Taxonomy & naming
Melanotaenia oktediensis was described by Gerald R. Allen and Norbert J. Cross in 1980, in their work on the rainbowfishes of New Guinea published through the Western Australian Museum. The specific epithet is geographic, taken directly from the Ok Tedi River, the type locality and the only river system from which the species is known. Eschmeyer's Catalog of Fishes governs the valid name, recording it as Melanotaenia oktediensis Allen & Cross, 1980; because it was described in Melanotaenia, where it remains, the authority carries no parentheses.
Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes of the order Atheriniformes, spread across New Guinea and northern and eastern Australia. The genus name combines the Greek melanos, 'black or dark', with the Latinised taenia (Greek tainia), 'band or stripe' — a nod to the dark lateral band common across the group. M. oktediensis is one of several rainbowfishes endemic to the Fly River drainage. Like many of its congeners, it is separated from close relatives more by colour and locality than by meristic characters, which overlap broadly within the genus. Reported counts include 6–7 dorsal spines, 18–19 dorsal soft rays, a single anal spine, and 24–26 anal soft rays.
Morphology
Melanotaenia oktediensis is a small-to-moderate rainbowfish, with a maximum recorded length of about 4 in (standard length, per Allen 1991). 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 — set on a body that deepens with age, most noticeably in mature males.
Colour data for this species are thin compared with the better-known aquarium rainbowfishes, and it is not among the genus's more intensely coloured members. As in Melanotaenia generally, dominant and displaying males should run deeper-bodied and more strongly marked than females and juveniles, with colour shifting by mood, condition, and light. Beyond the meristic counts noted above (dorsal spines 6–7, dorsal soft rays 18–19, anal spine 1, anal soft rays 24–26), precise life-colour descriptions of wild M. oktediensis are scarce in the accessible literature, so finer claims are best treated as data sparse.
Habitat
The Oktedi rainbowfish is endemic to the Ok Tedi River and its tributaries, one of the principal headwater streams of the upper Fly River system in southern Papua New Guinea. It is a fish of mountain streams, recorded at roughly 984–1640 ft elevation and a long way inland — some 559–590 mi upstream from the sea — at low latitudes around 7–8° S. Freshwater and benthopelagic, it lives in and near the flowing water of these upland catchment streams.
Reported water conditions are warm and near-neutral: about 73–79 °F and pH 6.5 to 7.5, in keeping with clear upland tropical streams. The restricted range — a single river and its feeders — is the central fact of the species' biology and the reason its status is so precarious. There is nowhere else for it to live, so any degradation of the Ok Tedi catchment lands on the entire global population at once.
Feeding
Melanotaenia oktediensis is omnivorous, with a reported trophic level of about 2.9 — the generalist slot typical of the genus. Rainbowfishes like it feed opportunistically in the upper and middle water column, taking small aquatic and terrestrial invertebrates along with algae and plant material grazed off submerged surfaces.
In the aquarium it is undemanding. Like its relatives, it will take good-quality flake and small pellets, plus live and frozen foods such as Daphnia, brine shrimp, and bloodworm, and some vegetable matter. As an active grazer it does best fed small amounts several times a day rather than one large meal. Dietary detail beyond the omnivorous classification is limited, so the guidance here follows the established pattern for the genus.
Mating
Like rainbowfishes generally, M. oktediensis lays eggs, and FishBase notes reproduction through distinct pairing — a receptive female pairing off with a displaying male rather than mass group spawning. Across the genus, courtship peaks in the hours after first light: dominant males flush with colour, flare their fins, and signal to females, often flickering a pale courtship band along the head and nape.
Courtship observations specific to the Oktedi rainbowfish are sparse in the accessible literature. What is reported fits the standard Melanotaenia pattern — a male displays to draw a female into fine cover, where the pair spawn. As with related species, keeping several males together tends to bring out the strongest display, since males signal hardest with rivals around.
Breeding
Melanotaenia oktediensis is an omnivorous egg-layer, and like other rainbowfishes should be a plant-spawner, attaching small adhesive eggs by fine filaments to fine-leaved vegetation or, in captivity, to synthetic spawning mops. Spawning in the genus typically runs over many days, the female releasing a few eggs at a time rather than one large clutch, with no parental care.
Breeding data specific to the Oktedi rainbowfish — clutch sizes, incubation times, fry-rearing notes — are not well documented in the accessible sources, so detailed figures should be treated as data sparse. Where breeders work with it, the genus approach applies: condition the adults well, give them fine-leaved plants or mops to spawn on, and move the eggs or mops to a separate container for rearing, since the adults will eat eggs and fry. Its medium resilience (an estimated population doubling time of 1.4 to 4.4 years) points to a reasonable reproductive capacity under good conditions.
In the aquarium
Reported aquarium guidance for M. oktediensis describes a peaceful community fish suited to a planted tank with rock and driftwood cover. FishBase cites preferred captive conditions near pH 7.3, hardness around 12, and a temperature near 73 °F — neutral, moderately hard, and on the cooler side of the tropical range, matching the species' upland-stream origins.
As with rainbowfishes generally, keep it as an active schooling group in a tank with open swimming space and some surface movement, alongside similarly sized, peaceful tankmates. Good filtration, stable water, and a varied diet keep it in condition. One caveat: this is a rare, narrowly endemic fish rather than a trade staple, so aquarium experience with it is limited and most husbandry advice is extrapolated from its better-known congeners.
Conservation
Melanotaenia oktediensis is assessed as Critically Endangered (CR) on the IUCN Red List, under criteria B1ab(iii,v), with the assessment dated 4 December 2019. The listing follows from its biology: a global range confined to one small river system — the Ok Tedi and its tributaries — leaves no resilience to local pressures, since there is no outlying population to buffer a decline.
The Ok Tedi catchment has been heavily affected by large-scale mining, and the resulting habitat degradation is the central threat. A narrowly endemic fish in a single mining-impacted basin is about as exposed as a freshwater fish gets. Australian government documentation on the genus likewise notes the species' occurrence in the Ok Tedi River and its limited distribution. Between the restricted range and the ongoing pressure on its only habitat, M. oktediensis is one of the most threatened rainbowfishes — a reminder that in a genus the hobby prizes for colour and hardiness, some members are clinging to a single vulnerable stretch of water.