Taxonomy & naming
Melanotaenia flavipinnis was described in 2014 by Gerald R. Allen, Renny K. Hadiaty and Peter J. Unmack, in the journal aqua, International Journal of Ichthyology (volume 20, number 1, pages 35–52). The type material was collected on Misool Island in West Papua Province, Indonesia, and the paper that named the species is the source for its size, fin-ray counts, and live colouration. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia flavipinnis Allen, Hadiaty & Unmack, 2014; because it was described in Melanotaenia, where it remains, no parenthetical authority is used.
The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes, an Australasian radiation centred on New Guinea and northern and eastern Australia. The genus name combines the Greek melan, 'black', with the Latinised taenia, 'stripe', a reference to the dark lateral band carried by many species. The specific epithet flavipinnis is Latin for 'yellow fin', chosen for the yellow dorsal, anal and pelvic fins of this species. Misool carries more than one endemic rainbowfish: M. flavipinnis shares the island with M. misoolensis, and the two are told apart on colour pattern and fine meristic and morphometric detail rather than any single diagnostic character — the usual story within this tightly knit genus.
Morphology
Melanotaenia flavipinnis is a small rainbowfish: the type series reaches about 3 in standard length in males and 3 in in females, smaller than most aquarium rainbowfishes. Adult males run slender for the genus, body depth going roughly 2.8 to 3.3 times into the standard length. Like all Melanotaenia it carries the family's divided dorsal fin — a short spinous anterior portion followed by a longer soft-rayed posterior portion — set on a laterally compressed body.
Meristic counts from the type description are diagnostic in combination rather than singly: dorsal spines 4–6, dorsal soft rays 11–14 (usually 13), anal soft rays 22–26, pectoral rays 12–14 (usually 14), lateral scales 34–36, predorsal scales 16–19, and gill rakers 16–19. In life the fish is greenish on the back, with a faint yellow stripe running between each horizontal scale row. A blue midlateral stripe marks the back half of the body, broadening to two rows on the caudal peduncle. The median fins — dorsal, anal and pelvic — are generally yellow, the feature that names the species; the caudal is a plainer greyish-brown. As with rainbowfishes generally, colour is brightest in displaying, well-conditioned males.
Habitat
The species is known only from the southeastern portion of Misool Island, in West Papua Province, Indonesia, where the type series came from two separate drainage systems: the Gam River and Ifaupan Creek, separated by only about 2 mi. Two short watercourses on one island: that restricted distribution is the defining fact of the species' biology and conservation.
Melanotaenia flavipinnis is a freshwater, benthopelagic fish of tropical streams. Detailed in-situ habitat data for this particular species are sparse, but Misool's rainbowfish habitats are typically clear streams and creeks running through dense jungle, and its co-endemic neighbour M. misoolensis is recorded from such clear forest streams. The estimated trophic level of about 2.9 is consistent with a small omnivore feeding low in the food web. Beyond these points the published record is thin, and finer details of water chemistry, flow and seasonality on Misool should be treated as data sparse rather than asserted.
Feeding
No species-specific feeding study is available for Melanotaenia flavipinnis, but its estimated trophic level of roughly 2.9 places it among the small omnivores typical of the genus. Rainbowfishes of this size are generalist micropredators and grazers, taking small aquatic and terrestrial invertebrates — insect larvae, tiny crustaceans, and insects fallen to the surface — together with algae and plant material gleaned from submerged surfaces.
Should anyone keep it, congeneric experience suggests it would be undemanding — good-quality flake and small pellets alongside frozen and live foods such as Daphnia, brine shrimp and small bloodworm, with some vegetable matter. An active grazer like this does best on small, frequent feedings rather than one large meal. These feeding notes are inferred from related rainbowfishes and the species' trophic level, not from direct observation of M. flavipinnis.
Mating
Reproductive behaviour has not been documented for Melanotaenia flavipinnis specifically. Within the genus, rainbowfishes are egg-scattering plant spawners with no parental care, and males court females with intense colour displays — body and fins at full saturation, often brightest in the hours after first light.
Given the yellow median fins and blue lateral stripe of mature males, courtship in this species would presumably feature those colours at their brightest, but this is an expectation drawn from congeners rather than a recorded observation. The available record for the species — its type description and database entries — does not describe spawning, courtship signalling, or seasonal timing, so these aspects are best regarded as data sparse.
Breeding
There is no published account of captive or wild breeding for Melanotaenia flavipinnis. The species was described in 2014 from a small type series and has minimal presence in the aquarium hobby, so no breeding protocol specific to it exists.
By analogy with other small Melanotaenia, expect a continuous egg-scatterer: small adhesive eggs hung by fine filaments from fine-leaved plants or spawning mops, hatching after roughly one to two weeks at tropical temperatures, the adults ignoring them. An aquarist would likely follow the standard rainbowfish routine — condition a group, offer spawning mops, lift the eggs out for separate rearing on infusoria and other tiny first foods. All of this is inferred from congeners; the breeding biology of M. flavipinnis itself remains undescribed.
In the aquarium
Melanotaenia flavipinnis is essentially absent from the aquarium trade. As a 2014-described endemic of two short Misool drainages, it has not entered the hobby in any meaningful way, and no dedicated care or husbandry account for it could be located. The notes that follow are therefore drawn from the general requirements of small Melanotaenia and should be read as inferred rather than species-confirmed.
Small rainbowfishes like this are active, peaceful schooling fish, best kept in groups in well-oxygenated planted tanks with open swimming room, at tropical temperatures broadly in the mid-20s Celsius. A male-skewed or balanced group pushes the males to display and hold colour. Stable, well-filtered water and a varied diet matter more than precise chemistry targets, which are unknown for this species' native streams. Anyone keeping it would be working from congeneric precedent, and should treat the specifics — temperature range, hardness, tankmates — as best-guess husbandry, not established fact.
Conservation
Melanotaenia flavipinnis is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 4 December 2019. Despite its very narrow known range — two drainage systems in the southeast of Misool — the species was not judged to face an imminent extinction risk at the time of assessment, and FishBase rates its resilience as medium.
Still, an endemic confined to one island's streams has little buffer against habitat change: forest clearance, pollution, or non-native fishes loosed into such small watersheds could hit the entire global population at once. The species is harmless to humans and is noted as having some aquarium and fisheries potential, though it supports no real fishery and is barely present in the ornamental trade. Keeping an eye on Misool's freshwater habitats is the obvious safeguard for a fish whose entire known world is two creeks.