Taxonomy & naming
Astyanax endy was described by Juan Marcos Mirande, Gastón Aguilera, and Mercedes Azpelicueta in 2006 in the journal Zootaxa (1286: 57–68), based on specimens from the upper Río Bermejo basin in northwestern Argentina. Mirande worked through CONICET, Aguilera through the Fundación Miguel Lillo, and Azpelicueta through the Universidad Nacional de La Plata, making this a collaboration among three of Argentina's principal ichthyological institutions.
In 2020, Terán and colleagues produced a comprehensive molecular phylogenetic revision of the sprawling Astyanax species complex across South America, finding that the genus as traditionally delimited was not monophyletic. That study transferred a large number of species — including A. endy — to the resurrected genus Psalidodon, so the currently accepted name in much of the phylogenetic literature is Psalidodon endy (Mirande, Aguilera & Azpelicueta, 2006). Eschmeyer's Catalog of Fishes (the governing authority for valid names) tracks this realignment; the authority remains parenthetical in Psalidodon to record the original description genus. For consistency with the site's family-level treatment and lingering use in databases including FishBase, the species is catalogued here under Astyanax.
Morphology
Astyanax endy is a small, laterally compressed characid reaching approximately 2 in standard length (SL) in males and up to 2.5 in SL in females — typical of the smaller members of the Astyanax/Psalidodon clade and suited to its stream habitat. Like other members of the group it has the characteristic body plan of a generalised characid: a moderately deep, silvery body, a single dorsal fin, an adipose fin, a forked tail, and the minute teeth and terminal mouth of an omnivore.
Females are slightly larger than males, and at sexual maturity they become noticeably deeper-bodied and rounder in the belly region — the most reliable indicator of sex in this group. Detailed meristic and morphometric data (fin-ray counts, scale rows, lateral-line pore counts) were established in the original 2006 description and distinguish A. endy from its congeners in the Bermejo drainage.
Habitat
Astyanax endy is endemic to the upper Río Bermejo basin in the Yungas ecoregion — the subtropical montane forest belt that descends the eastern slope of the Andes in Jujuy and Salta provinces, northwestern Argentina. Its extent of occurrence (EOO) has been calculated at 13,4 mi² and its area of occupancy (AOO) at approximately 1,465 mi² across just four known locations.
Within this range the species inhabits clear-water creeks and rivers with moderate flow — the cool, well-oxygenated headwater streams characteristic of Andean piedmont and foothill zones. The Yungas landscape around it is a forest of diverse tree species grading from cloud forest at higher elevations into subtropical dry forest lower down. Schools of A. endy move through these streams in mid-water and near-bottom zones (it is classed as benthopelagic by FishBase), exploiting the organic inputs and invertebrate communities associated with intact Yungas riparian forest.
Feeding
Astyanax endy has a trophic level of 3.0 recorded in FishBase, placing it in the lower range of carnivory and consistent with the generalised opportunistic omnivory typical of Astyanax and Psalidodon species. The diet of small characids in Andean streams typically includes aquatic invertebrates (insect larvae, microcrustaceans), terrestrial invertebrates that fall into the water, algae, and fine organic detritus.
No dedicated dietary study specific to A. endy appears to be available in the accessible literature, so diet is inferred from the trophic level figure and from what is known of ecologically similar small Astyanax species in comparable Andean piedmont habitats. High resilience is recorded for the species — an attribute that is partly a function of size and generalist feeding strategy, meaning populations can recover relatively quickly when pressure is reduced, if habitat quality is maintained.
Mating
Detailed reproductive behavioural studies on Astyanax endy specifically are not available in the accessible literature. As a member of the Characidae the species is expected to follow the egg-scattering, broadcast-spawning mode typical of the family: adults gather in groups and release eggs and sperm into the water column or over substrates including fine-leaved aquatic plants, with fertilisation occurring externally and no parental involvement thereafter.
Seasonal reproductive cues in Andean stream characids are typically tied to the onset of the rainy season and associated rises in water temperature, flow, and food availability. The Yungas rivers experience a marked seasonality in flow between the dry Andean winter and the summer wet season, and spawning in A. endy is presumed to peak during or just after the seasonal rise in water level, when dispersal opportunities and food resources for larvae are greatest.
Breeding
Astyanax endy is not reported in the aquarium hobby literature, and no captive breeding accounts are available. Based on the biology of closely related Astyanax and Psalidodon species that have been bred in captivity, the reproductive mode is expected to be that standard to the family: open-water egg-scattering with adhesive or semi-adhesive eggs settling among fine-leaved vegetation or bottom debris, a short incubation period of a few days at typical stream temperatures, and fry that are initially planktonic before becoming free-swimming and feeding on microorganisms.
Small characids from Andean streams typically need slightly cooler temperatures than lowland Amazonian relatives and may benefit from a seasonal temperature drop to stimulate conditioning and spawning. No parental care is provided; adult fish will prey on eggs and larvae if all are kept together, so separation of eggs or parents would be standard practice in any captive breeding attempt.
In the aquarium
Astyanax endy does not appear to be available in the ornamental fish trade. Its status as a range-restricted, IUCN-listed Vulnerable species from a remote Andean foothill basin in northwestern Argentina, combined with the absence of any established captive-breeding population, means it is effectively unknown to hobbyists.
Should it ever become available through legitimate captive-bred sources, the husbandry requirements would follow those of small Andean stream characids: clean, well-oxygenated water with moderate flow, temperatures in the cooler tropical to subtropical range (around 64–75 °F), a pH near neutral, and a diet of small live and frozen foods supplemented with quality dry food. A species tank or a community of similarly sized, non-aggressive stream fish would be appropriate. Its small size (under 2.5 in) and presumed schooling nature suggest it would be most comfortable and least stressed in a group of six or more individuals.
Conservation
Astyanax endy is assessed as Vulnerable (VU) on the IUCN Red List under criteria B1ab(iii)+2ab(iii); D2, with the assessment conducted on 16 December 2020 by Felipe Alonso and reviewed by J. M. Mirande (one of the species' original describers). The criteria reflect a small EOO (13,4 mi²), a small AOO (1,465 mi²) fragmented across just four locations, and a documented continuing decline in the extent and quality of its habitat.
The identified threats are multiple and largely driven by human activity in the Yungas: agricultural expansion and deforestation that degrade the riparian forest and increase sedimentation, water abstraction for irrigation and human use, and contamination from sewage, industrial effluents, and agrochemicals that degrade water quality in the Bermejo tributary streams. The species occupies 11–20% protected area according to the assessment, and local people occasionally take it as a food or bait fish. Recovery prospects depend on maintaining and restoring intact Yungas riparian forest and controlling agricultural runoff and contamination; the species' high intrinsic resilience (per FishBase) suggests that populations can recover if pressures are reduced, but the small number of known locations leaves limited room for error.