Taxonomy & naming
Rasbora nematotaenia was described by Carl L. Hubbs and Marvin R. Brittan in 1954. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Rasbora nematotaenia Hubbs & Brittan, 1954 with no subsequent change of genus. FishBase places the species in the family Danionidae (formerly treated as a subfamily of Cyprinidae) under the subfamily Rasborinae.
The genus Rasbora has been subject to sustained taxonomic revision over recent decades. Several familiar aquarium fishes were removed and reclassified into satellite genera — Boraras, Trigonopoma, Brevibora, Kottelatia and Rasboroides were all carved out of Rasbora by various workers, and Kottelat & Witte (1999) erected Trigonostigma for the harlequin-type rasboras. The celestial pearl danio, long sold as a rasbora, proved to belong in Danio. Rasbora nematotaenia itself has not been subject to published reclassification and is treated as a valid Rasbora species in current checklists. FishBase lists the species with FishBase ID 65354.
The epithet nematotaenia derives from the Greek nema (thread) and tainia (band or stripe), a reference to the fine linear markings along the flank characteristic of this species group.
Morphology
Specific morphological data for Rasbora nematotaenia are sparse in the published literature available at the time of writing. The species belongs to a large complex of small to medium-sized rasborins with an elongate, laterally compressed body, a small terminal to slightly upturned mouth, and a generally slender build typical of shoaling stream fishes.
The specific epithet alludes to a thread-like band along the body — a common pattern in Rasbora species groups and one of the features Hubbs and Brittan used to characterise this taxon. Beyond the original description, detailed meristic data have not been widely published in accessible secondary sources. FishBase records the species as benthopelagic (occupying the lower to mid water column) in freshwater habitats.
Habitat
Rasbora nematotaenia is recorded as a freshwater endemic of Indonesia. Based on FishBase and GBIF occurrence data, the species is associated with Indonesian freshwater systems, most likely on one of the major Sundaland islands — Borneo, Sumatra or Java — where the genus Rasbora reaches its greatest diversity. The precise catchment or catchments are not well-documented in secondary literature available to date.
Sundaland freshwater habitats encompass a wide range of conditions from fast, clear highland streams to slow, tannin-stained lowland rivers and peat-swamp drainages. Without confirmed habitat data specific to this species, it is not possible to characterise its preferences in detail. The benthopelagic designation suggests it occupies the lower or mid water column rather than being a strict surface or bottom dweller.
Feeding
No published dietary data specific to Rasbora nematotaenia have been located. Rasbora species as a group are generalised omnivores, taking a mix of small aquatic invertebrates — insect larvae, small crustaceans, worms — along with plant material and organic detritus depending on what is locally available.
In the aquarium, members of the genus typically accept standard small-particle foods including fine flakes, micro-pellets and live or frozen invertebrates such as daphnia and brine shrimp nauplii. It is reasonable to assume similar flexibility in this species, though care notes specific to R. nematotaenia are essentially absent from hobbyist literature.
Mating
Rasbora nematotaenia has not been specifically studied in reproduction, and no published accounts of its mating behaviour have been located. It is treated here according to the strongly conserved reproductive pattern of Rasbora and related rasborin genera, which are egg-scattering, non-guarding spawners.
In typical Rasbora spawning, a ripe male pursues a gravid female through vegetation or over the substrate. The pair aligns briefly and releases eggs and milt simultaneously with no external fertilisation structure and no pair bond. Adults do not guard the spawn site or attend eggs in any way, and in confined conditions will consume their own eggs readily.
Breeding
As with mating, no captive-breeding records specific to Rasbora nematotaenia are available in the literature consulted. The species almost certainly follows the standard Rasbora pattern: adhesive to weakly adhesive eggs are scattered over fine-leaved plants, moss clumps, root tangles or open gravel and are immediately abandoned. There is no parental care of any kind — no nest, no guarding, no mouthbrooding.
Eggs would be expected to hatch within two to three days at tropical temperatures, with larvae becoming free-swimming shortly thereafter and initially requiring infusoria-sized food. Breeders working with uncommon Indonesian Rasbora species typically use shallow, well-planted tanks with gentle filtration, slightly acidic and soft water, and remove adults after spawning to protect the eggs.
In the aquarium
Rasbora nematotaenia is not an established aquarium species and does not appear in mainstream hobby literature or commercial supply chains. It is essentially unknown in the trade, and care notes are correspondingly absent. The species would be of interest to specialists focused on Sundaland cyprinids or Indonesian endemics, but the combination of rarity in both the wild and in collections means practical experience has not been recorded.
Based on close relatives within the genus, any husbandry attempt would logically follow the approach used for similarly sized Indonesian Rasbora species: a planted tank with soft, slightly acidic water, gentle flow, subdued lighting and a varied diet of small live, frozen and prepared foods. Shoaling behaviour should be assumed, meaning groups of at least six individuals would be appropriate.
Conservation
Rasbora nematotaenia is assessed as Data Deficient (DD) on the IUCN Red List (2022 assessment). The Data Deficient designation reflects a genuine absence of information on population size, range extent and trend — not an inference of security. Many Indonesian freshwater endemics confined to single islands or limited catchments face serious threats from habitat loss driven by land clearing, peat-swamp drainage, agricultural expansion and water extraction, and the precautionary principle suggests that Indonesian endemic fishes should not be assumed safe simply because they have not been studied.
Freshwater biodiversity in Sundaland is under sustained and accelerating pressure. Until population surveys, distribution mapping and habitat assessments are carried out for species such as R. nematotaenia, their true conservation standing cannot be determined. The IUCN assessment (2022.2) explicitly flags the lack of adequate data rather than a positive finding of low risk.