Taxonomy & naming
Rasbora sarawakensis was described by M. R. Brittan in 1951, with the type locality in Sarawak, the Malaysian state that occupies the north-western third of Borneo. The valid name is confirmed by the Catalog of Fishes (Eschmeyer, CAS) and by GBIF taxonomy as Rasbora sarawakensis Brittan, 1951, with no subsequent change of genus or combination on record.
The genus Rasbora is a large, historically catch-all assemblage of small to medium cyprinids distributed across South and Southeast Asia. In recent decades a series of revisions have carved out numerous genera from what was once a broadly defined Rasbora — Boraras, Trigonopoma, Brevibora, Kottelatia, Rasboroides and others have been split off, and some species formerly placed in Rasbora now sit in entirely different subfamilies. Rasbora sarawakensis has not been reassigned and remains in the nominal genus under the current classification. The species epithet sarawakensis simply means 'from Sarawak,' a straightforward geographic adjective honouring the type locality.
Morphology
R. sarawakensis is a small, elongate, laterally compressed cyprinid reaching a maximum standard length of approximately 2 in, typical of the smaller members of the genus. The body is slender and torpedo-shaped, with a terminal mouth and the delicate build common to forest-stream rasboras.
The coloration is distinctive and attractive. The flanks carry a warm golden or yellowish-gold ground colour. A well-defined blue or blue-green longitudinal stripe runs from behind the operculum along the mid-flank to the base of the caudal fin, often bordering a narrow pale zone above and a golden zone below. The fins — particularly the dorsal, caudal and anal fins — show an orange or amber tint that intensifies in well-conditioned fish. The combination of the golden flank, lateral blue stripe and orange-washed fins is characteristic of the species and distinguishes it from sympatric rasboras.
Habitat
The species is restricted to Borneo, recorded from Sarawak in Malaysian Borneo and from Kalimantan Barat (West Kalimantan) in Indonesian Borneo. Its range encompasses the north-western lowlands of the island, a region of extensive lowland rainforest dissected by small forest streams and rivers draining into the South China Sea.
In the field it inhabits slow-moving to moderately fast-flowing small forest streams with dense canopy cover, fallen branches and accumulations of leaf litter on the substrate. The water is characteristically tannin-stained and clear, coloured amber to brown by humic acids leaching from decaying vegetation. Seriously Fish notes a temperature range of 75–82 °F (75–82 °F) and pH values of 6.0–7.5, with some accounts suggesting that the natural optimum may trend toward the softer, more acidic end of this range (pH 5.0–6.5), consistent with the soft and poorly buffered waters typical of lowland Bornean forest streams. Hardness is low, typically 2–8 °H.
Feeding
R. sarawakensis is an omnivore typical of small forest-stream rasboras. In its natural habitat it feeds on small invertebrates — aquatic and terrestrial insects, their larvae, small crustaceans and zooplankton — that fall into or live among the leaf litter and near the water surface under the forest canopy. Fine organic detritus and algal material also contribute to the diet.
In the aquarium it accepts all standard small foods readily. High-quality micro-pellets or fine-grade flakes form an adequate staple, but condition, colour and breeding readiness benefit from regular offerings of live or frozen foods: daphnia, baby brine shrimp, micro-worms and finely chopped bloodworm. Being a small fish, it feeds primarily in the middle to upper water column and should be given foods sized appropriately to its small mouth.
Mating
Like all Rasbora species, R. sarawakensis is an egg-scattering, non-guarding spawner. There is no nest-building, no pair bond, and no parental care of any kind. Spawning in the wild is likely triggered by seasonal cues — rising water temperature, increasing rainfall and altered water chemistry associated with the onset of the monsoon — which drive fish into shallow, vegetated marginal areas where eggs are scattered among fine-leaved plants and over soft substrate.
Courtship follows the pattern typical of small rasboras: a male in breeding condition pursues a gravid female actively through vegetation, nudging her flanks and driving her toward spawning sites. When she is receptive the pair press together briefly and release eggs and milt simultaneously. The eggs are weakly adhesive and settle among plant stems or on the substrate. Males show no further role after fertilisation, and adults will consume eggs they encounter.
Breeding
R. sarawakensis can be bred in the aquarium, though it is not among the frequently bred species in the hobby and detailed accounts are limited. The approach mirrors that used for other small forest-stream rasboras from Sundaland. A small, dimly lit breeding aquarium with very soft, acidic water (pH 5.5–6.5, hardness below 4 °H) at 79–82 °F provides suitable conditions. Fine-leaved plants such as Java moss or spawning mops offer sites for the scattered eggs to settle into and hide from the adults.
Conditioning the group on live and frozen foods over one to two weeks brings females into gravid condition. Introducing a small group or a conditioned pair into the breeding tank typically leads to spawning within a day or two. Because adults eat eggs given the opportunity, breeders are removed after spawning is observed. The eggs hatch in approximately 24–36 hours at tropical temperatures, and the fry become free-swimming two to three days after hatching. First foods are infusoria and paramecia, graduating to newly hatched brine shrimp nauplii and micro-worms as the fry grow.
In the aquarium
R. sarawakensis is a peaceful, active schooling fish well suited to a planted community aquarium designed to replicate Bornean forest-stream conditions. It should be kept in groups of at least eight to ten individuals; smaller numbers leave the fish nervous and prone to hiding, while a proper school produces natural shoaling behaviour and brings out the best colouration.
The ideal setup features a dark substrate, dense planting with fine-leaved species such as Java fern, cryptocorynes and mosses, and the addition of dried leaves or botanical material to introduce tannins and lower pH naturally. Lighting can be subdued. Water quality is important: the species does best in soft (2–8 °H), slightly acidic water (pH 6.0–7.0, ideally toward 6.0–6.5), kept clean with regular partial changes of similarly soft, conditioned water. Temperature should be maintained at 75–82 °F. It is compatible with other small, peaceful fish that share a preference for soft, acidic water — other small rasboras, small tetras (in appropriately acidic setups), pygmy corydoras and dwarf cichlids from soft-water habitats are all suitable companions. It is not recommended for hard, alkaline community tanks.
Conservation
R. sarawakensis is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment dated 17 January 2019. The assessment reflects the species' occurrence across both Malaysian and Indonesian Borneo, with no evidence of severe overall population decline at the time of assessment.
Nonetheless, the lowland rainforests of Borneo have experienced some of the fastest rates of deforestation and habitat conversion in the world over the past several decades, driven by oil palm expansion, logging, mining and agricultural encroachment. The small forest streams that R. sarawakensis inhabits are directly affected by deforestation: canopy removal raises water temperatures, increases sediment load and alters water chemistry in ways adverse to shade-adapted, tannin-water species. Collection for the ornamental fish trade adds a modest additional pressure, though the species is not among the most heavily traded Bornean rasboras. Continued monitoring of its range and habitat condition across Kalimantan in particular — where habitat loss has been most severe — is warranted.