Taxonomy & naming
Astyanax douradilho was described by Vinicius Abilhoa Bertaco in 2014, with the type locality given as the rio Tramandaí system, a coastal drainage of Rio Grande do Sul State, southern Brazil. The valid name, as recorded in Eschmeyer's Catalog of Fishes, is Astyanax douradilho Bertaco, 2014. The species belongs to the large and taxonomically complex genus Astyanax, one of the most speciose characid genera in the Neotropical region.
The original description distinguishes A. douradilho from congeners by a combination of meristic and morphometric characters: 37–39 perforated scales on the lateral line, 3–5 maxillary teeth that are tricuspid, 22–24 branched anal-fin rays, and the absence of a dark mid-lateral stripe running from the humeral region to the caudal peduncle — a feature present in many related species. Head length falls between 26.0 and 29.9% of standard length; the upper jaw measures 43.8–50.6% of head length; and the snout 23.0–28.6% of head length.
Two vertically elongated humeral spots are diagnostic for the species and serve as the most immediately visible field mark. Within the rio Tramandaí and rio Maquiné drainages, no other described Astyanax shares this precise combination of characters.
Morphology
Astyanax douradilho reaches approximately 3.5 in standard length, placing it in the mid-size range for the genus. Like other members of Astyanax it has a moderately compressed, oval body typical of small characids: a blunt snout, a terminal mouth with multicuspid teeth, an adipose fin, and a forked caudal fin.
The most distinctive external characters are the two humeral spots, positioned vertically on the shoulder region behind the operculum. These spots are elongated top-to-bottom rather than round, and their shape and arrangement differ from the single humeral blotch or horizontal stripe found in closely related species. The lateral line carries 37–39 perforated scales. Coloration in life is not extensively documented in the published description, but the species likely shows the pale silver flanks and yellow-toned fins common to many small Astyanax of southern Brazil. Data on live coloration remain sparse.
Habitat
The species is endemic to the rio Maquiné and rio Tramandaí systems, coastal drainages that flow to the Atlantic from the Serra Geral escarpment in Rio Grande do Sul, the southernmost state of Brazil. These are short, clear-water rivers draining rolling hills and remnant Atlantic Forest, quite distinct in character from the large lowland river systems that dominate much of the Neotropical region.
FishBase characterises the habitat as small, clear-water streams up to about 1 metre deep, with stony or rocky substrates and moderate riparian vegetation. The species is classed as benthopelagic, meaning it occupies the water column close to the bottom rather than being strictly demersal. Reported water parameters centre on a pH of approximately 6.5, consistent with the soft, slightly acidic water typical of clear Atlantic Forest streams in this part of Brazil. Temperature data for the specific streams are not extensively published, but Rio Grande do Sul's coastal drainages experience subtropical seasonal temperature variation.
Feeding
No detailed dietary studies of Astyanax douradilho have been published. Members of the genus Astyanax are broadly omnivorous, feeding opportunistically on small invertebrates, insect larvae, plant material, periphyton, and detritus according to what is seasonally and locally available. Small coastal-stream Astyanax in southern Brazil commonly ingest terrestrial insects that fall onto the water surface, as well as aquatic invertebrates associated with rocky substrates.
In the absence of species-specific data, aquarium care can follow the general Astyanax template: the fish are unfussy and accept a wide range of prepared foods, frozen invertebrates, and live foods. A varied diet that includes some live or frozen items — Daphnia, brine shrimp, bloodworm — keeps fish in good condition.
Mating
No species-specific mating behaviour has been documented for Astyanax douradilho in the literature. The genus Astyanax follows the typical small-characid reproductive strategy: egg-scattering open spawning with no parental care. Males are generally smaller and slimmer than ripe females; sexual dimorphism in this species has not been formally characterised beyond that pattern.
In related Astyanax species, males actively pursue females in the water column and spawning is triggered by rises in temperature or rainfall that mimic seasonal flood pulses in the wild. Eggs are scattered among vegetation or over the substrate and receive no parental attention. Whether A. douradilho shows any courtship-specific behaviour is not on record.
Breeding
Astyanax douradilho has not been reported in the aquarium literature as a regularly bred species, and no captive-breeding accounts have been traced in the available cache. Based on genus-level knowledge, spawning would follow the standard Astyanax pattern: adhesive eggs scattered among fine-leaved plants or over open substrate, with no parental care and a tendency for adults to consume eggs if not removed.
Incubation times for Astyanax eggs at subtropical temperatures typically range from one to three days, with larvae becoming free-swimming within a week of hatching. First foods would be infusoria or commercial fry food, followed by freshly hatched brine shrimp. Data on clutch sizes, fecundity, and any particular conditioning requirements specific to this species are not available.
In the aquarium
Astyanax douradilho is not an established aquarium fish and is unlikely to be encountered in the trade. Its small size — around 3.5 in SL — and clear-water stream origins suggest it would do well in a biotope or community aquarium set up to reflect the Atlantic Forest streams of southern Brazil: cool to subtropical temperatures, slightly acidic and soft water, moderate flow, a dark substrate of fine gravel, and some leaf litter or root tangle.
The species is probably peaceful enough to mix with other small characids and non-predatory bottom dwellers of similar origin. Being an active schooling characid, a group of six or more would be appropriate. The absence of hobby accounts means that care requirements should be approached cautiously, following the general guidance for small, clear-water Astyanax from southern South America rather than assuming the hardiness of tropical species. Data on temperature tolerance and long-term health in captivity are not available.
Conservation
Astyanax douradilho is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated November 2018. The Least Concern designation reflects the absence of evidence that the population is in decline at a rate that would qualify it for a threatened category, despite the species' restricted range within the coastal drainages of Rio Grande do Sul.
The rio Tramandaí and rio Maquiné systems are Atlantic Forest streams that face ongoing pressure from agricultural land-use change, deforestation of riparian corridors, and sedimentation. The Atlantic Forest biome is one of the world's most threatened biodiversity hotspots, with the great majority of its original cover lost. Endemics confined to small coastal drainages in this region are inherently vulnerable to habitat degradation even when not currently listed as threatened. No specific conservation measures targeting this species are on record.