Taxonomy & naming
Melanotaenia irianjaya was described by the Australian ichthyologist Gerald R. Allen in 1985. The holotype — an adult male of about 2.5 in standard length, catalogued as LBN 4952 — was taken in a stream at Fruata Village in what was then the Indonesian province of Irian Jaya, by G. Allen and H. Bleher on 16 November 1982. Paratypes were collected across a scatter of creeks and rivers near Merdai, along the Auk River near Suswa Village, and in a tributary of the Kamundan River near Senopi Village. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia irianjaya Allen, 1985; because the species was described in Melanotaenia, where it remains, no parenthetical authority is used.
The specific epithet refers to Irian Jaya, the name then in use for the easternmost province of Indonesia (now West Papua and Papua), within which the type locality lies. The genus name Melanotaenia is built from the Greek melan, 'black', and the Latinised taenia, 'stripe or band', a reference to the dark lateral band common to many species of the genus. Melanotaenia is the largest genus of the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia. Among its New Guinea congeners M. irianjaya holds one anatomical distinction: it is the only species of the genus that lacks palatine teeth.
Morphology
Melanotaenia irianjaya is a fusiform, laterally compressed rainbowfish of modest size. FishBase gives a maximum of about 3.5 in standard length after Allen's 1991 field guide; in practice well-conditioned males may approach 4.5 in in total length, while females usually remain under 4 in. Meristics fall within the normal range for the genus: roughly 5 to 7 dorsal spines and 12 to 16 dorsal soft rays, with a single anal spine followed by 21 to 26 anal soft rays. Like other Melanotaenia it carries the family's divided dorsal — a short anterior fin followed by a longer posterior one — and the deep body deepens with age in dominant males.
The colour pattern is restrained but diagnostic. A broad bluish midlateral band runs along the flank, most prominent on the rear half of the body. The dorsal and anal fins are reddish with narrow white margins, and mature males develop a large, elongated anal fin. The signature feature is the tail: the caudal fin is reddish but edged with a black band along both its upper and lower margins — a pattern unique among the rainbowfishes of this genus and the surest field mark for it. The pelvic and pectoral fins are largely transparent. Sexual dimorphism runs muted by Melanotaenia standards, but males are still the brighter, larger, and deeper-bodied sex.
Habitat
Melanotaenia irianjaya is found primarily in river systems that drain into Bintuni Bay, on the southern flank of the Vogelkop (Bird's Head) Peninsula of West Papua, Indonesia. It is a fish of rainforest creeks and small rivers. The habitats run a range — from turbid, slow-flowing reaches over flat terrain to clear, moderately fast water cutting through hills — but it turns up most often in moderate-flowing, reasonably clear creeks. Bottoms are gravel or sand, aquatic plants are few, and the fish keep to submerged logs and other woody cover.
For a New Guinea rainbowfish the wild water sits on the hard, alkaline side: pH from about 7.3 to 8.5, temperatures of roughly 75–84 °F, and very low conductivity in the range of about 10 to 60 µS/cm. FishBase's summary, drawn from broader genus data, cites a narrower and softer envelope of pH 6.0 to 7.0 and 73–79 °F; the field figures from the import-risk assessment are the more specific record for this species. The geographic range is small — an extent of occurrence on the order of 6,311 mi² — and that restricted distribution drives the species' conservation outlook.
Feeding
Melanotaenia irianjaya is an omnivore, taking small aquatic and terrestrial creatures along with plant matter. FishBase places it at a trophic level of about 2.9, consistent with a mixed diet rather than a strictly predatory or strictly herbivorous one. The fish carries villiform teeth around the lips that it uses to scrape algae and other growth from submerged surfaces, supplementing the small invertebrates and drifting items it gathers from the water column.
In the aquarium it is undemanding and feeds readily across the surface and middle water. Build the diet around good-quality flake and small pellets, then work in frozen and live foods — Daphnia, brine shrimp, bloodworm, and similar small invertebrates — plus some vegetable or algae-based matter to echo its natural grazing. As with rainbowfishes generally, a meaningful live or frozen component improves both colour and breeding condition, and the fish does best fed small amounts several times a day rather than a single large feeding.
Mating
Melanotaenia irianjaya reproduces by distinct pairing: a displaying male courts and pairs with a receptive female, in the egg-scattering, plant-spawning manner typical of the genus. There is no mouthbrooding, livebearing, or nest-guarding. Because the species shows less pronounced sexual dimorphism than many of its relatives, the male's courtship colour change — an intensifying of the reddish fins and bluish flank band — carries much of the signalling, and males are at their most vivid when displaying.
As in other rainbowfishes, spawning activity is concentrated in the early hours after first light, when males display most strongly and pairs move into fine-leaved vegetation or spawning mops to release eggs. Keeping several fish together encourages the competitive displays that bring males into peak colour and condition, and breeders typically condition groups rather than isolated pairs to bring fish into spawning form.
Breeding
Melanotaenia irianjaya is an egg-scatterer with no parental care. Spawning takes place over fine-leaved plants or synthetic spawning mops, to which the eggs attach by sticky filaments. The eggs are clear to light amber and small — reported at roughly 0.5 in, with most around 0.5 in in diameter — and the adhesive threads anchor them among the spawning medium until they hatch.
As is typical of the genus, a female releases a few eggs at a time over an extended period rather than depositing a single large clutch, and the adults offer no care and will eat their own eggs and fry. Breeders therefore harvest the mops or eggs to a separate rearing container. Detailed published rearing data specific to this species are sparse, but in line with closely related Melanotaenia the eggs hatch after about a week or more at typical breeding temperatures, and the tiny fry require very small first foods — infusoria or commercial liquid fry food — before they can take newly hatched brine shrimp and finely crushed flake. Live aquarium specimens were first collected near Fruata Village in 1982 by Heiko Bleher, and the species has been bred in captivity since its introduction to the hobby.
In the aquarium
The Irian Jaya rainbowfish is an active, peaceful schooling fish best kept in a group, ideally with several males so that they display against one another and stay coloured up. It is a strong, restless swimmer that appreciates a long, open swimming lane, so an aquarium with a generous footprint suits an adult group better than a tall, short one. A planted tank with open central swimming space, gentle to moderate flow, and some surface movement shows the fish to good effect and encourages the morning display behaviour.
Water chemistry should lean toward the species' hard, alkaline wild origins rather than the soft-water defaults often assumed for tropical fish. Field conditions of roughly pH 7.3 to 8.5 and 75–84 °F point to neutral-to-alkaline, warm water, though like most rainbowfishes it tolerates a fairly broad range provided things stay stable and well filtered. Pick tankmates that are similarly sized, peaceful, and active enough not to be outcompeted at feeding — other rainbowfishes, larger tetras, and peaceful barbs and rasboras all work. Hardy and undemanding once settled, it pays back good conditions with the black-edged red tail of a mature male.
Conservation
Melanotaenia irianjaya is assessed as Vulnerable (VU) on the IUCN Red List under criteria B1ab(iii) (version 3.1). The assessment was carried out on 3 December 2019 and published in 2020. The listing rests on the species' restricted distribution — an extent of occurrence of about 6,311 mi² across an estimated 3 to 10 locations defined by the threats it faces — combined with a continuing decline in the quality of its inland-wetland habitat.
The threats are specific: expanding oil-palm plantations, gas drilling, and deforestation across the species' range in the Bintuni Bay catchment of southern Vogelkop. A fish tied to a small set of river systems has little buffer against such pressure, with no large outlying population to offset local declines. The species is native to Indonesia and has not been evaluated under CITES. As with other restricted-range rainbowfishes, maintaining captive-bred stock in the hobby helps reduce collecting pressure on the wild population and keeps the species secure outside its shrinking native habitat.