Taxonomy & naming
Melanotaenia vanheurni was described by Max Weber and Lieven Ferdinand de Beaufort in 1922, on the basis of material collected in 1920 during Dutch zoological exploration of northern New Guinea. Eschmeyer's Catalog of Fishes treats the valid name with the original authors in parentheses — (Weber & de Beaufort, 1922) — reflecting a change of genus since the original description. The species sits in Melanotaenia, the largest genus of the family Melanotaeniidae, the true rainbowfishes that radiate across New Guinea and northern and eastern Australia.
The specific epithet is a patronym honouring Willem Cornelis van Heurn (1887–1972), the Dutch zoologist and collector who gathered the original specimens. The genus name combines the Greek melas (black) with the Latinised taenia (stripe or band), a reference to the dark lateral stripe that runs the length of the body in many species of the genus and is especially well marked in this one. Like most Melanotaenia, M. vanheurni is told from its relatives more by colour pattern and proportions than by hard meristic characters, which overlap broadly across the genus; reported counts include five to seven dorsal spines, eighteen to twenty-eight soft dorsal rays, a single anal spine, and twenty-one to twenty-six soft anal rays.
Morphology
Van Heurn's rainbowfish is among the larger rainbowfishes. FishBase records a maximum standard length of about 6.5 in in males, with females reaching roughly 4 in; older hobby accounts describe specimens approaching 8 in in total length, so this is a substantial fish by the standards of the genus. The body is fusiform and only moderately compressed — less deep-bodied than show species such as the Boeseman's or Ayamaru rainbowfishes — giving it the streamlined outline of a fish adapted to flowing water.
The colour pattern is the species' most reliable field mark. The back is brown to olive, the lower sides are yellow, and a distinct blue-black stripe runs along the mid-line of the body, bordered above and below by narrow yellow to whitish stripes. Like other Melanotaenia it carries the family's divided dorsal fin, a short anterior section followed by a longer posterior one. A breeding male intensifies and develops a feature hobbyists single out: a pulsing golden glow on the top of the head, a courtship signal akin to the flickering 'courtship band' of many rainbowfishes. Females and non-displaying fish are plainer — the usual sexual dimorphism of the genus.
Habitat
Melanotaenia vanheurni is endemic to northern West Papua (Indonesian New Guinea) and is known with certainty only from two river systems of the Mamberamo basin: the Doorman River and the Taritatu River, the latter formerly known as the Idenburg. Recorded latitudes lie just south of the equator, around 2 to 4 degrees south. It is a freshwater, benthopelagic species of tropical lowland to foothill country.
Within these systems the fish favours small, clear rainforest creeks and tributaries rather than the large, turbid main channels; hobby accounts note that it avoids the big silt-laden rivers, including the Mamberamo itself, in favour of clearer feeder streams with flow ranging from slow to swift over rock or sand bottoms. Reported water conditions are warm and slightly alkaline — temperatures of roughly 77–82 °F and pH around 7.1 to 7.5. Tied to clear, well-oxygenated tributaries of a single basin, it has drawn little fieldwork, and the data stay sparse.
Feeding
Detailed dietary studies of Van Heurn's rainbowfish are lacking, but the species is an omnivore in the manner typical of the genus, with a reported trophic level of about 2.9. Rainbowfishes of flowing rainforest streams feed opportunistically in the upper and middle water column, taking small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects fallen onto the surface — alongside algae and plant material grazed from submerged surfaces.
In captivity the species would be expected to behave as other large Melanotaenia do, accepting good-quality flake and pellet foods together with frozen and live items such as Daphnia, brine shrimp, and bloodworm, supplemented with some vegetable matter. An active fish from clear, productive water, it does best on a varied diet fed in small amounts through the day, which also keeps colour and condition up. Since it is rarely kept, feeding notes specific to it are thin, and the above reflects genus-level practice rather than data for this fish.
Mating
Reproduction in Melanotaenia vanheurni is reported as distinct pairing — the species forms pairs at spawning rather than broadcasting in a loose aggregation. This matches the broad pattern across the rainbowfishes, in which a displaying male courts a receptive female with intensified colour and fin display before the pair spawn together among vegetation.
The signal recorded for this species is that pulsing golden glow on the spawning male's head, the equivalent of the flashing nape or 'courtship stripe' of many congeners, and brightest at peak display. As in relatives, the signalling should be strongest in the first hours after light and sharpest when rival males are about. Direct observations of spawning, wild or captive, are scarce here, so most of what can be said is inferred from close relatives rather than documented for M. vanheurni itself.
Breeding
Van Heurn's rainbowfish has not been widely bred in aquaria, and at the time of the main hobby accounts it was not known to have been bred in captivity at all. As a result there is little species-specific breeding data, and what follows is the general pattern for the genus rather than confirmed observation for this fish.
Rainbowfishes of the genus Melanotaenia are egg-scattering plant spawners with no parental care. Conditioned fish spawn over fine-leaved vegetation or synthetic spawning mops, depositing small adhesive eggs that hang from the spawning medium by fine filaments, typically a few eggs released daily over an extended period rather than a single clutch. Eggs hatch after roughly a week or more depending on temperature, and the tiny fry require very small first foods — infusoria and similar — before taking newly hatched brine shrimp. Given the large adult size, any breeding effort would need correspondingly large conditioning and rearing space; with no established hobby record, these expectations stay untested for M. vanheurni specifically.
In the aquarium
Van Heurn's rainbowfish is uncommon in the aquarium trade and is not a fish most hobbyists will encounter. The accounts that exist describe it as peaceful and well suited to a species tank or a community of similarly non-aggressive companions, but stress that its large adult size — well beyond that of the popular show rainbowfishes — demands a correspondingly large aquarium with a long, open swimming lane.
Water should reflect the species' clear-stream, slightly alkaline origins: a pH around 7.1 to 7.5 and a temperature near 81 °F are the figures cited, with good filtration and water movement to mirror the flowing tributaries it inhabits. An active, schooling fish, it is best kept in a group, with tankmates chosen for peaceable temperament and a size that won't be bullied or outcompeted. So seldom maintained, it leaves husbandry advice resting on a thin base of hobby reporting and general rainbowfish practice rather than a deep store of keeping experience.
Conservation
Melanotaenia vanheurni is assessed as Least Concern (LC) on the IUCN Red List, in an assessment dated 6 December 2019 (species ID 13077). It is neither CITES- nor CMS-listed. Some older hobby sources still report the species as Data Deficient, reflecting a pre-2019 assessment; the current FishBase-linked status is Least Concern, and that more recent assessment should be regarded as authoritative.
The Least Concern listing is consistent with the species occupying clear tributaries across the Doorman and Taritatu systems within the large and comparatively remote Mamberamo basin, away from the concentrated collecting and habitat pressures that threaten narrowly endemic lake species such as the Vogelkop rainbowfishes. FishBase rates the species' resilience as medium, with an estimated population doubling time of roughly 1.4 to 4.4 years. Like much of this poorly studied fish's biology, though, the assessment rests on limited field data, and more documentation of its range and habitat would firm up confidence in the status.