Taxonomy & naming
Rasbora atranus was described by Maurice Kottelat and Heok Hui Tan in 2011 in Ichthyological Exploration of Freshwaters (volume 22, issue 3, pages 215–220). The species is placed in the large and taxonomically complex genus Rasbora, within the sumatrana–elegans species complex — a cluster of similarly patterned mid-sized rasboras distributed across the Sunda shelf.
The genus Rasbora itself has undergone significant revision over the decades. Several genera previously lumped within it — Boraras, Trigonopoma, Brevibora, Kottelatia and Rasboroides among them — were carved out by Kottelat and others from the 1990s onward as molecular and morphological work clarified relationships. R. atranus remains in the core Rasbora assemblage under the Catalog of Fishes (Eschmeyer, CAS), the authority for valid names followed here. No synonyms are recorded for this species.
The specific epithet atranus is Latin for 'black' or 'very dark', in reference to the prominent dark anal-fin blotch and midlateral stripe that distinguish the species within its complex.
Morphology
Rasbora atranus is a moderately elongate, laterally compressed rasbora. The largest known specimen from the type series measures approximately 2.5 in standard length, placing it in the mid-size range for the genus.
The species is distinguished from close relatives in the sumatrana–elegans complex by a combination of colour features: a dark midlateral stripe that runs from the opercle to the base of the caudal fin (often present but sometimes faint), an elongated triangular dark blotch at the end of the caudal peduncle, and most distinctively a conspicuous oval-to-triangular black blotch occupying the anterior half of the anal-fin base. In life the fins are orange, lending the fish a striking two-tone appearance of dark-marked body and warm-coloured fins. No published data on sexual dimorphism specific to this species is available; in Rasbora generally, females are deeper-bodied when gravid.
Habitat
Rasbora atranus is known from the upper Mahakam drainage in Kalimantan Timur (East Kalimantan), central Borneo, Indonesia. The Mahakam is one of the major river systems of eastern Borneo, draining a large catchment through montane and lowland forest before reaching the Makassar Strait. The species was described from the upper portion of this drainage.
Borneo's forested stream habitats typically feature soft, moderately acidic water with low mineral content, particularly in blackwater tributaries draining peat-swamp and heath forest. General conditions for rasboras in this region run soft and acidic — pH broadly in the range of 5.5–7.0 and temperatures in the tropical range of 73–82 °F (73–82 °F). Specific water-parameter data for the type locality of R. atranus have not been published, but conditions consistent with lowland Bornean streams should be assumed.
Feeding
No specific dietary study exists for Rasbora atranus. Like the great majority of small Rasbora species, it is presumed to be an omnivorous micropredator in the wild, taking small aquatic and terrestrial invertebrates — insects and their larvae, small crustaceans, and zooplankton — along with algal films and organic detritus, as opportunity allows.
In an aquarium setting, captive Rasbora species of similar size accept a wide range of prepared foods: high-quality small flakes, micro pellets, and live or frozen items such as daphnia, bloodworm and baby brine shrimp. The latter are particularly valuable for conditioning fish for spawning and for maintaining coloration in the orange fins that characterize this species.
Mating
Rasbora atranus has not been specifically studied in courtship or in captivity, but the reproductive biology of Rasbora as a genus is well established. All members are egg-scattering, non-guarding spawners with no parental care and no nest construction of any kind.
In the genus generally, a gravid female is pursued through vegetation by one or more males. The pair or group come together briefly, eggs and milt are released simultaneously, and the adhesive or weakly adhesive eggs settle onto fine-leaved plants, moss, or substrate. Neither parent takes any further interest in the eggs. Adults in a group will readily eat scattered eggs, so separation is necessary if any spawn are to be reared.
Breeding
Rasbora atranus has not been reported in captivity in the published hobby literature. Breeding, should it be attempted, would follow the pattern standard for mid-sized Rasbora species: a shallow, warm, soft and acidic tank with fine-leaved plants or a mesh spawning substrate, well-conditioned adults fed live and frozen foods, and the adults removed once spawning is observed.
Eggs are adhesive and would be expected to adhere to plant surfaces or fall to the substrate. Incubation typically takes two to three days at tropical temperatures in related species, with larvae becoming free-swimming a few days after hatching and initially taking infusoria or commercially available fry foods before graduating to nauplii. Soft, acidic water mimicking the Bornean lowland stream habitat would be appropriate. The species' rarity in the trade means captive husbandry data are essentially absent.
In the aquarium
Rasbora atranus is essentially unknown in the aquarium hobby. No commercial availability has been recorded, and it is unlikely to appear in the trade given its restricted known range in the upper Mahakam drainage and the general inaccessibility of that region. Should specimens become available, they would best be maintained in a soft, slightly acidic, well-planted aquarium mimicking the shaded, gently flowing streams of Bornean forest.
A temperature of 75–82 °F (75–82 °F), pH of 5.5–7.0, and low to moderate water flow would be appropriate. As with all rasboras, a group of at least six to eight individuals is recommended to bring out natural shoaling behaviour and reduce individual stress. Fine-leaved plants, floating vegetation for diffuse lighting, and a dark substrate would suit the species' presumed natural habitat and showcase its orange finnage.
Conservation
Rasbora atranus is listed as Data Deficient (DD) on the IUCN Red List, assessed in 2019. The Data Deficient rating reflects the near-total absence of field survey data for the species following its original description — it is known from the type locality in the upper Mahakam drainage and little beyond, and no population estimates or trend data exist.
The upper Mahakam region of Kalimantan Timur has experienced significant deforestation, agricultural expansion, and resource extraction pressure over recent decades, pressures that affect stream-dwelling fish through siltation, altered hydrology and chemical pollution. Until dedicated surveys establish the species' actual range, abundance and habitat dependencies, a meaningful threat assessment is not possible. Its restricted known range and the scale of land-use change in central Borneo make precautionary attention warranted.