Taxonomy & naming
Melanotaenia mubiensis was described by the Australian ichthyologist Gerald R. Allen in 1996, in the journal Revue française d'Aquariologie (23(1–2): 9–16), in a paper presenting two new rainbowfishes from the Kikori River system of Papua New Guinea. The species was founded on 36 specimens, 0.5–4 in standard length, collected during a 1995 WWF biological survey of the region. The holotype (Western Australian Museum P.30984-001, 3.5 in SL) came from a side channel of the Mubi River near its junction with the Digimu River, in a limestone grotto at roughly 6°40′S, 143°30′E and about 1312 ft elevation. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia mubiensis Allen, 1996; the species was described in Melanotaenia, where it remains, so no parenthetical authority is used.
The specific epithet mubiensis is a toponym, naming the fish for the Mubi River where the type material was collected. The genus name Melanotaenia combines the Greek melas (black) with taenia (band or stripe), a reference to the dark midlateral stripe that runs along many members of the genus and is conspicuous on this species' caudal peduncle. Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia.
Allen identified M. mubiensis as most closely related to M. lacustris, the rainbowfish of Lake Kutubu, which lies about 43 mi farther upstream and drains into the same Mubi system. Large adults of the two share a similar blue coloration, but M. mubiensis is the more slender fish and has a smaller eye — shorter than or equal to the snout length, versus longer than the snout in lacustris. That eye is the readiest way to tell the two apart.
Morphology
Melanotaenia mubiensis is a comparatively slender rainbowfish, a build that distinguishes it from the deeper-bodied lacustris of nearby Lake Kutubu: males average a body depth of roughly 38% of standard length against about 47% in lacustris. It reaches a maximum recorded length of about 3.5 in standard length (the male holotype measured 3.5 in SL). Like other Melanotaenia it carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — and a laterally compressed body.
Meristic counts from the original description are: first dorsal IV–VII spines, second dorsal 13–15 rays (typically V–I,14); anal fin with one spine and 21–23 rays (typically I,21); pectoral rays 13–16; and 36–37 lateral scales. The eye is small, shorter than or equal to the snout — the character that separates the species from its larger-eyed relative lacustris.
In coloration the upper half of the body is a dusky brown to charcoal, grading to yellow-tan or whitish below, with a prominent blackish midlateral stripe that is most distinct on the caudal peduncle. The first dorsal fin is whitish with a blackish tip. Preserved specimens show these muted tones, but Allen noted that large live adults take on the blue cast characteristic of lacustris — a handsome blue fish at full size.
Habitat
The Mubi rainbowfish is known only from a small section — on the order of 12 mi — of the middle Kikori drainage in the Southern Highlands of Papua New Guinea, where every collection site is a tributary of the Mubi River. The Mubi is a primary mountain tributary of the Kikori and the outlet of Lake Kutubu, which lies about 43 mi upstream; recorded localities sit at elevations of roughly 1247–1312 ft and include Au Creek, a spring beside the Digimu River, and Ofake Creek near Kantobo.
The habitat is unusual: narrow, clear streams running through closed-canopy rainforest over limestone (karst) hills. The holotype came from a series of sinkholes linked by short tunnels to the main channel of the Mubi — a limestone grotto rather than open river. Allen recorded water temperatures of 72–74 °F at the collection stations, with clear, slow-flowing water reaching depths of about 10 ft over bottoms of rock, mud, and bedrock strewn with fallen logs. These are cool, shaded, mineral-rich waters that reflect the limestone catchment, not the warm lowland conditions many tropical rainbowfishes inhabit.
The population appears to be genetically isolated from fishes downstream: below Kantobo the river drops some 1148 ft over 4 mi through large waterfalls and rapids, a barrier that likely cuts the Mubi rainbowfish off from any populations farther down the Kikori. With the small area occupied, that isolation drives both the species' biology and its conservation concern.
Feeding
No detailed dietary study of Melanotaenia mubiensis in the wild has been published, and the cache notes that data here are sparse. FishBase places the species at a modelled trophic level of about 2.9, consistent with an omnivorous diet weighted toward small invertebrates — the general pattern across the genus, in which rainbowfishes take aquatic and terrestrial insects, small crustaceans, and some algae and plant material from the water column and submerged surfaces.
The clear, log-strewn karst streams supply drifting and surface insects, larvae among the fallen wood and rock, and small crustaceans — the sort of prey a fish of this build, feeding in the mid and upper water column, would take. Without stomach-content data, though, the specifics here are inferred from congeners rather than documented for mubiensis itself.
Mating
Reproductive behaviour in Melanotaenia mubiensis has not been studied in detail in the wild, and observations specific to the species are limited. FishBase records its reproductive mode simply as one of distinct pairing during spawning, the courtship arrangement typical of the genus, in which a male displays to and pairs with a female rather than spawning in an undifferentiated group.
Across Melanotaenia generally, the male intensifies his colour and flares his fins to lead a receptive female into vegetation or cover to spawn, with displays often most vivid in the early morning. The original description noted dimorphism that fits such a display system: males are deeper-bodied than females (average body depth about 38% of standard length versus about 31%), carry slightly more elongate posterior second-dorsal rays, and have the depressed first dorsal overlapping the second dorsal. Beyond those inferences from morphology and congeners, the pairing, timing, and spawning site for mubiensis remain undocumented.
Breeding
There is little published information on the breeding of Melanotaenia mubiensis specifically, whether in the wild or in aquaria, and the species has had only minimal presence in the hobby. What can be said follows from the genus rather than from species-level study, and is noted as such.
Melanotaenia rainbowfishes are egg-scattering, plant-spawning fishes. They attach adhesive eggs by fine filaments to fine-leaved vegetation — or, in captivity, to spawning mops — releasing a few eggs at a time over weeks rather than one clutch, and giving no parental care. The cool habitat temperatures recorded for mubiensis (72–74 °F) suggest that, if bred, it would be spawned and reared at the lower end of the rainbowfish range rather than in warm water. Clutch sizes, incubation times, and fry-rearing notes for this species are not on record, and are left unstated rather than guessed.
In the aquarium
Melanotaenia mubiensis is rare in the aquarium hobby and has never been a commercial staple; FishBase notes potential for the aquarium trade, but in practice the species is seldom encountered outside specialist rainbowfish circles. Its requirements have therefore been little documented, and what guidance exists is extrapolated from its natural habitat and from the husbandry of related highland species.
Temperature is what sets it apart from most tropical rainbowfishes: its native streams run at about 72–74 °F, decidedly cool, so it belongs at the low end of the rainbowfish range rather than in warm water. The clear, mineral-rich, limestone-influenced streams point to neutral-to-hard, alkaline water with good clarity and movement. A slender, active fish of around 3.5 in, it would — like its relatives — want a long tank with open swimming room, a group to bring out natural behaviour and colour, and similarly sized peaceful tankmates.
Given its Endangered status and tiny wild range, any aquarium stock should be captive-bred, not wild-collected. For most aquarists it stays a fish of interest chiefly for its rarity and its place alongside the better-known Lake Kutubu rainbowfish in the Kikori system.
Conservation
Melanotaenia mubiensis was assessed as Endangered (EN) on the IUCN Red List in 2019 (published 2020; Allen & Hammer) under criteria B1ab(v)+2ab(v). The species is a restricted-range Papua New Guinea endemic, known only from a roughly 20-km section of the middle Kikori/Mubi River system — the assessment gives an extent of occurrence of about 466 mi² and an area of occupancy of about 25 mi², treated as a single threat-based location.
The primary threat identified by the assessment is invasive fishes: tilapia (Oreochromis) and Common Carp (Cyprinus carpio) introduced for aquaculture, together with invasive water hyacinth (Eichhornia crassipes), are inferred to be driving a continuing decline in the population through predation, competition, disease and habitat degradation. Because these invaders move freely throughout the species' tiny range, the whole population is effectively exposed at once. Allen recorded the fish as common in the 1990s, but the current population trend is unknown. FishBase records its fishing vulnerability as low (10 of 100) and notes it is harmless to humans, but neither factor offsets the vulnerability of so narrow a range.
The most useful thing an aquarist can do is avoid creating any demand for wild-caught fish and support captive maintenance instead. As with other narrowly endemic rainbowfishes, the fish's long-term prospects rest on two things: secure aquarium populations, and protection of the karst-stream habitat in the Mubi drainage.