Taxonomy & naming
Melanotaenia ogilbyi was described by the Dutch zoologist Max Weber in 1910, on material collected during the Dutch scientific expeditions to southern Netherlands New Guinea (now the Indonesian province of South Papua). Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia ogilbyi Weber, 1910; the species was described in Melanotaenia, where it remains, so no parenthetical authority is used. FishBase places it in the order Atheriniformes, family Melanotaeniidae — the rainbowfishes and blue eyes.
The specific epithet honours James Douglas Ogilby (1853–1925), an Irish-born ichthyologist who worked in Australia and published widely on the fishes of the Australasian region. Like most rainbowfishes, M. ogilbyi is told apart from its relatives by colour pattern and geography rather than by hard meristic characters, which overlap broadly within the genus; recorded counts are 6–7 dorsal spines, 11 dorsal soft rays, a single anal spine, and 17–18 anal soft rays. Because the species is known from so few specimens, its relationships within the southern New Guinea Melanotaenia assemblage remain only loosely resolved, and some hobby literature has noted a probable resemblance to the neighbouring Melanotaenia papuae.
Morphology
Melanotaenia ogilbyi is a small rainbowfish. The maximum recorded size is about 3 in standard length in the material examined by Allen (1991), with one hobby source citing around 3 in total length — the smaller end of the genus. Like other Melanotaenia it would have the family's compressed, fairly deep body and divided dorsal fin: a short anterior dorsal followed by a longer posterior dorsal, set ahead of a similarly long anal fin.
The live colouration of this species is poorly documented. The original description rests on preserved museum specimens, and published accounts of its pattern lean heavily on faded material rather than on detailed records of living fish, even though the species has since been collected again in the Timika region and exists in small numbers as captive-bred stock. Hobby accounts speculate that it may resemble Melanotaenia papuae but with more strongly developed dark midlateral striping — possibly merging into a single broad dark band along the flank — yet this remains inference rather than documented fact. So little reliable information exists that the species' appearance in life is data sparse, and any account of its colours should be treated with caution.
Habitat
Ogilby's rainbowfish is known from two areas of southern Papua Province, Indonesia: the lower Lorentz River drainage and, some 217 mi to the northwest, the Timika region (the Kuala Kencana–Kopi River area). The IUCN assessment characterises its habitat as small, slow-flowing rainforest streams, and notes that the species may prove more widespread across the largely unsurveyed country between Etna Bay and the Lorentz River. The Lorentz is one of western New Guinea's largest southward-flowing rivers, draining the southern flank of the central cordillera toward the Arafura Sea; the original type specimens came from its lower reaches.
FishBase lists the species as freshwater and benthopelagic, occupying a tropical latitudinal band of roughly 5–6° S. No measured water-chemistry or temperature figures are recorded for its habitat — none of the available sources give specific values — so any statement of preferred parameters would be a guess. The recorded range remains small (the IUCN estimates an extent of occurrence of about 16,311 mi²) and the easternmost, Lorentz population lies within Lorentz National Park. That narrow range, with habitat loss around Timika, drives its conservation status.
Feeding
No direct dietary study of Melanotaenia ogilbyi exists, and its feeding habits in the wild have not been documented. FishBase places it at a trophic level of about 2.9, which is consistent with the omnivorous, low-on-the-food-chain feeding typical of rainbowfishes generally.
By analogy with better-studied members of the genus, it would be expected to take small aquatic and terrestrial invertebrates — insect larvae, micro-crustaceans, and insects from the surface film — together with algae and plant detritus grazed from submerged surfaces. With effectively no aquarium history, there are no captive feeding observations to add, and the specifics of its diet remain data sparse.
Mating
Reproductive behaviour in Ogilby's rainbowfish has not been observed directly. FishBase records its reproductive style only in general terms, noting distinct pairing during courtship — a pattern shared across the rainbowfishes, in which a male courts and pairs with a female to spawn rather than broadcasting eggs en masse.
In the absence of any field or aquarium observation specific to this species, the details of its courtship — display behaviour, timing, and any colour signalling — are unknown. Other Melanotaenia typically spawn after a male displays intensely to a female in the early morning, the pair moving into fine vegetation to release eggs; whether M. ogilbyi follows the same routine is a reasonable inference but not a documented fact.
Breeding
There is no published account of Melanotaenia ogilbyi breeding, in the wild or in captivity. It is not established in the aquarium trade and appears never to have been bred under controlled conditions, so nothing is on record about its eggs, incubation period, fry, or fecundity.
Rainbowfishes as a group are egg-scattering, plant-spawning fishes that attach adhesive eggs by fine filaments to fine-leaved vegetation or spawning mops, releasing a few eggs at a time over an extended period and giving no parental care; eggs typically hatch after roughly a week to a fortnight depending on temperature. It is reasonable to suppose M. ogilbyi follows this general template, but for this particular species the breeding biology is genuinely unknown and should not be stated with false precision.
In the aquarium
Ogilby's rainbowfish has only a marginal presence in the aquarium hobby. The IUCN assessment records that it is found in the international aquarium trade, but that the fish offered come from captive-bred stock rather than ongoing wild collection (P. Unmack pers. comm. 2019); it is a specialist's rainbowfish, not a commonly seen one, and detailed published husbandry is thin.
The species would be expected to respond to the same broad husbandry that suits other small southern New Guinea rainbowfishes — a planted tank with open swimming space, a group of conspecifics so that natural display behaviour can be expressed, warm tropical temperatures, and a varied omnivore diet. But these are largely inferences from the genus rather than a well-documented body of experience specific to M. ogilbyi, which remains far better known from museum material than from the aquarium.
Conservation
Melanotaenia ogilbyi is assessed as Vulnerable (VU) on the IUCN Red List under criterion B1ab(iii), assessed on 6 December 2019 and published in 2020 (Kadarusman, Allen & Hammer). The listing reflects a small extent of occurrence (about 16,311 mi²) split across a few threat-based locations, together with a continuing decline in habitat: the species was formerly common in the Timika region (Kuala Kencana–Kopi River area) but is now considered rare there, largely because of habitat destruction tied to mining-driven population growth. The easternmost, Lorentz population lies within Lorentz National Park and faces no major present threats.
The species had previously been assessed by the IUCN as Indeterminate (1994) and Data Deficient (1996); the 2019 Vulnerable assessment supersedes those and is the current standing. The population trend is recorded as decreasing. The chief obstacle to its conservation is how little it has been studied — research into its population size and distribution is the recommended priority, and renewed field survey of the southern Papuan lowlands between Etna Bay and the Lorentz would be the necessary first step toward understanding its true range and status.