Taxonomy & naming
Rasbora atridorsalis was described by Maurice Kottelat and Xueying Chu in 1988 from material collected in the Mekong basin. The Catalog of Fishes (Eschmeyer, the authority for valid names) recognises the species in the genus Rasbora, and no subsequent revision has moved it. GBIF and FishBase both carry the same authorship: Kottelat & Chu, 1988.
The genus Rasbora has undergone substantial reclassification over the past two decades. Kottelat & Witte (1999) and subsequent workers carved out several satellite genera — Boraras, Trigonopoma, Brevibora, Kottelatia, Rasboroides and Trigonostigma — from what was once a loosely defined Rasbora, and the celestial pearl danio moved into Danio itself. Rasbora atridorsalis was never reassigned through any of these splits and sits firmly in the core genus. No other valid combinations exist in the primary literature for this species.
Morphology
R. atridorsalis reaches a maximum recorded standard length of about 3 in, making it a larger-than-average member of the genus. The body is elongate and laterally compressed, with the silvery-bluish flanks typical of open-water schooling rasboras.
The most distinctive feature is the black midlateral stripe, which is diagnostically tapered: narrow over the anterior flanks and operculum, widening progressively toward the caudal peduncle, and terminating as a conspicuous dark blue blotch on the caudal fin base. This graduated stripe differs from the uniform bands seen in many of the smaller, peat-swamp rasboras and gives the species its epithet. The back is darker, the belly paler and silvery.
Habitat
The species is confirmed from the Mekong basin in two areas: Xishuangbanna Prefecture in Yunnan, southern China, and Laos. It is a freshwater, benthopelagic fish of tropical river systems within that drainage.
The Mekong in Xishuangbanna flows through subtropical hill and valley terrain before entering the lowland Indochinese plain downstream. In this part of the basin, rasboras typically occupy moderately to gently flowing stretches of mainstream tributaries and associated clear-water streams, often with structured banks and submerged vegetation. General care references for the genus suggest neutral to slightly acidic water in the range of 75–81 °F (75–80 °F), with soft to moderate hardness; hobbyist data indicate pH 6.8–7.8 under aquarium conditions, with wild-caught fish often preferring the lower end of that range around pH 5.0–7.0.
Feeding
No dedicated dietary study is available for R. atridorsalis in the wild. Mid-sized Rasbora species of similar build and habitat are generalised omnivores and opportunistic micropredators, taking small invertebrates — aquatic insect larvae, small crustaceans, worms — from the water column and surface, supplemented by plant matter and organic detritus.
In the aquarium, rasboras of this size group accept high-quality dried foods readily. Condition is best maintained with regular offerings of live or frozen invertebrates such as daphnia, bloodworm and mosquito larvae. The species' deep-water benthopelagic tendency suggests it feeds through the water column rather than strictly at the surface.
Mating
Like all core Rasbora species, R. atridorsalis is an egg-scattering, non-guarding spawner. There is no nest construction, no bubble nest, no mouthbrooding and no parental care beyond the act of fertilisation. Spawning is typically triggered by environmental cues — a rise in temperature, addition of soft fresh water, or changes in photoperiod — that mimic seasonal conditions in the Mekong basin.
A receptive male pursues a gravid female actively through open water and among vegetation. When the female is ready she holds position briefly while the pair release eggs and milt simultaneously; the adhesive eggs settle onto plants, fine substrate or leaf litter. Adults give the eggs no further attention and, if not removed from a breeding tank, will consume the spawn.
Breeding
Breeding R. atridorsalis in captivity follows the standard approach used for mid-sized egg-scattering rasboras. A separate spawning tank with a layer of fine-leaved plants, java moss or a mesh above a bare bottom allows eggs to fall beyond the reach of the adults. Hobbyist data drawn from closely related rasbora species suggest conditioning at a pH around 6.0, soft water (GH 3–4), and a temperature near 79 °F (78 °F).
Females that are well-conditioned on varied live and frozen foods fill noticeably with eggs. After spawning the adults should be removed promptly. Eggs are adhesive and typically hatch within two to three days; larvae become free-swimming several days later and accept infusoria and freshly hatched brine shrimp nauplii. Specific data on clutch size and fry development remain sparse in the published record for this species.
In the aquarium
R. atridorsalis is a rarely encountered species in the hobby but is well-suited to a species or community aquarium housing other peaceful mid-sized fishes from the Mekong region. Its size — up to 3 in — means it needs a longer tank than typical nano rasboras; a 35.5 in or larger aquarium provides adequate swimming room for a proper shoal.
Like all schooling rasboras it should be kept in groups of at least seven individuals. Hobbyist experience with the genus suggests a well-planted tank with a dark substrate, gentle filtration, tannin-stained or lightly coloured water, and a temperature of 75–81 °F (75–80 °F) at neutral to mildly acidic pH. Under these conditions the species' metallic flanks and the bold contrasting stripe show at their best. It is an active mid-water swimmer that is unlikely to trouble smaller tankmates.
Conservation
Rasbora atridorsalis is assessed as Least Concern on the IUCN Red List (assessment of 24 February 2012). The evaluation reflects a range confined to the upper Mekong basin — Xishuangbanna in Yunnan and Laos — without evidence of a significant decline that would trigger a threatened category.
The Mekong faces growing pressure from hydropower development, agricultural runoff, sand extraction and commercial fishing across its length, and fish communities in Yunnan and Laos are affected by river modification and habitat loss. The conservation picture for this narrowly distributed rasbora should be revisited as more survey data from its specific range become available; its long-term security is tied to the health of upper Mekong tributaries.