Taxonomy & naming
Astyanax acatlanensis was formally described by Juan Jacobo Schmitter-Soto in 2017 within a large revisionary work covering Astyanax (Characiformes: Characidae) across Central and North America, published in the Journal of Natural History. That revision erected nine new species and substantially reorganised relationships within the genus across the region; A. acatlanensis was one of the new taxa, characterised by a combination of meristic counts (dorsal soft rays 10; anal soft rays 21–24; vertebrae 32–33) and its restricted type locality in the Río Acatlán drainage, Puebla, Mexico.
Astyanax is one of the most species-rich genera in Characidae, ranging from the southern United States deep into South America, with a particularly diverse array of lineages in Mexico and Central America. Within that sprawling genus, A. acatlanensis occupies a peripheral corner of highland drainage in the Balsas–Papaloapan watershed. The valid name as registered in the Catalog of Fishes is Astyanax acatlanensis Schmitter-Soto, 2017, and the species has not been placed in synonymy or reclassified since its description.
Morphology
Astyanax acatlanensis is a slender, fusiform tetra reaching a maximum recorded standard length of 3 in, which places it in the mid-size range for North American Astyanax. The body plan is typical for the genus: a compressed, torpedo-shaped trunk, a forked caudal fin, a single dorsal fin, and the Characidae hallmark of a small adipose fin behind the dorsal. Meristic data from the original description give dorsal soft rays 10 and anal soft rays 21–24, the latter a useful character in distinguishing it from sympatric or geographically proximate congeners.
Like most Astyanax, the species is silvery overall with a prominent dark lateral stripe running from behind the operculum to the caudal peduncle, often terminating in a caudal spot. Coloration is otherwise unremarkable — no striking pigment patches or iridescent banding — and the species has not attracted selective breeding or line improvement in the aquarium trade, so wild-type appearance is all that is documented.
Habitat
The species is known from two river systems in highland southern Mexico: the Río Acatlán (type locality) in the state of Puebla, and the Río Jía in Oaxaca. Both belong to small drainages within the broader Balsas–Papaloapan basin. The IUCN estimates the extent of occurrence at 249–373 mi², underlining how geographically circumscribed the known range is.
Field data gathered at the time of description note that A. acatlanensis occupies stream reaches with rocky-gravelly substrates and flows cooler than those tolerated by most other Astyanax encountered at lower elevations in Mexico. This cooler, faster-flowing microhabitat may partially explain why the species went undetected as a distinct taxon until the 2017 revision — it occupies a niche that differs meaningfully from the warm, lentic or sluggish habitats frequented by many lowland congeners.
Feeding
No dedicated dietary study of Astyanax acatlanensis has been published. Like the broader Astyanax complex, it is presumed to be an opportunistic omnivore, taking small aquatic invertebrates, insect larvae, terrestrial insects fallen to the surface, and plant or algal material.
In the aquarium, Astyanax species of comparable size generally accept a wide range of prepared foods — flake, micro-pellets, and frozen or live invertebrates such as Daphnia and brine shrimp. A varied diet supports good condition and colour. Given the species' highland stream origins and reported cooler habitat, it may prefer somewhat cooler water temperatures than lowland Astyanax, and care should be taken not to overheat aquarium specimens.
Mating
Courtship behaviour in A. acatlanensis has not been formally documented. Astyanax species in general are open-water egg scatterers. Males pursue females through the water column, and spawning is triggered by the female broadcasting eggs while the male fertilises them in open water or among fine-leaved vegetation. There is no territory defence and no pair bond.
In the aquarium, breeding is typically initiated by a water change that slightly lowers temperature or raises the pH, mimicking a rain event, followed by active chasing and pursuit by males. Spawning mops or fine-leaved plants provide attachment points for semi-adhesive eggs. Because the adults show no restraint in consuming eggs, breeders routinely remove eggs or spawning mops to a separate vessel immediately after spawning.
Breeding
Egg-scattering open spawning is the reproductive mode for Astyanax, and A. acatlanensis almost certainly follows this pattern. Eggs are small, semi-adhesive, and laid among fine-leaved vegetation or substrate crevices rather than attached firmly as in some plant spawners. No parental care is provided; adults will eat their own eggs given the opportunity.
Incubation times and fry development data specific to this species are not available in the published literature. By analogy with other small Astyanax, eggs likely hatch within 24–48 hours at moderate temperatures, with the yolk-sac larvae becoming free-swimming within a few days. First foods for fry would typically be infusoria or very small live foods, followed by newly hatched brine shrimp nauplii as the fry grow. The species has not been established in the hobby in meaningful numbers, so captive breeding experience is essentially undocumented.
In the aquarium
Astyanax acatlanensis is not an established aquarium fish and is rarely, if ever, available in the trade. Its small size (to 3 in SL), peaceful schooling temperament, and adaptation to cool highland stream water make it theoretically suitable for a species tank or a temperate-leaning community, but its absence from the hobby means that husbandry data are almost entirely inferred from related Astyanax species.
If kept, conditions replicating the cool, well-oxygenated highland stream habitat seem appropriate: moderate current, good filtration, rocky or gravelly substrate, and water on the cooler end of the tropical range or even unheated in temperate rooms. The species should be maintained as a group rather than in pairs or trios, consistent with the shoaling behaviour typical of Astyanax. Any specimens entering the trade should be confirmed as captive-bred given the species' Data Deficient status and restricted wild range.
Conservation
The IUCN Red List assessed Astyanax acatlanensis as Data Deficient (DD) in 2018 (published 2019). The DD designation reflects that the species was formally described only in 2017 and that no systematic population surveys have been conducted: scientists lack the information needed to make a reliable assessment of extinction risk. The known range — two small river systems in highland Puebla and Oaxaca — is itself a warning sign, as geographically restricted Mexican freshwater fishes are disproportionately threatened by water extraction, agricultural run-off, and stream degradation.
A listing in a broader IUCN assessment of Mexican freshwater fishes also records the species as Data Deficient, consistent with the primary assessment. Until dedicated fieldwork establishes population size, trends, and the extent of any habitat degradation, the species remains a data gap in the conservation record of highland Mexican ichthyofauna. Its small extent of occurrence (249–373 mi² EOO) would qualify it for threatened status under certain scenarios if population data were available.