Taxonomy & naming
Astyanax unitaeniatus was described by Garutti in 1998 from material collected at the Ribeirão Macambira, municipality of Iaciara, Goiás, Brazil (approximately 14°08'S, 46°37'W). The holotype is MZUSP 40542, a specimen of 2 in standard length, collected as part of a type series of 45 specimens. The valid name is Astyanax unitaeniatus Garutti, 1998, as recorded in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong), the governing authority for nomenclatural questions in this group.
Astyanax is one of the most species-rich genera in the family Characidae and has long presented a taxonomic challenge: many species are superficially similar, meristic characters overlap broadly, and the genus as historically constituted is almost certainly not monophyletic. Ongoing revisionary work continues to shift species in and out of Astyanax. Until a formal revision addresses unitaeniatus specifically, it is retained in Astyanax on the basis of available evidence. Its distribution in the upper Tocantins basin places it in the central Brazilian cerrado drainage system, a region that has yielded a number of geographically restricted Astyanax species.
Morphology
Astyanax unitaeniatus is a small, elongate characin. The original description records the holotype at 2 in standard length, with the type series spanning approximately 1.5–3 in SL. Like other Astyanax, the body is moderately compressed and fusiform — built for open-water schooling rather than bottom-hugging or hovering among vegetation. The lateral line carries 41–45 scales, and the anal fin has 20–25 rays, meristic features documented in the original description.
The defining feature, and the source of the species name, is the single continuous black lateral stripe. In Astyanax unitaeniatus this band is bold and unbroken, running along the flank from the posterior edge of the operculum to the base of the caudal fin, where it typically expands into a dark caudal spot. The ground colour is silvery, and the dorsal surface is darker. This striped pattern sets it apart from many congeners, which show only a caudal spot or a more diffuse or shorter lateral marking.
Habitat
Astyanax unitaeniatus is known from the upper Tocantins River drainage in the state of Goiás, Brazil, in the cerrado (tropical savanna) biome. The Ribeirão Macambira type locality is a tributary of the broader upper Tocantins system, a river that drains the central Brazilian plateau before flowing north into the Amazon basin via the Araguaia confluence.
Streams and rivers of the cerrado are characterised by seasonal rainfall patterns, often with pronounced wet and dry seasons. During the dry season, many smaller tributaries contract and clear; during the wet season, floodplains expand and connectivity between drainages increases. Astyanax species in such systems are typically found in open water and at the margins of streams, schooling in the water column. FishBase classifies A. unitaeniatus as freshwater, benthopelagic, and tropical in its habitat requirements.
Feeding
Like most small Astyanax, Astyanax unitaeniatus is presumed to be an opportunistic omnivore. The genus as a whole is well studied in terms of feeding ecology: species typically consume small invertebrates (insect larvae, microcrustaceans, and terrestrial insects that fall onto the water surface), algae, plant matter, and organic detritus, with the relative proportions varying by season, habitat, and availability.
No dedicated dietary study specific to A. unitaeniatus has been located in the available literature. In the aquarium, small Astyanax species are undemanding feeders that accept high-quality flake and micropellet foods readily, and benefit from supplementary live or frozen foods such as Daphnia, cyclops, and brine shrimp nauplii to maintain condition and promote natural foraging behaviour.
Mating
Astyanax species are egg-scattering open spawners with no parental care, and A. unitaeniatus is expected to follow this pattern. In wild Astyanax populations, spawning is often associated with the onset of the wet season, when rising water temperatures, increasing photoperiod, and expanded habitat trigger reproductive behaviour. Schools congregate and pairs or small groups scatter adhesive eggs among fine-leaved aquatic vegetation or over sandy substrates.
No direct observation of mating behaviour specific to A. unitaeniatus has been documented in the literature reviewed. Sexual dimorphism in Astyanax is typically subtle; females in breeding condition are usually deeper-bodied and more rounded in ventral profile than males of equivalent length, a trait common across the genus. In aquarium settings, mature individuals of both sexes can often be distinguished by abdominal fullness in females.
Breeding
Breeding biology specific to Astyanax unitaeniatus has not been formally described, but the genus exhibits a well-established and consistent reproductive strategy that can guide husbandry. Astyanax are egg-scatterers that deposit small, adhesive eggs among fine-leaved plants or spawning mops, with no subsequent parental care; adults will consume both eggs and newly hatched fry if not separated.
For aquarists wishing to attempt breeding, conditioning adults on a varied diet of live and frozen foods and providing a spawning tank with clumps of fine-leaved plants or synthetic mops is the recommended approach. Eggs should be removed promptly after spawning or the adults removed. Incubation times and fry size are consistent with other small Astyanax — hatching within 24–48 hours at tropical temperatures, with fry initially feeding on infusoria or commercial fry foods before accepting newly hatched brine shrimp. The species is not known to be in regular production in the hobby.
In the aquarium
Astyanax unitaeniatus is not a commonly traded species in the aquarium hobby; it lacks the widespread availability of the more familiar Astyanax such as the blind cave tetra (A. mexicanus) or the red-eye tetra (A. fasciatus complex). Specimens occasionally appear as bycatch in shipments of wild-caught South American tetras, particularly from the central Brazilian interior.
Care requirements for this species should be consistent with other small, schooling Astyanax from the central Brazilian cerrado: a well-filtered aquarium of at least 23.5–31.5 in length, soft to moderately hard neutral-to-slightly-acidic water, tropical temperatures in the range of 72–82 °F, and a school of at least six individuals to allow natural social behaviour and reduce stress. The striking lateral stripe is best appreciated in a well-lit tank with a dark substrate, where the bold black band contrasts clearly against the silvery body. Suitable tankmates include similarly sized, peaceful tetras, small rasboras, and calm catfishes.
Conservation
The conservation status of Astyanax unitaeniatus has not been formally assessed by the IUCN Red List as of the time of writing; FishBase reports the species as Least Concern, though a dedicated IUCN assessment with a published assessment date could not be verified from available sources. The species is endemic to the upper Tocantins drainage in Goiás, Brazil.
The cerrado biome, where A. unitaeniatus is found, is one of the world's most threatened tropical savanna ecosystems. Agricultural expansion — particularly soy and cattle — has caused extensive deforestation and riparian degradation in the region, with associated impacts on stream water quality, flow regime, and aquatic biodiversity. The construction of dams and diversions in the Tocantins basin has altered hydrological connectivity and habitat availability for river-dependent fishes. How these pressures specifically affect A. unitaeniatus is not well documented, but the broader context of cerrado fish conservation warrants attention for any narrowly distributed endemic species in the basin.