Taxonomy & naming
Rasbora rubrodorsalis was formally described by Donoso-Büchner and Schmidt in 1997, a relatively late date for a species from the heavily collected lower Mekong. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the valid combination as Rasbora rubrodorsalis Donoso-Büchner & Schmidt, 1997. No authority parentheses are used, as the species has not been moved out of the original genus.
The genus Rasbora has been substantially revised over the past two decades. Kottelat and others have carved out several satellite genera — Boraras, Trigonopoma, Brevibora, Kottelatia and Rasboroides among them — from what was formerly a broadly defined Rasbora. Rasbora rubrodorsalis remains in the core genus under the current treatment. It sits in informal species-group association with other small, schooling lowland rasboras of the greater Mekong and Chao Phraya drainage systems, including R. borapetensis, which it superficially resembles but can be distinguished by the red dorsal-fin coloration and its considerably smaller adult size.
GBIF (taxon key 2359535) confirms the accepted binomial as Rasbora rubrodorsalis Donoso-Büchner & Schmidt, 1997 with no recorded synonyms.
Morphology
Rasbora rubrodorsalis is a slender, laterally compressed cyprinid with a gently arched back and a subterminal, slightly upturned mouth suited to feeding in the mid-water and at the surface. Standard length reaches approximately 1.5 in at most, making it one of the smaller members of its genus. The body is translucent silver with a faint lateral stripe and a golden sheen along the midline.
The species' clearest distinguishing feature is the orange-red coloration concentrated in the dorsal fin — the rubrodorsalis of its name (Latin: red-backed or red-dorsaled). This red patch is absent or much reduced in the similar R. borapetensis and allows field identification in areas where the two species co-occur. The remaining fins are largely clear or faintly pigmented. The caudal fin is forked, as typical of active-swimming schooling rasboras.
Sexual dimorphism is modest and typical for the genus: females develop a fuller, rounder body when gravid, while males remain slimmer. No notable fin-extension dimorphism has been recorded.
Habitat
The species is native to the lower Mekong basin, including waters within Thailand, Laos and Cambodia, and also occurs in the Mae Klong basin and the Chao Phraya system in Thailand. FishBase records its presence across the Mekong, Chao Phraya and Mae Klong river basins, all draining to the Gulf of Thailand. These are lowland tropical systems subject to marked seasonal flood pulses.
Within these basins, Rasbora rubrodorsalis favours the quieter, more vegetated margins: shallow swamps, marshes, slow reaches of streams, ditches and seasonally flooded areas rather than the fast main channels. Aquatic macrophytes and emergent or overhanging bank vegetation provide both refuge and foraging ground. Water chemistry in these habitats tends toward the neutral-to-slightly-acid: Seriously Fish records pH 6.5–7.5, water temperature 75–82 °F, and hardness of 2–10 °dH — soft to moderately soft, warm water typical of lowland tropical Southeast Asia.
Feeding
Rasbora rubrodorsalis is a micropredator, taking a diet of small invertebrates in the wild. Seriously Fish lists small insects, worms, crustaceans and zooplankton as its primary prey. Like other small schooling rasboras it forages actively through the water column and at the surface, exploiting the rich invertebrate fauna of densely vegetated, shallow margins.
In the aquarium the species accepts dried foods readily, but performs best when the diet is varied to include small live or frozen items such as daphnia, newly hatched brine shrimp, micro-worms and tubifex. Its small mouth means that particle size matters: standard community flake crushed or micro-pellets sized for nano fish are appropriate. Feeding in a relaxed school — not in competition with larger, faster tankmates — allows all individuals to feed evenly.
Mating
Rasbora rubrodorsalis is an egg-scattering, non-guarding spawner. Like the great majority of danionin cyprinids, there is no pair bond, no nest, and no parental investment beyond the act of spawning itself. In courtship, a ripe male drives a gravid female through vegetation, the two fish pressing close together briefly among plants or fine substrate as eggs and milt are simultaneously released.
Spawning is typically cued by rising temperature, increased water volume or rainfall — conditions that signal the seasonal flood pulse in the lowland habitats where this species lives. Eggs are weakly adhesive or non-adhesive and fall among plants and substrate, where the adults take no further interest in them and will consume them if they encounter them.
Breeding
Breeding in the aquarium follows the standard rasbora protocol for egg-scatterers. A small, bare or gently planted spawning tank with fine-leaved plants, spawning mops or a mesh bottom to protect eggs from the adults is the usual setup. A conditioned pair or small group is introduced; a small temperature increase and fresh, slightly acidic, soft water help trigger spawning, which commonly occurs in the morning.
Eggs number in the tens to low hundreds per session and should hatch within two to three days at 79–82 °F. Adults are removed after spawning to prevent egg predation. Larvae become free-swimming a few days after hatching and require infusoria or commercial micro-fry foods initially, progressing to newly hatched brine shrimp nauplii as they grow. There is no parental care at any stage.
In the aquarium
Rasbora rubrodorsalis is described by Seriously Fish as highly peaceful and suitable for a nano or community planted aquarium. Its small adult size (under 1.5 in) and completely peaceful temperament make it an excellent choice for tanks housing other small, non-aggressive species: nano cyprinids, small tetras, pygmy corydoras, otocinclus, and small livebearers all make compatible neighbours. It is best kept in schools of eight to ten or more — smaller numbers produce nervous, pale fish that stay hidden; a proper school brings out the characteristic active, confident schooling behaviour and the best red dorsal fin colour.
Water parameters are straightforward for a Southeast Asian lowland fish: 75–82 °F, pH 6.5–7.5, soft to moderately hard water (2–10 °dH). A planted tank with dark substrate, some driftwood and areas of gentle flow replicates the vegetated margins it occupies in the wild and enhances coloration. Feeding is uncomplicated; the only practical requirement is ensuring food is small enough for the fish's proportionally small mouth. It adapts readily to captivity and is described as very adaptable to aquarium conditions.
Conservation
Rasbora rubrodorsalis is assessed as Least Concern (LC) on the IUCN Red List, most recently evaluated in 2022. The species has a reasonably broad range across several major river systems — the Mekong, Chao Phraya and Mae Klong — spanning Thailand, Laos and Cambodia, and shows no evidence of significant range-wide population decline that would warrant a threatened category.
The lowland floodplain, swamp and marsh habitats it favours are nonetheless subject to ongoing pressure across the Mekong region: drainage for agriculture, conversion of floodplain wetlands, water extraction, and general degradation of slow-water, vegetated habitats all reduce available microhabitat. These pressures are diffuse rather than targeted at this species specifically, and its adaptability and broad distribution provide some buffer. Continued monitoring of lowland wetland integrity in its range is warranted as development pressures in the lower Mekong intensify.