selection and genetic drift. One of the most striking changes to evolve was the loss of eyes. This is referred to as a "regressive trait" because the surface fish that originally colonized caves possessed eyes. In addition to regressive traits, cave forms evolved "constructive traits". In contrast to regressive traits, the purpose or benefit of constructive traits is generally accepted. Active research focuses on the mechanisms driving the evolution of regressive traits, such as the loss of eyes, in A. mexicanus. Recent studies have produced evidence that the mechanism may be direct selection, or indirect selection through antagonistic pleiotropy, rather than genetic drift and neutral mutation, the traditionally favored hypothesis for regressive evolution.
The blind form of the Mexican tetra is different from the surface-dwelling form in a number of ways, including having unpigmented skin, having a better olfactory sense by having taste buds all over its head, and by being able to store four times more energy as fat, allowing it to deal with irregular food supplies more effectively.
Darwin said of sightless fish:
By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have affected other changes, such as an increase in the length of antennae or palpi, as compensation for blindness.—Charles Darwin, Origin of Species (1859)
Modern genetics has made clear that the lack of use does not, in itself, necessitate a feature's disappearance.  In this context, the positive genetic benefits have to be considered, i.e., what advantages are obtained by cave-dwelling tetras by losing their eyes? Possible explanations include:
- Not developing eyes allows the individual more energy for growth and reproduction.
- There remains less chance of accidental damage and infection, since the previously useless and exposed organ is sealed with a flap of protective skin.
- The lack of eyes disables the body clock, which is controlled by periods of light and dark, conserving energy.
Another likely explanation for the loss of its eyes is that of selective neutrality and genetic drift; in the dark environment of the cave, the eyes are neither advantageous nor disadvantageous and thus any genetic factors that might impair the eyes (or their development) can take hold with no consequence on the individual or species. Because there is no selection pressure for sight in this environment, any number of genetic abnormalities that give rise to the damage or loss of eyes could proliferate among the population with no effect on the fitness of the population.
Among some creationists, the cave tetra is seen as evidence 'against' evolution. One argument claims this is an instance of "devolution"—showing an evolutionary trend of decreasing complexity. But evolution is a nondirectional process, and while increased complexity is a common effect, there is no reason why evolution cannot tend towards simplicity if that makes an organism better suited to its environment.
Research by MIT biology professor Susan Lindquist shows that inhibition of the HSP90 protein has a dramatic effect in the development of the blind tetra. This research is seen by creationists as evidence of "built-in adaptability, not slow and gradual evolution."
In the aquarium
The blind cave tetra is a fairly hardy species. Their lack of sight does not hinder their ability to get food. They prefer subdued lighting with a rocky substrate, like gravel, mimicking their natural environment. They become semi-aggressive as they age, and are by nature schooling fish.
- Froese, Rainer and Pauly, Daniel, eds. (2006). "Astyanax mexicanus" in FishBase. March 2006 version.
- "Astyanax mexicanus". Integrated Taxonomic Information System. Retrieved 1 July 2006.
- Yoshizawa, M.; Yamamoto, Y.; O'Quin, K. E.; Jeffery, W. R. (Dec 2012). "Evolution of an adaptive behavior and its sensory receptors promotes eye regression in blind cavefish". BMC Biology 10: 108. doi:10.1186/1741-7007-10-108.
- Gross, J.B. (Jun 2012). "The complex origin of Astyanax cavefish". BMC Evolutionary Biology 12: 105. doi:10.1186/1471-2148-12-105.
- Retaux, S.; Casane, D. (Sep 2013). "Evolution of eye development in the darkness of caves: adaptation, drift, or both?". Evodevo 4: 26. doi:10.1186/2041-9139-4-26.
- Soares, D.; Niemiller, M. L. (Apr 2013). "Sensory Adaptations of Fishes to Subterranean Environments". Bioscience 63 (4): 274–283. doi:10.1525/bio.2013.63.4.7.
- Wilkens, H (Nov 2012). "Genes, modules and the evolution of cave fish". Heredity 105 (5): 413–422. doi:10.1038/hdy.2009.184.
- Protas, M; Tabansky, I.; Conrad, M.; Gross, J. B.; Vidal, O.; Tabin, C. J.; Borowsky, R. (Mar–Apr 2008). "Multi-trait evolution in a cave fish, Astyanax mexicanus". Evolution & Development 10 (2): 196–209. doi:10.1111/j.1525-142x.2008.00227.x.
- Jeffery, WR (2009). "Regressive Evolution in Astyanax Cavefish". Annual Review of Genetics 43: 25–47. doi:10.1146/annurev-genet-102108-134216.
- Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 315, 1997, ISBN 0-86542-256-7
- Dawkins, R.: Climbing Mount Improbable, W. W. Norton & Co, 1997, ISBN 0-393-31682-3
Names and Taxonomy
Comments: Formerly included as a subspecies of A. fasciatus. Includes eyed surface fishes with relatively high genetic variability and eyeless cave forms with relatively low genetic variability; in at least one cave the two forms interbreed (see Nelson 1984); there is some question as to whether cave and noncave populations are conspecific, though available evidence suggests that they are (see Lee et al. 1980). Removed from Cypriniformes and placed in Characiformes by Nelson (1984) and in the 1991 AFS checklist (Robins et al. 1991).