Yucatán Tetra

Astyanax altior reaches a maximum length of 5.75 cm (2.26 in) standard length (SL). The body is notably deep for a member of Astyanax, within the range of 37–48% SL; other species inhabiting the Yucatán never have a body depth greater than 42% SL. A. altior has a dark patch on the caudal peduncle, a dark-gray or silvery lateral stripe, and a somewhat diffuse, oval-shaped humeral spot. The body color is most often gray or silver, but orange is not unknown; in populations living alongside congener A. aeneus, individuals of A. altior are a brighter yellow. In the original description of A. altior, its visual similarities to congener A. aeneus were remarked upon. The main differentiating factor between the two at the time was the number of anal-fin rays, which are fewer in A. altior than A. aeneus (22 to 23 vs. 24, respectively). The original description also makes note of its sharp and "unusually high (or long)" fins.

Astyanax altior is endemic to the Yucatán Peninsula of Mexico, specifically to the northernmost third. It has been collected from a variety of habitats, including both freshwater and brackish conditions, but it is most commonly cited from the cenotes therein. It also seems to be abundant in karstic wetland systems. Its type locality, a site near Progreso, has been destroyed. Mangrove swamps are also a part of A. altior's habitat, though it is sensitive to the conditions therein, and mangrove swamps that have deteriorated in general health have smaller populations. Cenotes are natural sinkholes in carbonate rock that fill with water, most often referring specifically to the formations in the Yucatán Peninsula.These are either connected to an aquifer or other body of water (and thus have sufficient water cycling to maintain clear waters), or are isolated (and therefore turbid). A. altior has a preference for the cenotes with clearer waters. The anchialine cenotes located in Tulum are a specific example, but A. altior is generally easy to find within most cenotes in its range.

Astyanax altior is known to be omnivorous. Juveniles are more often planktivorous, and adults lean slightly towards carnivorous tendencies, though specimens will take supplemental plant materials (largely in the form of periphyton). There may be an element of cannibalism in its diet. Astyanax altior forms schools. Schooling behavior is more common in younger fish. It is also known to live syntopically with congener A. aeneus in at least one cenote; this is part of why they are recognized as different valid species. Otherwise, they would likely be considered synonymous due to considerable morphological overlap, but the fact that they remain distinct when in sympatry supports their species status. Reproduction occurs during the rainy season.

The genus name "Astyanax" is an allusion to Astyanax, a Trojan warrior of the same name in Homer's Iliad. While the reasoning therein was not made clear in the original text, this possibly originates in the large, armor-like scales of type species of the genus, A. argentatus, which was formerly considered a synonym of A. mexicanus by taxonomists. The specific name "altior" means "higher" (consider "altitude", which means "height"), and is likely in reference to the fins, which were described as "unusually high" by Hubbs.