Taxonomy & naming
Rasbora naggsi was formally described by Silva, Maduwage and Pethiyagoda in 2010. The type locality is a stream on the grounds of the Sabaragamuwa University Campus at Belihul-Oya in the Sabaragamuwa Province of Sri Lanka. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Rasbora naggsi Silva, Maduwage & Pethiyagoda, 2010.
The specific epithet naggsi honours Fred Naggs, a British malacologist at the Natural History Museum, London, whose contributions to South Asian invertebrate systematics have benefited the broader understanding of the island's fauna. No synonyms or prior combinations are recorded for this species.
Rasbora remains a large, paraphyletic genus in the family Cyprinidae; extensive revisionary work since Kottelat and Witte (1999) has carved out numerous genera (Boraras, Trigonopoma, Brevibora, Kottelatia, Rasboroides and others) from the old broad Rasbora, but the Catalog of Fishes retains R. naggsi within Rasbora. Its phylogenetic placement within the genus is not fully resolved.
Morphology
Rasbora naggsi reaches approximately 3 in standard length, placing it among the larger members of its genus. As with most rasboras it has the characteristic slender, laterally compressed cyprinid body with a subterminal to slightly inferior mouth suited to feeding in fast current.
Detailed morphometric data from the original description have not been widely reproduced in secondary literature available to researchers, and the species remains data-sparse for meristics and colour pattern beyond its size class. Its rheophilic ecology — fast-flowing boulder-and-sand streams — suggests a streamlined build adapted to maintaining position in current, consistent with the body form seen in other swift-water South Asian rasboras.
Habitat
Rasbora naggsi occupies fast-flowing, shaded hill streams with a substrate of boulders, cobbles and sand. It is strictly rheophilic — a fish of moving water with high dissolved oxygen, cool temperatures and clear to lightly tannin-tinged water. The type and, as far as is known, only locality is a stream associated with the Sabaragamuwa University Campus near Belihul-Oya, situated in the wet-zone uplands of Sri Lanka.
This habitat contrasts sharply with the slow, acidic, darkly stained blackwaters associated with peat-swamp rasboras of Sundaland. Sri Lankan hill streams tend toward neutral to mildly acidic pH and moderate hardness influenced by the island's geology, and the cool, well-oxygenated character of the water reflects the elevated, shaded catchment. Water temperatures are likely in the range of 72–79 °F, with the aquarium-care guidance for similar species suggesting 75–81 °F as a working range.
Feeding
No dedicated dietary study of Rasbora naggsi has been published in the sources available. As a rheophilic stream cyprinid it likely feeds on drifting invertebrates — aquatic insect larvae, small crustaceans and terrestrial invertebrates falling into the current — supplemented by biofilm, algae and fine organic matter.
In the aquarium, rasboras of similar ecology accept a broad range of prepared foods including quality flakes, small pellets and frozen or live invertebrates such as daphnia, bloodworm and brine shrimp. A varied diet supports condition and natural foraging behaviour.
Mating
Rasbora naggsi is an egg-scattering, non-guarding spawner. No field or aquarium observations of its courtship behaviour have been documented in the available literature, but its breeding strategy follows the pattern shared by rasboras broadly: a driving male pursues a gravid female through the water column or among submerged vegetation and root structures, and the pair releases eggs and milt together in brief, close contact.
There is no pair bond and no territory. After spawning the adults disperse and ignore the eggs entirely, and in captivity they readily consume them if given the opportunity.
Breeding
No aquarium breeding of Rasbora naggsi is documented in the sources available, and the species is not established in the hobby trade. By analogy with rheophilic stream rasboras of comparable size, breeding in captivity would require a well-oxygenated, mature tank with gentle to moderate current, clean water, and fine-leaved plants or smooth substrate over which scattered adhesive eggs can settle out of the adults' reach.
The eggs receive no parental care; adults should be removed after spawning. Larvae would likely require infusoria or very fine prepared foods upon becoming free-swimming. Given the species' restricted wild population and Endangered status, any captive propagation would have conservation significance if pursued by specialist aquarists working with reputable institutions.
In the aquarium
Rasbora naggsi is not currently established in the aquarium hobby and is unlikely to be available commercially given its extremely restricted range and Endangered conservation status. Any specimens would require authorised collection under Sri Lankan wildlife regulations and CITES documentation if applicable.
For aquarists maintaining Sri Lankan endemic species for conservation purposes, its rheophilic habits indicate a need for strong filtration providing current, high dissolved oxygen, and temperatures in the 72–79 °F range. A species tank or carefully chosen community of stream-adapted fishes from the same region would be more appropriate than a generalised community setup. Its relatively large size for a rasbora means it requires a suitably spacious aquarium.
Conservation
Rasbora naggsi is assessed as Endangered (EN) on the IUCN Red List, with the assessment dated 7 August 2019. The primary driver of its threat status is its known occurrence from a single locality — a stream on the Sabaragamuwa University Campus near Belihul-Oya — which makes the species acutely vulnerable to any localised habitat disturbance.
Sri Lanka's wet-zone hill streams face ongoing pressure from agricultural expansion, deforestation of riparian corridors, sedimentation, water extraction and pollution from domestic and agro-industrial sources. A strictly rheophilic species with no documented secondary localities has essentially no buffer against any of these impacts at its single known site. The species has not been recorded from the aquarium trade in significant numbers, so collection pressure is not currently a documented concern, but the lack of captive stock also means there is no insurance population outside the wild.
Effective conservation requires protection of the Belihul-Oya watershed and its riparian forest, as well as targeted surveys to determine whether populations exist at undocumented sites elsewhere in the Sabaragamuwa hills.