Taxonomy & naming
Astyanax vermilion was described by Ana Maria Zanata and Priscila Camelier in 2009, alongside a second new species, Astyanax burgerai, from the same coastal drainages of northeastern Bahia, Brazil. The original description was published in Neotropical Ichthyology and diagnoses the species within the large, heterogeneous genus Astyanax (Baird & Girard, 1854) on the basis of a combination of fin-ray counts, scale counts, and the distinctive reddish coloration of the posteroventral body and fins in preserved males. The type locality lies within the rio Salgado–rio Almada system.
A 2024 phylogenomic revision by Melo and colleagues proposed removing several Atlantic Forest Astyanax from that genus and placing them in Psalidodon, transferring this species to Psalidodon vermilion within a reinstated Acestrorhamphidae. This reclassification is noted in FishBase and on the Etyfish name-etymology database. Eschmeyer's Catalog of Fishes remains the governing authority for valid names used on this site; readers should be aware that both name combinations appear in the current literature.
Morphology
Astyanax vermilion is a small, slender characid reaching a maximum of 1.5 in standard length in males. Meristic counts include 10–11 dorsal soft rays, 23–26 anal rays, and 33 vertebrae — counts consistent with allied Atlantic Forest Astyanax but combined with diagnostic colour and fin characters. The species is distinguished from congeners by dark distal coloration on the pelvic fins, an inconspicuous humeral blotch, an inconspicuous caudal spot, and — most strikingly — a reddish-orange suffusion of the posterior body and all paired fins in living males.
Females lack the red coloration, providing clear external sexual dimorphism. Body form is typical of the genus: a compressed, fusiform profile, a terminal mouth, and the characteristic adipose fin present between dorsal and caudal. The species is benthopelagic in habit, occupying the lower to mid-water column in shallow streams.
Habitat
The species is known exclusively from the rio Salgado and rio Almada drainages of northeastern Bahia state, Brazil — a narrow endemic range within the Atlantic Forest biome. Within these coastal river systems it occupies clear-water streams at elevations of 617–846 ft, where current is slow to moderate and depth rarely exceeds 3 ft. Substrate is characteristically rocky, pebbly, or sandy.
The Atlantic Forest streams of coastal Bahia are seasonally variable, and riparian vegetation provides a key input of allochthonous material — terrestrial insects, plant fragments, and seeds — that shapes the diet of small characids like this species. Research in the Cachoeira River basin, within the same regional fauna, has documented how riparian cover directly governs the trophic composition of similar Psalidodon populations, implying that intact gallery forest is integral to the habitat quality this species requires.
Feeding
Astyanax vermilion is omnivorous, with a diet drawn from both aquatic and terrestrial sources. FishBase records filamentous algae, plant fragments, seeds, insects, arthropods, and organic debris among its gut contents — a breadth typical of Atlantic Forest stream characids that exploit both benthic resources and riparian input.
Peer-reviewed research on the same species in the Cachoeira River basin confirms that feeding ecology is tightly linked to the degree of riparian vegetation coverage: in reaches with intact forest, allochthonous material (insects and plant matter falling from overhanging vegetation) forms a larger proportion of the diet, while in degraded reaches the fish shift toward aquatic algae and benthic detritus. This dietary flexibility likely reflects both the species' opportunism and the fragmented, seasonally fluctuating nature of Atlantic Forest stream resources.
Mating
No detailed field studies of mating behaviour have been published for Astyanax vermilion specifically. Based on its membership in Astyanax and allied Atlantic Forest characids, spawning is assumed to follow the open egg-scattering pattern typical of the group: sexually mature adults congregate and release adhesive eggs among fine-leaved aquatic plants or over suitable benthic substrate without forming lasting pair bonds or territorial arrangements.
Sexual dimorphism — males with vivid red-orange posterior coloration, females without — suggests that colour plays a role in mate assessment, as in other characids where males display breeding coloration to females. The extent to which riparian seasonality or hydrology cues spawning in this species' narrow upland stream habitat has not been reported.
Breeding
Detailed captive-breeding records for Astyanax vermilion are not documented in the available literature. Extrapolating from closely related Atlantic Forest Astyanax and the general characid pattern, the species almost certainly spawns by scattering small, adhesive eggs among fine-leaved plants or submerged vegetation, with no parental care — adults should be considered egg predators and separated from spawning media.
Water temperature in the natural range (upland streams, 617–846 ft elevation in coastal Bahia) is likely modestly cooler than lowland tropical norms, though precise thermal data for the breeding season are not available from the cache. Any aquarist attempting breeding should replicate the clear, well-oxygenated, low-to-moderate-current conditions of upland Atlantic Forest streams, with soft to moderately hard, slightly acid to neutral water and a temperature in the low to mid-20s Celsius.
In the aquarium
Astyanax vermilion is a rare fish in the ornamental trade and is not commonly available outside specialist circles or Brazilian domestic supply. Its small adult size (under 1.5 in SL) makes it suitable for modestly sized aquaria, and its clear-water, upland-stream origins call for well-oxygenated, clean water with gentle to moderate current. Soft to neutral water chemistry reflecting the Atlantic Forest coastal drainages would be appropriate.
As a schooling characid it should be kept in groups of six or more. The vivid red-orange male coloration is expressed in life but can fade under poor conditions or in preserved specimens, suggesting that diet quality, lighting, and stress levels will influence display. Tankmates should be small, peaceful species compatible with cool, oxygen-rich Atlantic Forest stream conditions. Given the species' restricted wild range and the ecological sensitivity of its habitat, sourcing from captive-bred stock is preferable to any wild collection.
Conservation
Astyanax vermilion is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 7 November 2018. While the status indicates that no immediate extinction risk was identified at that assessment, the species is a narrow endemic confined to two coastal river drainages (rio Salgado and rio Almada) in northeastern Bahia — a region where Atlantic Forest cover has been severely reduced to a fraction of its original extent.
The Atlantic Forest is one of the world's most threatened and species-rich biomes, and the freshwater fish communities of its coastal Bahia drainages are poorly known and unevenly protected. Small-stream endemics dependent on intact riparian vegetation — precisely the condition documented to govern feeding ecology in this species — are structurally vulnerable to land-use change, deforestation, and stream degradation even when current population data do not trigger a threatened listing. The IUCN LC assessment should be read as a snapshot, not a guarantee of long-term security.