Taxonomy & naming
Melanotaenia manibuii was described in 2015 by Kadarusman, Slembrouck and Pouyaud, in a paper by Nugraha, Kadarusman, Hubert and colleagues that introduced eight new rainbowfishes (family Melanotaeniidae) from the Bird's Head Region of West Papua, Indonesia (Cybium 39(2): 99–130). The authors combined molecular and morphological evidence — a cytochrome oxidase I (COI) phylogeny, twelve microsatellite loci, and classical morphometrics and meristics — to delimit the new species from one another and from their congeners. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia manibuii; because the species was described in Melanotaenia, where it remains, no parenthetical authority is used.
Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia. The Bird's Head (Vogelkop, or Doberai) Peninsula is a hotspot of rainbowfish diversity, where many narrowly endemic species are each tied to a particular stream or lake basin. M. manibuii belongs to this Bird's Head assemblage. No single dramatic trait sets it apart; it is diagnosed by a combination of meristic and proportional characters.
The specific epithet honours Alfons Manibui, the Bupati (regency head, a local government leader) from Bintuni, the town near the type locality — a patronym recognising regional support for the work. The genus name combines the Greek melan (black or dark) with the Latin taenia (band or stripe), a reference to the dark lateral band seen in many members of the genus.
Morphology
Melanotaenia manibuii is a small rainbowfish; the type material reaches about 4 in in standard length. As in other members of the genus, the body is laterally compressed and the dorsal fin is divided into a short anterior portion and a longer posterior portion. The original description gives fin counts of roughly IV–V spines plus I,13–16 rays in the dorsal fin and I,22–25 rays in the anal fin, with 35–37 lateral scales and 18–19 gill rakers.
Several proportional features help separate it from related Bird's Head species: a relatively short head (about 24.0–25.1% of standard length), a small eye (about 6.8–8.0% of standard length), and a comparatively deep caudal peduncle. The species also lacks the dark stripes along the margins of the dorsal and anal fins seen in some congeners, and its body is notably compressed in cross-section.
The most telling feature of M. manibuii is morphological rather than meristic. In most rainbowfishes males are markedly more colourful and deeper-bodied than females; this species shows little or no obvious sexual dimorphism, and that departure from the genus norm was one of the diagnostic points the describing authors flagged. Live-colour descriptions are thin for this obscure, recently described fish, so colour is not elaborated here beyond what the original account supports.
Habitat
Melanotaenia manibuii is endemic to the Bird's Head (Vogelkop) region of West Papua, Indonesia, and is known from the type locality: a stream flowing through secondary forest over a gravel and sandy bottom, near Bintuni on Teluk Bintuni (Bintuni Bay). It is a freshwater, benthopelagic species of tropical lowland waters.
Clear, forest-shaded streams over gravel and sand describe many Bird's Head rainbowfish habitats, where small endemics hold to the slow-to-moderate flow among submerged structure and marginal cover. Specific physico-chemical data — temperature ranges, pH, and hardness measured in the field — are not recorded in the sources available for this species, so its water chemistry can only be inferred loosely from regional norms, not stated as fact.
What governs the species' biology is the narrowness of its range. Known only from a small area near Bintuni, M. manibuii has nowhere else to go, which is why any pressure on its single habitat translates straight into a precarious conservation status.
Feeding
No diet study specific to Melanotaenia manibuii has been published; FishBase lists no recorded food items, and its trophic level is only an estimate (around 2.9). What can be said is drawn from the genus. Rainbowfishes are generally opportunistic omnivores that feed in the upper and middle water column, taking small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects from the surface — along with algae and plant material.
In a forest stream over gravel and sand, a small rainbowfish like this would pick drifting and surface invertebrates and graze fine algae and detritus from submerged surfaces. No manibuii-specific feeding observations exist, so this account keeps to genus-level generalities rather than asserting particulars the record does not support.
For any aquarium context, the same caution applies: the species is not established in the hobby, so its captive feeding behaviour is undocumented and would, in practice, follow the undemanding omnivorous pattern typical of Melanotaenia.
Mating
There are no published observations of courtship or mating in Melanotaenia manibuii; FishBase's reproduction and spawning fields are empty for the species. As a result, any description must be framed as genus-typical expectation rather than recorded fact.
Melanotaenia rainbowfishes are generally egg-scattering, plant-spawning fishes with no parental care. In most species, males court females with intense colour displays in the early hours after first light, often flashing a pale courtship band along the head and nape. M. manibuii apparently lacks the strong sexual dimorphism that underpins those male displays in most rainbowfishes, which raises an open question: how does its courtship signalling differ? The sources do not answer it.
So the honest position is that the species' mating behaviour is undocumented. The genus pattern offers a reasonable default, but the reduced dimorphism is a flag that manibuii may not follow it neatly.
Breeding
No breeding data exist for Melanotaenia manibuii: there are no records of spawning, egg counts, incubation times, or fry development for this species, and it is not bred in captivity. The discussion here is therefore limited to what the genus reliably shows, offered only as context.
Rainbowfishes are generally continuous or near-continuous spawners under good conditions, depositing a few adhesive eggs at a time onto fine-leaved plants or spawning mops, the eggs hanging by fine filaments from the medium. Spawning runs over many days rather than as a single clutch. The adults give no parental care and will eat their own eggs, and the tiny fry need very small first foods before graduating to larger fare. Incubation in the genus commonly runs on the order of one to two weeks, shortening with warmth.
Whether M. manibuii follows this pattern is unknown. Recently described, endangered, and absent from the hobby, its reproductive biology is one of the clearest gaps in the record — data sparse, and best left so rather than filled with guesses.
In the aquarium
Melanotaenia manibuii is not established in the aquarium hobby. No species-specific care, breeding, or habitat guide could be located for it; the only available husbandry material is generic, genus-level rainbowfish guidance written for common species such as M. boesemani. As a recently described (2015), narrowly endemic, and Endangered fish, it is essentially absent from the trade, and what little can be said about keeping it is extrapolated from the genus rather than observed in this species.
As a rule, Melanotaenia rainbowfishes do best as active, peaceful, mid-water schoolers kept in groups, in a long tank with open swimming space and good filtration. Most do well in warm water in the mid-20s Celsius and tolerate a fairly wide chemistry range, though the right parameters for a Bird's Head forest-stream species are undocumented and should not be stated with false precision. Given the conservation status, the most useful aquarium point here is the ethical one: there is no captive-bred supply, and a narrowly endemic Endangered fish is no candidate for casual collection.
Conservation
Melanotaenia manibuii is assessed as Endangered (EN) on the IUCN Red List, under criterion B1ab(iii). The assessment was carried out on 5 December 2019 (published 2020). The listing reflects the species' very restricted distribution — a small extent of occurrence around its single known locality near Bintuni — together with a continuing decline in the quality of its habitat.
A species confined to one small area has little resilience. There is no large outlying population to buffer local losses, so degradation of the type-locality stream and its catchment bears directly on the whole species. Australian regulatory material (a DCCEEW draft terms-of-reference document) also notes the species' limited distribution, corroborating the concern. It is not evaluated under CITES.
Unlike widely traded rainbowfishes, M. manibuii has no captive-bred safety net in the hobby to offset wild pressures. That leaves habitat protection in the Bird's Head region as the practical key to its survival, and explains why a fish this small and this little-known is still treated as Endangered despite how recently it entered the scientific record.