Taxonomy & naming
Melanotaenia kokasensis was described by Gerald R. Allen, Peter J. Unmack and Renny K. Hadiaty in 2008, in a paper titled "Two new species of rainbowfishes (Melanotaenia: Melanotaeniidae), from western New Guinea (Papua Barat Province, Indonesia)" published in aqua, International Journal of Ichthyology (volume 14, part 4, pages 209–224). It was placed directly in the genus Melanotaenia, the largest in the family Melanotaeniidae — the true rainbowfishes that radiate across New Guinea and northern and eastern Australia. Because the species has remained in its original genus, the authority is cited without parentheses.
The specific epithet kokasensis refers to the village of Kokas, the major landmark near the type locality on the northern Fakfak Peninsula. The genus name Melanotaenia combines the Greek melan, meaning black, with the Latin taenia, a stripe or band — for the dark midlateral stripe many species carry, and this one shows plainly. Among the West Papuan rainbowfishes it is told apart more by its slender build, small size, and fine colour detail than by its meristic counts, which overlap broadly with related species.
Morphology
Melanotaenia kokasensis is a small rainbowfish. The original description records a maximum standard length of about 2.5 in in males and unsexed specimens and roughly 2.5 in in females, one of the smaller members of the genus. Like all rainbowfishes it has the family's divided dorsal fin — a short anterior section followed by a longer posterior one — set on a laterally compressed body.
The meristic characters reported in the description fall within typical Melanotaenia ranges: dorsal spines five to six, dorsal soft rays eleven to fourteen (usually thirteen or fourteen), anal fin with one spine and twenty-one to twenty-four soft rays (usually twenty-two or twenty-three), pectoral rays thirteen to fifteen (usually fourteen), lateral scales thirty-five to thirty-seven (usually thirty-six), predorsal scales sixteen to eighteen, and total gill rakers on the first arch sixteen to nineteen.
The males carry the colour. The body is pale blue-grey, crossed by a blackish midlateral stripe at the level of the upper pectoral-fin base. A narrow orange stripe runs between each scale row on the upper half of the body, and a broad horizontal bluish streak sits above the abdomen. The pelvic and anal fins are pale yellowish, the second dorsal fin carries a narrow white margin, and the caudal fin has narrow blackish margins along its upper and lower edges. As is usual in the genus, females and juveniles are plainer, and these descriptive details refer chiefly to mature, displaying males.
Habitat
Melanotaenia kokasensis is known only from its type locality, a small stream flowing along a limestone creek bed through primary forest on the northern Fakfak Peninsula of West Papua (Papua Barat Province), Indonesia. The setting is unusual: the stream plunges down a steep ramp roughly 66 ft high right beside the sea, emptying into a mangrove-lined inlet near the village of Kokas. The fish were collected about 1 mi upstream, in a circular pool some 49–66 ft across and no more than about 2 ft deep, roughly 66 ft below limestone fissures that appear to be the stream's underground source.
The water is tropical freshwater, and the species is recorded as pelagic — active in open water — within its small habitat. A limestone catchment points to clear, mineral-rich water, the kind of karstic setting several West Papuan rainbowfishes favour. FishBase assigns the species a trophic level of about 2.9 and rates its resilience as medium, with an estimated population doubling time of 1.4 to 4.4 years.
The entire known range is one short limestone stream, so the habitat is the central fact of the species' biology. No outlying population has been documented, and that tiny area of occupancy is what drives the conservation concern.
Feeding
No detailed dietary study of Melanotaenia kokasensis has been published, and on this point the data are sparse. Its FishBase trophic level of approximately 2.9 marks it as an omnivore feeding low on the food web, in line with rainbowfishes generally — small aquatic and terrestrial invertebrates plus algae and plant material.
Going by better-studied members of the genus, M. kokasensis likely feeds opportunistically in the upper and middle water column on insect larvae, small crustaceans, and surface-fallen insects, with some grazing on algae and biofilm. In the absence of direct observations, these inferences should be treated as general expectations for a small stream-dwelling rainbowfish rather than as documented facts for this species.
Mating
The reproductive behaviour of Melanotaenia kokasensis has not been documented in the wild or, so far as the available sources record, in captivity. As with feeding, the data here are sparse and any account rests on the well-established patterns of the genus rather than on direct study of this species.
Rainbowfishes generally are egg-scattering plant-spawners: males court females with intense colour, often brightest at first light, and pairs drop a few eggs at a time into fine-leaved vegetation. The orange scale-row lines and other colours of male M. kokasensis presumably figure in that display, as in related species, but no courtship description for this fish has been published.
Breeding
There are no published accounts of captive or wild breeding for Melanotaenia kokasensis, and FishBase notes only that the species has aquarium potential rather than an established breeding record. The data are sparse and no clutch sizes, incubation times, or rearing notes specific to this fish can be reported without fabrication.
What can be said is general to the genus: rainbowfishes are usually continuous spawners that lay small, adhesive eggs which attach by fine filaments to fine-leaved plants or spawning mops, give no parental care, and readily eat their own eggs. M. kokasensis probably follows the same pattern, though for this species it remains unverified.
In the aquarium
Melanotaenia kokasensis is essentially unknown in the aquarium hobby. FishBase records its human use only as having aquarium potential, and its extreme rarity in the wild — a single Critically Endangered population — means it is not a fish that should be sought from wild sources.
No husbandry literature exists for the species, so any keeping advice is extrapolated from related West Papuan rainbowfishes rather than tested. Its small adult size, around 2.5–3 in, would in principle suit a modest planted tank, and its limestone-stream origin suggests clear, well-oxygenated, mineral-rich and likely neutral-to-alkaline water. Beyond those inferences the husbandry data are sparse. For now this is a fish for conservation and field interest, not the aquarium.
Conservation
Melanotaenia kokasensis is assessed as Critically Endangered (CR) on the IUCN Red List under criteria B2ab(iii), assessed in December 2019 and published in 2020 by Allen and Kadarusman (IUCN Red List version 2025-2). The listing reflects the species' known occurrence at only a single location — the small limestone stream near Kokas on the Fakfak Peninsula of West Papua — giving it a very restricted area of occupancy, combined with an inferred continuing decline in the quality of that habitat.
A species confined to one short stream has essentially no buffer against local disturbance: degrade the water, alter the limestone catchment, or apply any pressure at the type locality, and the whole known population is at risk at once. The Australian Department of Climate Change, Energy, the Environment and Water has documented the type-locality habitat in draft import-risk assessment materials for the genus — a reminder that the dominant concern is the species' wild status, not any presence in trade.
Given the narrow range and Critically Endangered listing, the priority for M. kokasensis is protecting its one known habitat. No captive population exists to offset risks to the wild fish, which makes the integrity of the Kokas stream and its limestone source all the more important.