Taxonomy & naming
Betta nuluhon was formally described by Kamal, Tan & Ng in 2020, published in Zootaxa (volume 4819). The type locality is a stream within the Crocker Range Forest Reserve in western Sabah, Malaysian Borneo. Eschmeyer's Catalog of Fishes governs the valid name: Betta nuluhon Kamal, Tan & Ng, 2020.
The species belongs to the akarensis group, a clade of Bornean mouthbrooding bettas that includes B. chini and B. balunga, from which B. nuluhon is distinguished by a combination of morphological characters: fewer lateral scales (29–31, mode 30), predorsal scale count of 20–21, a slender body depth of 22.1–25.2% standard length, yellow iris in life, and a distinctive pattern of transverse bars on the caudal fin. The greenish-blue iridescence on the opercle of mature males is particularly notable in live fish and provides a useful field mark within the group.
The specific epithet nuluhon is a noun taken from the Kadazandusun language, the indigenous tongue of the Kadazan-Dusun people of Sabah, meaning 'hill' — a direct reference to the hill-stream habitat where the species was collected.
Morphology
Betta nuluhon is a slender betta, with body depth recorded as 22.1–25.2% of standard length — notably more streamlined than many of its relatives, a build consistent with life in flowing hill streams. Standard length in the type series ranges from 1.5–2.5 in, with FishBase summarising the maximum as approximately 2.5 in SL.
Live coloration is striking in mature males: the body is brown to dark brown with blue-rimmed scales giving a faint reticulate impression, and the opercle carries a patch of greenish-blue iridescence. The iris is yellow. The caudal fin bears distinct transverse bars. Females are less vividly marked, as is typical across mouthbrooding Betta species, where intense male coloration plays a role in courtship signalling.
Habitat
Betta nuluhon inhabits clear-water hill streams within the Crocker Range Forest Reserve, a protected montane forest reserve in western Sabah. The type-locality stream was shallow, with overhanging riparian vegetation and a substrate of pebbles, sand, and silt. This is a habitat very different from the still, blackwater peat swamps associated with many lowland Betta species: the water is flowing, well-oxygenated, and relatively cool for a tropical fish.
Water-chemistry data recorded at the type locality give a temperature of 75 °F, pH 6.57, and dissolved oxygen of 6.25 mg/L — slightly acidic, cool, and oxygen-rich conditions reflecting the montane elevation. Like all labyrinth fishes, B. nuluhon possesses a suprabranchial organ that supplements gill respiration with atmospheric air, but in a well-oxygenated stream environment this organ is likely less critical than it is for lowland swamp species. Surface access is nonetheless important in captivity.
Feeding
No specific dietary study has been published for Betta nuluhon. Based on the feeding ecology of other small akarensis-group bettas in similar hill-stream habitats, the diet most likely consists of small invertebrates — aquatic insect larvae, microcrustaceans, and terrestrial insects that fall onto the stream surface. The clear, flowing stream habitat suggests a fish that hunts by sight in relatively high-visibility conditions, unlike the murky-water foragers of peat swamps.
In captivity, small live and frozen foods (Daphnia, Artemia nauplii, chironomid larvae, and small worms) are the most appropriate diet. Dry foods can be accepted once the fish are acclimated, but live prey is preferable both for nutritional balance and to sustain natural foraging behaviour.
Mating
Betta nuluhon is a paternal mouthbrooder. Courtship follows the pattern typical of mouthbrooding bettas in the akarensis group: the male displays to the female with spread fins and intensified coloration, particularly the iridescent opercular patch. After a series of approaches the pair engage in an embrace — the male wrapping around the female — during which eggs are fertilised.
Following spawning, the male collects the fertilised eggs into his buccal cavity. Coloration and the greenish-blue opercular iridescence likely function as honest-quality signals during mate choice, with females selecting males on the basis of condition and display intensity.
Breeding
After collecting the fertilised eggs into his mouth, the male B. nuluhon incubates the clutch buccally, eating nothing during the incubation period. In related akarensis-group species this incubation typically lasts one to two and a half weeks, after which the male releases free-swimming fry. The fry are relatively large and well-developed at release compared with the larvae of bubble-nesting bettas, having completed early development in the protected environment of the buccal cavity.
Because the species was described only in 2020 and remains rare in the hobby, detailed captive breeding data specific to B. nuluhon are sparse. Aquarists working with closely related mouthbrooding bettas such as B. chini and B. balunga generally provide a well-planted, low-flow aquarium with slightly acidic, soft-to-moderately-soft water, dim lighting, and ample cover. The brooding male should not be disturbed; some keepers separate him into a species tank to prevent females from harassing him during the mouthbrooding period.
In the aquarium
Betta nuluhon is extremely rare in the aquarium hobby. Given its description date of 2020 and its restricted wild range, captive specimens will be encountered only through specialist wild-betta societies or from hobbyists who have obtained wild-caught or F1 stock. It should be regarded as an advanced species requiring dedicated husbandry rather than a community-tank fish.
The hill-stream origin has important care implications. Unlike lowland peat-swamp bettas, B. nuluhon comes from cool (around 75 °F), slightly acidic (pH ~6.5), well-oxygenated flowing water. A species tank with gentle but consistent water movement, stable cool temperatures, and a soft, slightly acidic water chemistry is more appropriate than a warm, stagnant setup. As a labyrinth fish it requires unobstructed surface access for air breathing. Tankmates, if any, should be peaceful and similarly sized; many keepers maintain small mouthbrooding bettas in species-only setups to facilitate breeding observation and prevent egg or fry predation. Overhanging plants and a dark substrate help replicate the shaded, riparian stream environment and reduce fish stress.
Conservation
Betta nuluhon has not yet been formally assessed by the IUCN Red List; the species is too recently described for a completed evaluation to have been published as of 2026. Its conservation situation is nonetheless worth considered attention. The Crocker Range Forest Reserve, while a protected area, is not immune to the pressures — logging at its margins, agricultural encroachment, and stream sedimentation — that affect highland Sabah forest generally.
The species' restricted known range from a single type locality, combined with its hill-stream habitat specialisation, means that any significant degradation of the Crocker Range streams could affect the entire known population. Responsible aquarists should ensure any captive specimens have been legally obtained and, given the rarity of the fish in the hobby, contribute to captive propagation efforts through specialist wild-betta groups.