Taxonomy & naming
Rasbora semilineata was described by Max Weber and Lieven Ferdinand de Beaufort in 1916, making it one of the many Bornean cyprinids catalogued during the Dutch naturalist expeditions of the early twentieth century. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Rasbora semilineata Weber & de Beaufort, 1916, with no change of genus since the original description.
The genus Rasbora sensu lato was a large, heterogeneous wastebasket for small to medium danionine cyprinids throughout most of the twentieth century. Beginning in the 1990s a series of revisions by Kottelat, Rüber and collaborators carved out several natural groups into their own genera — Boraras, Trigonostigma, Trigonopoma, Brevibora, Rasboroides and others were each recognised as distinct from the Rasbora core. R. semilineata was not reassigned in any of these revisions and remains in Rasbora under the current Catalog of Fishes treatment. Whether it belongs to the residual Rasbora core or to one of the several informally recognised species groups within the genus has not been resolved.
The specific epithet semilineata derives from Latin semi- ('half') and lineata ('lined'), referring to a lateral stripe or line marking that is partial or incomplete — a descriptive allusion to the species' flank pattern.
Morphology
Rasbora semilineata is a small fish, reaching a recorded maximum of 1.5 in standard length (SL) according to FishBase — placing it among the diminutive members of the genus rather than the larger mahseer-affiliated forms. The body is slender and laterally compressed in the typical danionine manner, with a subterminal mouth suited to picking small prey from the water column and surface.
The defining character implied by the name is an incomplete or half-length lateral stripe, presumably running along part of the flank rather than from opercle to caudal base. Detailed colour descriptions and diagnostic morphological data are not available in the accessible literature; the species is known primarily from its original description and brief treatment in regional checklists. Its overall form is consistent with the typical Bornean Rasbora gestalt: a small, streamlined schooling cyprinid with a plain or lightly patterned silvery body.
Habitat
Rasbora semilineata is confirmed endemic to northern Borneo, placing it within the Sundaland bioregion — one of the world's most biodiverse freshwater fish provinces. Northern Borneo encompasses the Malaysian states of Sabah and Sarawak and the Sultanate of Brunei, a landscape of lowland and foothill rainforest drained by numerous rivers running off the island's central highland spine.
Its congeners in Bornean Rasbora assemblages (including R. sarawakensis, R. tornieri and R. dusonensis, among others) occupy a spectrum from clear, fast-flowing hill streams to tannin-stained peat-swamp waters and sluggish blackwater tributaries. In common with the broader Bornean freshwater fish fauna, R. semilineata is expected to prefer soft, slightly acidic water with minimal suspended sediment, often shaded by overhanging riparian canopy and with a substrate of coarse sand, gravel or accumulated leaf litter. FishBase classifies it as freshwater and benthopelagic, consistent with a midwater shoaling life in slow to moderate current.
Feeding
No specific dietary data for Rasbora semilineata are available in the published literature. By analogy with its congeners and with the broader danionine cyprinid guild, it is assumed to be an opportunistic omnivore and micropredator, feeding on small invertebrates — chironomid and other insect larvae, microcrustaceans, terrestrial insects falling to the water surface — as well as algae and fine organic detritus.
Bornean Rasbora species of comparable size are active midwater foragers that exploit both the water column and the surface film. In an aquarium setting, small live and frozen foods such as daphnia, cyclops and baby brine shrimp are likely to be accepted alongside high-quality fine flake or micro-pellet foods, given the species' diminutive maximum size of under 1.5 in.
Mating
Rasbora semilineata has not been observed spawning in either the wild or captivity in the accessible literature, but as a member of the genus Rasbora it is expected to follow the egg-scattering, non-guarding reproductive strategy typical of the danionine cyprinids. In related species, a ready male closely pursues a gravid female through vegetation or over the substrate, pressing alongside her as eggs and milt are released in brief, repeated bursts.
There is no pair bond, nest construction or any form of parental care. The weakly adhesive eggs settle among plant stems, moss or leaf litter after spawning, and both parents will readily consume them. Spawning in northern Bornean streams is likely tied to rainfall and water-level cycles, with peak reproductive activity corresponding to the wetter months when streams rise and spread into flooded forest margins.
Breeding
No captive breeding records for Rasbora semilineata appear in the hobby literature, reflecting its obscurity as an aquarium subject. The approach used successfully for comparable small Bornean Rasbora species — a shallow, well-planted breeding tank with soft, slightly acidic water, fine-leaved plants or Java moss as egg-catching substrate, and removal of adults after spawning — would be the logical starting point.
Eggs of small Rasbora species typically hatch within one to two days at tropical temperatures, and the free-swimming larvae take infusoria or commercial fry food before graduating to freshly hatched brine shrimp nauplii. Successful breeding in captivity would benefit the species' profile and potentially contribute to an understanding of its requirements — information currently absent from both the scientific and hobbyist literature.
In the aquarium
Rasbora semilineata is essentially unknown in the ornamental fish trade and there are no established care guidelines in the hobby literature. Its small size (under 1.5 in SL) and its membership in a genus that includes many popular and tractable aquarium species suggest it should adapt to a planted community tank designed for small Southeast Asian cyprinids, with soft, moderately acidic water, gentle filtration and a dark substrate that showcases the fish's natural colouration.
Given its Bornean forest-stream origins, water parameters in the range of pH 6.0–7.0 and 72–79 °F (72–79 °F) would be appropriate starting points. Like all small shoaling rasboras it should be kept in groups of at least six individuals; singletons and pairs become stressed and less active. Peaceful, slow-moving tankmates of similar scale — other small danionines, small Corydoras, dwarf rasboras or micro-loaches — would be appropriate companions. It is rarely if ever commercially available and would most likely be encountered through specialist importers handling Bornean wild-caught material.
Conservation
Rasbora semilineata is assessed as Data Deficient (DD) on the IUCN Red List, with an assessment date of 19 October 2020. The DD listing reflects an absence of adequate population, distribution and trend data rather than confirmed security; it does not preclude significant conservation concern.
As a Bornean endemic, R. semilineata is subject to the same landscape-level pressures bearing on the island's freshwater biodiversity: deforestation for oil palm and timber, peat-swamp drainage, sedimentation from agricultural and mining runoff, and the conversion of lowland rivers for irrigation and hydropower. Northern Borneo retains more intact forest cover than the southern and western parts of the island, but habitat loss is ongoing. Until dedicated field surveys establish its distribution, abundance and habitat requirements, the species cannot be meaningfully assessed, and targeted conservation planning remains impossible.