Taxonomy & naming
Melanotaenia mamahensis was described in 2016 by Gerald R. Allen, Peter J. Unmack and Renny K. Hadiaty in the journal aqua, International Journal of Ichthyology (22(1):1–32), in a paper revising the 'Goldiei group' of rainbowfishes from the Bird's Neck region of New Guinea. That paper described five new species at once and also recognised Melanotaenia dumasi Weber; M. mamahensis is one of the five. Because it was described in Melanotaenia, where it remains, the valid name carries no parenthetical authority. Eschmeyer's Catalog of Fishes governs the valid name as Melanotaenia mamahensis Allen, Unmack & Hadiaty, 2016.
The holotype is a male of 2.5 in standard length (MZB 22779), taken with a seine net in the Sungai Mamah at 03°20.489'S, 134°59.971'E in West Papua Province, Indonesia, on 14 January 2015 by J. Christian, H.-G. Evers and A. Wagnitz. The paratype series ranges from 2–3 in standard length. The genus Melanotaenia is the largest in the family Melanotaeniidae — the true rainbowfishes — which radiate across New Guinea and northern and eastern Australia; within that genus M. mamahensis sits among the Goldiei-group species of the Bird's Neck.
The specific epithet mamahensis is a place name, referring directly to the type locality: the Mamah River (Sungai Mamah). The genus name combines Greek melanos, 'black' or 'dark', with taenia, 'band' or 'stripe', a reference to the dark midlateral stripe seen across the genus. The standard English common name in FishBase is the Mamah rainbowfish.
Morphology
Melanotaenia mamahensis is a small rainbowfish; the type series spans roughly 2–3 in standard length, with the largest paratype near 3 in SL (about 3 in). Like other members of the genus it is laterally compressed and carries the family's characteristic divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — over a moderately deep body. FishBase does not populate a maximum total length for the species, so size statements here rest on the measured type material rather than on a separately reported maximum.
The species' description rests on the adult male, the colourful sex. The top of the head and the predorsal midline are brown, grading to bluish-grey on the nape and upper back. A blue midlateral stripe runs from the eye to the base of the tail, becoming darkest and widest on the caudal peduncle. Above this blue stripe sits a broad yellow zone, and a narrow orange stripe follows the line between scale rows on the upper half of the side. The lower side is pale bluish to light grey, with a whitish spot on each scale, and there is a small red spot — about half the diameter of the pupil — at the upper-rear corner of the operculum. The first dorsal fin is reddish.
As is typical of the genus, females and juveniles are far plainer than displaying males, and male colour intensity is mood- and condition-dependent. Detailed meristic and morphometric data for the species are given in the original 2016 description; beyond the type material, published natural-history detail is sparse.
Habitat
Melanotaenia mamahensis is, on current knowledge, a single-stream endemic. It is known only from the Mamah River, a small stream roughly 31 mi west of Nabire near the edge of Cenderawasih Bay, on the Bird's Neck of West Papua, Indonesia. No other localities have been reported, and this extremely restricted range is the central fact of the species' biology and its conservation assessment.
The type habitat is a clear, slow-flowing rainforest stream. When it was collected in January 2015 the fish was abundant — swimming in large groups, favouring the deeper, shaded sections. Water at that visit measured 79 °F and pH 7.9: warm, clear, slightly alkaline.
Because the species is known from a single visit to one stream, the published environmental record is essentially that single set of measurements; seasonal variation, depth range, and downstream extent are not documented. Statements about its preferences should be read as a snapshot of the type locality rather than a full ecological profile.
Feeding
No diet study specific to Melanotaenia mamahensis has been published; FishBase places it at a trophic level of about 2.9, consistent with an omnivore feeding low on the food chain. By analogy with the genus, it can be expected to be an opportunistic omnivore of the upper and middle water column, taking small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and surface-fallen insects — together with algae and plant material.
In the aquarium, rainbowfishes of this kind are undemanding feeders that accept good-quality flake and small pellets alongside frozen and live foods such as Daphnia, brine shrimp, bloodworm and Cyclops, with some vegetable matter. A varied diet with a meaningful live or frozen component improves both colour and breeding condition.
Given the sparse record for this species in particular, these notes are extrapolated from general Melanotaenia husbandry rather than from observations of M. mamahensis itself.
Mating
No courtship study specific to Melanotaenia mamahensis has been published. Across the genus, rainbowfishes are egg-scattering plant-spawners in which males court females with intense colour displays — flaring the fins and flashing the body so that the bright zones reach maximum saturation — and in which display is strongest in the early hours after first light and most vigorous when rival males are present.
Many rainbowfish males also show a flickering pale 'courtship band' that flashes along the head and nape during display. Whether M. mamahensis shows this exact signal is not documented, but the reddish first dorsal fin, the blue midlateral stripe and the red opercular spot of the adult male are the kind of bright, sexually dimorphic markings used in such displays.
These remarks come from the genus as a whole; direct observations of mating in this species are, on the available record, lacking.
Breeding
There is no published account of captive or wild breeding for Melanotaenia mamahensis specifically. The genus as a whole follows a well-known pattern: continuous or near-continuous egg-scattering over fine-leaved plants or spawning mops, with adhesive eggs that hang by fine filaments from the spawning medium, no parental care, and a tendency for adults to eat their own eggs and fry.
In that generic pattern, a female releases a few eggs daily over many days rather than a single clutch, and larvae hatch at around 0.5 in and are difficult to identify to species until they reach roughly 1 in. First foods for the tiny fry are correspondingly small — infusoria, rotifers, or liquid fry foods — before graduating to newly hatched brine shrimp.
Until the species is bred and documented, these expectations should be treated as the default Melanotaenia template applied to M. mamahensis, not as observed facts for this fish.
In the aquarium
Melanotaenia mamahensis is a recently described, range-restricted species, and it is not an established fixture of the ornamental trade in the way that better-known rainbowfishes are. Husbandry guidance therefore rests on its measured wild conditions and on general rainbowfish care rather than on a body of aquarium experience with this species.
The single set of habitat measurements — about 79 °F and pH 7.9 in a clear, slow-flowing rainforest stream — points to warm, clean, slightly alkaline water as a sensible target, with the strong filtration, open swimming space and stable conditions that suit active, schooling rainbowfishes generally. As a shoaling species reported to swim in large groups in the wild, it would be kept in a group, with males displaying against one another to bring out colour.
Given its conservation status and limited availability, anyone keeping this species should prioritise responsibly sourced, captive-bred stock and accurate record-keeping over wild collection. The care details above are deliberately conservative because species-specific aquarium data are sparse.
Conservation
Melanotaenia mamahensis is assessed as Vulnerable (VU) on the IUCN Red List under criterion D2, with the assessment dated 5 December 2019 (IUCN species ID 161100311; assessment 161100331). Criterion D2 applies to species with a very restricted area of occupancy or number of locations such that a plausible future threat could drive them to Critically Endangered or Extinct in a short time — a status that fits a fish known from a single small stream.
The basis of the listing is the species' tiny known range: it has been recorded only from the Mamah River, near the edge of Cenderawasih Bay west of Nabire. A single-stream endemic has essentially no buffer against localised events such as habitat alteration, pollution, or the introduction of non-native fishes, and any such pressure on that one stream would bear on the entire known population at once.
The species' restricted distribution has also drawn the attention of biosecurity authorities outside its range — for example, an Australian import risk assessment treats Bird's Neck rainbowfishes of this group on a species-by-species basis. For aquarists, the responsible course is to favour captive-bred stock and to avoid contributing to collection pressure on a fish whose entire documented world is one rainforest creek.