Taxonomy & naming
Melanotaenia papuae was described by the Australian ichthyologist Gerald R. Allen in 1981. The holotype was collected by Allen near Port Moresby, Papua New Guinea, in a rainforest stream at Mt. Diamond, about 14 mi (roughly 14 miles) east of the city (near 9°27'S, 147°17'E), taken with a shrimp seine on 29 September 1978. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia papuae Allen, 1981; the species was described in Melanotaenia, where it remains, so no parenthetical authority applies.
The specific epithet papuae refers to Papua — the southern region of Papua New Guinea in which the species occurs. The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes — an Atheriniformes group that radiates across New Guinea and northern and eastern Australia. M. papuae is one of several rainbowfishes around Port Moresby. What sets it apart within the genus is its small adult size and restricted southern-PNG range rather than any hard meristic character; the counts overlap broadly among congeners.
Meristic counts recorded for the species are typical of the genus: 5–7 dorsal spines, 9–12 dorsal soft rays, a single anal spine, and 14–18 anal soft rays, set in the divided dorsal-fin arrangement characteristic of rainbowfishes.
Morphology
Melanotaenia papuae is a small rainbowfish. FishBase records a maximum length of about 2.5 in SL in males and unsexed fish and 2.5 in SL in females, placing it among the more diminutive members of the genus rather than the deep-bodied giants of the Vogelkop lakes. Like all Melanotaenia it is laterally compressed and carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — together with an anal fin of similar extent along the underside.
Detailed colour data is sparse; the available sources emphasise distribution and habitat over pattern. As a benthopelagic creek fish, it is understated next to its line-bred relatives in the trade. As across the genus, mature males run deeper-bodied and more strongly coloured than females, with display intensity rising and falling with mood and condition — but no precise colour description is asserted here beyond what the genus-level pattern supports.
Habitat
The Papuan rainbowfish has a tightly circumscribed range: numerous streams within roughly a 22 mi radius of Port Moresby, on the southern coast of Papua New Guinea, around latitude 9–10° S. It inhabits small rainforest creeks, where it is a freshwater, benthopelagic fish. Recorded localities include the type stream at Mt. Diamond east of the city, the Sogeri Plateau some 20 mi to the northeast, the Brown River forestry reserve, and tributaries of the Laloki, Brown, and Goldie rivers — material gathered across 1970–1971 and 1978.
Reported water conditions are warm and near-neutral to mildly alkaline. FishBase gives a tropical temperature range of about 72–90 °F, pH from 7.2 to 7.8, and a hardness from around 10 dH upward, consistent with clear, forested lowland creeks rather than soft blackwater. Everything about this fish turns on its small footprint. Confined to a cluster of streams around one growing city, it has little buffer against local habitat change — the central fact behind its conservation listing.
Feeding
Melanotaenia papuae is an omnivore with a recorded trophic level of about 2.9, in line with the rest of the genus. No species-specific dietary study exists for it, but rainbowfishes as a group are opportunists, taking small aquatic and terrestrial invertebrates — insect larvae, crustaceans, insects fallen onto the surface — alongside algae and plant matter. Villiform teeth around the lips let them scrape growth off hard surfaces.
In the tank it is undemanding. It takes good-quality flake and small pellets readily, and does well on frozen and live foods such as Daphnia, brine shrimp, and small bloodworm, with some vegetable content. Like most rainbowfishes it forages constantly in mid-water, so frequent small feedings suit it better than one large meal. A varied diet keeps both colour and breeding condition up.
Mating
Like rainbowfishes generally, Melanotaenia papuae reproduces by distinct pairing without parental care, and FishBase notes distinct pairing during reproduction for the species. Spawning behaviour follows the familiar Melanotaenia pattern: a displaying male courts a female near fine-leaved vegetation or submerged cover, and the pair release small numbers of eggs at a time rather than a single large clutch.
In rainbowfishes display and spawning peak in the first hours after light, when males throw their strongest colour. No courtship observations specific to M. papuae are on record, so this stays at the level the genus supports: small-batch, plant-associated spawning driven by male display, triggered most reliably by warm, stable water and well-conditioned adults.
Breeding
Melanotaenia papuae is an omnivorous egg layer, and like all Melanotaenia it provides no parental care. At the genus level the eggs are clear to light amber and roughly 0.5 in across — mostly around 0.5 in — each bearing sticky filaments that anchor it to fine-leaved plants or spawning mops. Larvae hatch at about 0.5 in.
Breeders use the standard rainbowfish method. Conditioned adults deposit eggs over fine plants such as Java moss or over synthetic mops, and the medium (or the eggs themselves) goes to a separate rearing container, since the adults will eat both eggs and fry. Females release a few eggs daily over a long spawning period rather than a single batch. Rainbowfishes generally start breeding at around six months and live roughly four years in the wild, up to about eight in captivity, so a well-kept group stays productive over several seasons. The fry are tiny and need very small first foods — infusoria, rotifers, or liquid fry food — before graduating to newly hatched brine shrimp and crushed flake.
In the aquarium
A peaceful community fish, harmless to keepers. FishBase's aquarium notes point to conditions close to its native creeks: pH around 7.5, hardness near 10 dH, temperature about 79 °F, with rock, plant, and driftwood cover. Keep it in a group — that settles the fish and gets the males displaying.
At under about 2.5 in it doesn't need the long tanks larger rainbowfishes demand, though as a restless open-water swimmer it still wants a clear swimming lane and some surface movement. A planted tank with driftwood and rockwork, neutral to mildly alkaline water of moderate hardness, and stable warm temperatures suits it. Pick tankmates of similar size and temperament; it competes for nothing, and it's hardy once the water echoes its forested-stream origins.
Conservation
Melanotaenia papuae is assessed as Vulnerable (VU) on the IUCN Red List, under criterion B1ab(iii) — a listing driven by its restricted range. Confined to small streams within roughly a 22 mi radius of Port Moresby, it is at risk from that narrow distribution rather than any single catastrophic threat: a fish tied to a handful of creeks around a growing capital has little resilience to degradation in its catchments.
The Australian Government's Department of Climate Change, Energy, the Environment and Water (DCCEEW) examined it in a 2023 risk report — interest that runs to both conservation and biosecurity, since the species appears in the ornamental-fish trade. FishBase notes it as of interest to both commercial fisheries and the aquarium hobby. As with other narrowly endemic rainbowfishes, breeding it in the tank eases collecting pressure on the small wild population and hedges against local habitat loss, so captive-bred stock is the conservation-minded choice.