Taxonomy & naming
Astyanax troya was described by María de las Mercedes Azpelicueta, Jorge Casciotta, and Adriana Almirón in 2002, in a paper treating two new species of the genus Astyanax from the Paraná river basin in Argentina, published in the Revue suisse de zoologie (volume 109, pages 243–259). The type locality lies within the Paraná drainage of northeastern Argentina. The valid name as recognised by Eschmeyer's Catalog of Fishes is Astyanax troya Azpelicueta, Casciotta & Almirón, 2002.
The genus Astyanax, family Characidae, is one of the most species-rich characid genera in the Neotropics, and its circumscription has long been contentious. Mirande's 2010 molecular and morphological phylogeny of the Characidae (Neotropical Ichthyology 8(3):385–568) recovered A. troya within the subfamily Gymnocharacinae and treated Astyanax sensu lato as non-monophyletic, providing phylogenetic context for subsequent revisions. Some treatments transfer species into the resurrected genus Psalidodon, and FishBase lists the species under Psalidodon troya; the Catalog of Fishes remains the nomenclatural authority followed here.
A. troya is most closely allied to A. leonidas, described in the same paper. The two species share similar anal-fin ray counts, perforated lateral-line scale counts, one maxillary tooth, a deep dentary, and secondary sexual hooks on the fins of males — characters that link them within the group but distinguish them from many other Astyanax.
Morphology
Astyanax troya reaches a maximum recorded standard length of 3 in. The body plan is a moderately deep-bodied, laterally compressed characid typical of the genus. Diagnostic characters from the original description include a markedly concave dorsal profile over the supraoccipital region, a single broad, low pentacuspid maxillary tooth, and premaxillary inner-row teeth that are gently expanded distally with cusps in the same plane and a central cusp only scarcely longer than the others. The dentary carries 8–10 teeth decreasing in size from front to back.
An autapomorphy recovered by Mirande's (2010) phylogenetic analysis is a fourth infraorbital bone that is approximately square or more developed in the longitudinal than in the dorsoventral direction — a character distinguishing it at the bone-level from related forms. Males bear bony hooks on all fins, including the pectoral, pelvic, anal, and caudal fins, as well as a pelvic axillary scale; these hooks are a secondary sexual character widespread in certain characid lineages and are absent in females.
Habitat
Astyanax troya is endemic to the Paraná river basin in northeastern Argentina. FishBase classifies it as a benthopelagic subtropical freshwater species, inhabiting the lower and middle water column of riverine habitats within the Paraná drainage.
The Paraná basin of northeastern Argentina is a lowland system with warm subtropical conditions. Specific physicochemical data for A. troya's microhabitat are not available in the published record, but the broader Paraná basin hosts diverse characid assemblages in river channels, floodplain lakes, and associated tributaries across a range of water types.
Feeding
No detailed dietary study specific to Astyanax troya is available in the current literature. Like other small Astyanax, it is presumed to be an opportunistic omnivore, taking small aquatic invertebrates, insect larvae, algae, and plant material in the wild — a generalist feeding mode that characterises most members of the genus across South American riverine habitats.
In aquarium settings, members of this genus typically accept a wide range of prepared foods including flake and small sinking pellets, and benefit from supplemental offerings of frozen or live foods such as Daphnia and brine shrimp.
Mating
Astyanax troya has not been studied in detail with respect to mating behaviour, but the presence of bony fin hooks on males — documented in the original description on all fins — is consistent with the spawning embrace behaviour recorded in other hook-bearing characids, in which hooks anchor the male to the female during the spawning act. This reproductive morphology is a secondary sexual character fully absent in females.
As with most small characids, open egg-scattering over fine-leaved vegetation or substrate is the presumed mating mode, with no pair bond or territory defended. Spawning in related Astyanax typically peaks in the warmer months of the Paraná basin's seasonal cycle.
Breeding
No captive-breeding account specific to Astyanax troya is on record. By analogy with other small Astyanax from the Paraná system, breeding would involve egg-scattering over fine-leaved plants or substrate with no parental care; eggs are likely adhesive or semi-adhesive, and adults should be separated from eggs to prevent predation after spawning.
Temperatures in the mid-to-upper 20s Celsius and slightly acidic to neutral water would reflect the subtropical Paraná conditions. First foods for fry would follow the standard characid protocol: very fine live or commercial micro-foods until the fry are large enough for newly hatched brine shrimp.
In the aquarium
Astyanax troya is effectively unknown in the mainstream aquarium hobby and is not commercially available; no trade or husbandry literature covers it. As a small, schooling characid from a subtropical South American river basin it would be expected to behave in a manner typical of the broader Astyanax complex: an active, relatively peaceful fish suited to a community of similar-sized species, kept in groups and offered a mixed diet.
Its restricted range and Vulnerable conservation status mean that any specimens encountered would most plausibly be wild-caught, and the ethical weight of that should be considered. There is no established captive population known to exist outside Argentina.
Conservation
Astyanax troya is assessed as Vulnerable (VU, criterion B1ab(iii)) on the IUCN Red List, with the assessment conducted on 23 November 2020 by Jorge Liotta of the Museo de Ciencias Naturales 'A. Scasso'. The qualifying criterion B1 flags a restricted extent of occurrence, with subcriterion ab(iii) indicating continuing decline in area, extent, or quality of habitat.
Endemism to the Paraná drainage of northeastern Argentina is the core vulnerability: the species has nowhere to expand if its local habitat is degraded or fragmented. Pressures on the lower and middle Paraná system include agricultural runoff, wetland drainage, and river engineering associated with one of South America's most intensively used river basins. FishBase notes high population resilience, which offers some buffer, but resilience alone does not offset range restriction when habitat quality is in decline.