Taxonomy & naming
Melanotaenia iris was described by the Australian ichthyologist Gerald R. Allen in 1987, in a paper that introduced the species alongside notes on the fish fauna of the headwaters of the Strickland River in Papua New Guinea. The type series comprised only five specimens, all collected from the Logatyu River, a mountain tributary of the Strickland near Wankipe. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia iris Allen, 1987; because it was described in Melanotaenia, where it remains, no parenthetical authority is used.
The genus Melanotaenia is the largest in the family Melanotaeniidae — the true rainbowfishes — a group that radiates across New Guinea and northern and eastern Australia. The genus name combines the Greek melan (black) with the Latin taenia (stripe), a reference to the dark lateral band carried by many species in the group. The specific epithet iris points to the rainbow itself — both the family's common name and Iris, the Greek goddess of the rainbow and messenger of the gods. M. iris is one of the more obscure species in the genus, known from very few specimens and rarely turning up in collections or in the hobby.
Morphology
Melanotaenia iris is a small-to-medium rainbowfish, reaching a maximum recorded length of about 4.5 in SL (Allen 1987; G.R. Allen pers. comm. 2019, per the 2020 IUCN assessment); FishBase lists a slightly smaller 4 in SL figure. Like other members of the genus, it has the laterally compressed, moderately deep body of a rainbowfish and the family's characteristic divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal. Recorded meristics include 5 to 6 dorsal spines, 17 to 19 dorsal soft rays, a single anal spine, and 21 to 24 anal soft rays.
Life-colour information is thin — the fish is known from a handful of preserved specimens and has barely been studied alive. Rainbowfishes show sexual dimorphism, with males growing deeper-bodied and more intensely coloured than females, especially when displaying; the specifics for M. iris are not documented. Its precise coloration is best treated as data sparse rather than asserted in detail.
Habitat
The Strickland rainbowfish was first described from the Logatyu River, a mountain tributary of the Strickland near Wankipe in Papua New Guinea, at roughly 1,656 ft elevation — high for a rainbowfish — about 528 mi upstream from the mouth of the Fly River. Later survey work (reported in the 2020 IUCN assessment) added records from the Lagaip River and its major tributaries, including the Ok Om, Kuru and Pori Rivers, and the species may be more widespread still in unsampled tributaries of the upper Fly–Strickland system. The IUCN describes it as locally common in mountainous terrain, in boulder-bottom pools interspersed between shallow rapids, occurring in both main channels and the smaller, clearer tributaries it is suspected to prefer.
This is a cool, clear, flowing mountain-stream environment rather than the warm lowland lakes and slow rivers that many of its relatives inhabit. FishBase classifies the species as freshwater, benthopelagic, and tropical, occurring at latitudes around 6 to 7 degrees south. Beyond the elevation and the upland setting, the physical conditions of its habitat — water chemistry, temperature, and seasonal regime — are not well documented.
Feeding
The natural diet of Melanotaenia iris has not been studied directly, and FishBase notes that its biology is poorly known. It carries a trophic level of about 2.9, consistent with the omnivorous-to-mildly-carnivorous feeding typical of rainbowfishes, which take small aquatic and terrestrial invertebrates from the water column and surface along with algae and plant material.
In a cold, fast mountain stream, drifting insect larvae and other small invertebrates would be the expected mainstay, supplemented by whatever algae and detritus the current delivers. No aquarium feeding records exist for this species, which is essentially absent from the hobby; any husbandry expectations would have to be extrapolated from better-known relatives rather than from observation of M. iris itself.
Mating
Information specific to the reproductive behaviour of the Strickland rainbowfish is very limited. FishBase records that the species shows distinct pairing during reproduction, a trait shared with many rainbowfishes in which a receptive female follows a displaying male to spawn. Beyond this brief note, courtship displays, spawning seasonality, and site selection have not been described for M. iris.
Across the genus, male rainbowfishes typically court with intense colour displays — flaring fins and flashing the body, often most vividly in the early-morning hours — and pairs deposit small numbers of adhesive eggs at a time among fine vegetation. Whether the cold, high-altitude habitat of M. iris changes any of this is unknown. With no direct observation on record, its mating biology is inferred from relatives, not documented.
Breeding
No breeding records exist for Melanotaenia iris, in the wild or in captivity. The species has not, so far as the literature shows, been bred in the aquarium, and its early life history — egg size, incubation period, fry development — is undescribed.
Rainbowfishes as a group are egg-scattering, plant-spawning fishes that attach small adhesive eggs by fine filaments to fine-leaved vegetation or spawning mops, releasing a few eggs daily over an extended period rather than a single clutch, with no parental care. If M. iris follows the family pattern, similar behaviour would be expected, but this is an extrapolation. Given how few specimens are known and the remoteness of its single locality, the species' reproductive biology remains genuinely data sparse.
In the aquarium
The Strickland rainbowfish is effectively unknown in the aquarium hobby. It is not established in the ornamental trade, has no recorded body of captive-husbandry experience, and is rarely if ever available to aquarists. As such, there is no reliable, species-specific guidance to offer on keeping it.
Its cool mountain-stream origin would set it apart from the warm-water rainbowfishes most aquarists keep: expect a need for cooler, well-oxygenated, flowing water rather than the warm still conditions that suit lowland species. The fish is recorded as harmless to humans and is not listed under CITES or CMS. Any keeping advice would be guesswork — this is a wild-only species with no aquarium track record.
Conservation
Melanotaenia iris was assessed as Vulnerable (VU) on the IUCN Red List in 2020, under criterion B1ab(iii). The listing reflects a restricted extent of occurrence — estimated at more than 5,000 km2 within a 1,125–12,900 km2 range across the upper Fly–Strickland system — divided among about five locations, combined with an inferred continuing decline in habitat quality. The current population trend is assessed as stable, and the species is regularly monitored by nearby mining operations with no present evidence of decline.
The principal threats are habitat-based: gold mining and the spread of settlements in the upper Fly–Strickland catchment are driving ongoing habitat degradation, and the recent arrival of introduced tilapia (Oreochromis) in the Fly River system is flagged as a plausible future threat. Because the species is still relatively little studied, further research into its true distribution and the pressures acting on it is recommended. The species is not listed under CITES or CMS.