Taxonomy & naming
Melanotaenia sembrae was described in 2015 by Kadarusman, Carman and Pouyaud as part of a larger study — Nugraha, Kadarusman, Hubert and colleagues, 'Eight new species of rainbowfishes (Melanotaeniidae) from the Birds Head Region, West Papua, Indonesia,' published in the journal Cybium (volume 39, issue 2). That paper named a whole suite of Bird's Head rainbowfishes at once, the product of intensive survey work across the peninsula's karstic river systems. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia sembrae; because it was described in Melanotaenia and remains there, no parenthetical authority is used.
The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia. The Bird's Head Peninsula is a hotspot of rainbowfish endemism, many species each restricted to a single river or lake basin. M. sembrae is one of those narrow endemics, named for the Sembra River where the type series was collected.
Like most of the genus, M. sembrae is separated from its relatives less by any single feature than by a combination of meristic and morphometric characters and subtle differences in male coloration — hard counts overlap broadly across Melanotaenia. The original description gives the diagnostic fin-ray and scale counts that set it apart from the other Bird's Head species named alongside it.
Morphology
Melanotaenia sembrae is a small rainbowfish, with the type material reaching about 4 in in standard length. FishBase notes that this figure rests on the type specimen and should ideally be replaced once a larger sample is available, so the maximum size is best regarded as approximate. The body is laterally compressed, as in the rest of the genus, and the species carries a relatively pointed head with a long snout (around 9.1 to 9.7 percent of standard length), a deep caudal peduncle (about 10.7 to 11.6 percent of standard length), and a long pectoral fin (roughly 20.2 to 21.9 percent of standard length).
The meristics given in the original description are: a divided dorsal fin with IV–V spines in the first dorsal and one spine plus 12–14 rays in the second; an anal fin with one spine and 22–24 rays; 36–37 lateral scales; 10 transverse scales; 17–19 predorsal (anterior) scales; 12–15 cheek scales; and 17–18 gill rakers on the first arch. Together, these counts are what separate it from the other rainbowfishes of the Bird's Head Region.
One colour feature stands out: a distinct red margin stripe along the second (posterior) dorsal fin. Beyond that, live-colour data are thin for M. sembrae, and — as with many rainbowfishes — the fullest coloration shows in mature, displaying males rather than in preserved type material. Females and juveniles are presumably plainer, in line with the strong sexual dimorphism across the genus, but published descriptions of female colour are limited.
Habitat
Melanotaenia sembrae is known only from its type locality on the Sembra River, in the Bird's Head (Vogelkop) Region of West Papua, Indonesia. At that site it was found at depths down to roughly 4 metres, in moderate to fast-flowing, clear water running over a limestone rocky substrate. This is a rheophilic, clear-water habitat — a flowing stream over rock, not a still lake — and the limestone catchment implies hard, mineral-rich water, a recurring feature of the karstic Bird's Head drainages.
FishBase classifies the species as a freshwater, benthopelagic fish of tropical climates, with an estimated trophic level around 2.9, consistent with a small invertebrate-and-algae feeder. The single known locality is the central fact of its biology. As far as the published record shows, the species has been confirmed nowhere else, which limits what can be said about its ecological range and drives its conservation concern.
Fast, clear, limestone-bedded water sets it apart from the still, vegetated lake systems some better-known rainbowfishes inhabit. In-situ water-chemistry measurements (temperature, pH, hardness) for the Sembra River are not recorded in the cached sources, so precise figures are best treated as data-sparse rather than guessed; the limestone setting nonetheless points to neutral-to-alkaline, hard water.
Feeding
No direct dietary study of Melanotaenia sembrae is recorded in the available sources, but its estimated trophic level of about 2.9 and its placement among the rainbowfishes indicate an opportunistic omnivore that takes small aquatic and terrestrial invertebrates together with algae and plant material. In a fast-flowing, clear, rocky stream this would most plausibly mean drifting insect larvae and small crustaceans, terrestrial insects falling onto the surface, and aufwuchs — the film of algae and micro-invertebrates grazed from rock surfaces.
In captivity, where it is kept at all, it would behave like other Melanotaenia: an undemanding feeder taking good-quality flake and small pellets alongside frozen and live foods such as Daphnia, brine shrimp, bloodworm and Cyclops, with some vegetable matter. A varied diet with a live or frozen component should improve colour and condition. These are genus-level inferences, though, not documented experience with M. sembrae, which remains rare in the hobby.
Mating
There are no published reproductive or breeding observations for Melanotaenia sembrae in the cached sources — FishBase explicitly records no reproduction data for the species — so its mating behaviour can only be inferred from the well-documented patterns of the genus. Rainbowfishes are characteristically egg-scattering, plant-spawning fishes with no parental care, and males court females with intense colour displays, flaring fins and intensifying their pattern, often flashing a pale 'courtship band' along the head and nape.
Display and spawning in Melanotaenia run hardest in the early hours after first light, dominant males competing for display areas while receptive females follow a male into vegetation or other fine cover to deposit eggs a few at a time. Males show their strongest colour around rivals, so the species would likely be at its most vivid in a group rather than as an isolated pair.
Whether M. sembrae departs from this template is unknown. Its fast-flowing, rocky native habitat differs from the still, vegetated waters where rainbowfish spawning has most often been watched, which leaves open the possibility of habitat-specific spawning substrates. Without direct data, anything past the genus-level pattern is speculation.
Breeding
Captive breeding of Melanotaenia sembrae is not documented in the cached sources, and the species is uncommon in the aquarium trade, so there is no established account of rearing it. By analogy with its well-bred relatives in the genus, it would be expected to be an adhesive-egg, plant- or mop-spawner: conditioned adults depositing small, filament-anchored eggs over fine-leaved plants or synthetic spawning mops, a female releasing a few eggs daily across an extended spawning period rather than a single clutch.
In related Melanotaenia, the relatively large adhesive eggs hatch after roughly 7 to 12 days at typical breeding temperatures in the mid-to-upper 20s Celsius, with the incubation time shortening in warmer water. Adults offer no parental care and will eat their own eggs and fry, so breeders of other rainbowfishes harvest mops or eggs to a separate rearing container, then start the tiny fry on very small first foods such as infusoria, rotifers or commercial liquid fry food before moving them onto newly hatched brine shrimp.
All of this is genus-level expectation, not recorded fact for M. sembrae. Given the narrow wild range and Endangered status, any captive population would carry conservation as well as hobby interest, but the published record currently holds no breeding data specific to it.
In the aquarium
Melanotaenia sembrae is a rare fish in the aquarium hobby and is not a standard trade species, so dedicated husbandry information for it is scarce; FishBase notes only a general aquarium and minor fisheries potential. Where it is kept, it would be expected to need the same broad care as other rainbowfishes of its size: an active, peaceful, schooling fish best maintained in a group, in a tank with open swimming space and good water movement.
Its natural habitat — fast, clear water over limestone — argues for well-oxygenated, gently to moderately flowing water that is neutral to alkaline and on the harder side, mirroring the mineral-rich karstic streams it comes from. Stable, clean, oxygen-rich water matters more than precise numbers. Temperatures in the low-to-mid 20s Celsius, typical of tropical highland-edge streams, are a reasonable starting point, though specific figures for the Sembra River are not recorded.
With documented keeping experience so limited, much of this is extrapolated from the genus. Treat the species as a flowing-water rainbowfish — favouring current, oxygenation and hard, alkaline water — and you would be adding to a thin husbandry record simply by recording its behaviour and any breeding.
Conservation
Melanotaenia sembrae is assessed as Endangered (EN) on the IUCN Red List, under criterion B1ab(iii), with the assessment dated 5 December 2019. That criterion reflects a very restricted extent of occurrence — here, a single known locality — together with a continuing decline in habitat quality. The listing follows from the species' biology: a fish known from one river has essentially no buffer against any deterioration of that one place.
The Bird's Head rainbowfishes are vulnerable as a group for the same reason. Many were described together in 2015 because survey work revealed how finely partitioned the fauna is, separate species confined to individual rivers and basins. Pressures on these karstic clear-water streams — shifts in land use, water quality, sedimentation, or non-native fishes introduced elsewhere in the region — translate quickly into risk for a single-locality endemic.
The available sources show no significant captive or trade population that might buffer the wild stock, the kind that exists for more popular relatives. The conservation picture comes down to a narrow range and the continuing-decline basis of the Endangered listing. Beyond that the species remains poorly documented, and better survey and monitoring of the Sembra River would be the obvious next step.