Taxonomy & naming
Gerald R. Allen, Peter J. Unmack and Renny K. Hadiaty described Melanotaenia grunwaldi in 2016, one more name in the ongoing revision of New Guinea rainbowfishes — a fauna that has grown fast as molecular work and fresh fieldwork pry apart long-confused populations. Eschmeyer's Catalog of Fishes, the authority for valid names, records it as Melanotaenia grunwaldi Allen, Unmack & Hadiaty, 2016. It was described in Melanotaenia and remains there, so the authority carries no parentheses.
The epithet honours Norbert Grunwald, a German aquarist long tied to the European rainbowfish hobby and its specialist study group, for his work keeping and studying the family. The genus name pairs the Greek melas (black or dark) with the Latinised taenia, a band or stripe — a nod to the dark lateral markings common across the group. Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes and blue-eyes radiation of New Guinea and Australia, and M. grunwaldi is one of many regional forms from western New Guinea. Later ichthyological literature treated comparative material further, including Hadiaty, Rahardjo and Allen's 2019 paper in the Jurnal Iktiologi Indonesia.
Morphology
Grunwald's rainbowfish is small and laterally compressed, reaching at least about 3 in in standard length. It carries the family's divided dorsal fin — a short first dorsal followed by a longer second — set on a moderately deep body that deepens with age in dominant males.
The feature most often cited to identify it is the broad yellowish area on the body. That colour zone helps separate it from neighbouring Melanotaenia in the western New Guinea complex, where many species overlap closely in fin-ray and scale counts and are told apart mainly by adult male colour. Colour intensity tracks mood, dominance and condition: a displaying male is far brighter and more saturated than a stressed or subordinate one. Beyond that yellow zone and the general rainbowfish build, finer morphometric and meristic detail is thin in readily available sources and is best taken from the original 2016 description.
Habitat
Melanotaenia grunwaldi is a freshwater fish endemic to western New Guinea, Indonesia. The Catalog of Fishes records the type locality as the Topo River in Papua Province — the Cenderawasih Bay drainage of the central north coast — at roughly 3°27'S, 135°33'E. FishBase classifies it as a tropical, benthopelagic freshwater species, which puts it in the slow-flowing streams, creeks and associated still waters typical of the region's rainbowfish fauna rather than open lakes or fast torrents.
Western New Guinea's rainbowfishes generally occupy clear to slightly turbid lowland and foothill streams, living among aquatic vegetation, submerged roots and marginal cover in warm tropical water. Measured water chemistry for M. grunwaldi's type locality is not in the cached sources, so treat any temperature, pH and hardness figures as data sparse. The genus-level expectation is warm tropical freshwater in the low-to-mid 20s Celsius, with chemistry running anywhere from soft and slightly acidic to harder and more alkaline depending on local geology. Its restricted range — shared by many narrowly endemic New Guinea rainbowfishes — bears on both its biology and its conservation status.
Feeding
No species-specific feeding study for Grunwald's rainbowfish exists in the cached sources. FishBase estimates its trophic level at roughly 2.9 from size and close relatives, a value pointing to a general omnivore rather than a specialist.
Melanotaenia are opportunistic omnivores that forage in the upper and middle water column, taking small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, insects fallen to the surface — alongside algae and plant material grazed off submerged surfaces. With no grunwaldi-specific observations, that genus pattern is the reasonable expectation in the wild, and a similarly varied diet suits it in captivity.
Mating
No direct observations of courtship in Melanotaenia grunwaldi appear in the cached sources, so its behaviour is read off the well-documented, conservative habits of the genus rather than species-specific study.
Melanotaenia are egg-scattering, plant-spawning fishes with no parental care. Males are social and competitive — dominant fish stake out display areas and court females with heightened colour, flared fins and active display. In many species a flickering pale 'courtship band' flashes along the head and nape of a displaying male, brightest in the morning, and spawning tends to peak in the hours after first light. By that pattern, Grunwald's rainbowfish should display hardest with rival males present and chase receptive females into fine vegetation to spawn.
Breeding
The cached sources hold no grunwaldi-specific breeding account, so what follows is the standard Melanotaenia pattern — the appropriate expectation for the species until species-level data are published.
Given good conditions, these rainbowfishes spawn continuously or nearly so, dropping small adhesive eggs that hang by fine filaments from fine-leaved plants or synthetic mops. A female releases a few eggs a day over a long spawning run rather than one big clutch. The eggs hatch in roughly a week to a week and a half at typical tropical temperatures, sooner in warmer water. Adults give no care and will eat their own eggs and fry, so breeders lift the spawning medium to a separate rearing container. The tiny fry need very small first foods — infusoria, rotifers or commercial liquid fry food — before graduating to newly hatched brine shrimp. Clutch sizes, egg dimensions and hatch times for M. grunwaldi itself are undocumented in the cached material and should be regarded as data sparse.
In the aquarium
FishBase notes aquarium potential for Melanotaenia grunwaldi, but the cached research turned up no species-specific care or breeding page; hobby searches returned only generic Melanotaeniidae guidance. So its requirements are best approached at the genus level.
These are active, peaceful, sociable schooling fish. They do best in groups of six or more in a long tank with open swimming space, where the males display against one another and hold their best colour. They are hardy and adaptable across a range of stable conditions, generally thriving in warm tropical freshwater in the low-to-mid 20s Celsius with clean, well-filtered water and some surface movement; soft to moderately hard, slightly acidic-to-neutral water suits the western New Guinea stream species. Peaceful tankmates of similar size — other rainbowfishes, larger tetras, peaceful barbs and rasboras — are natural companions. With no measured tank parameters specific to M. grunwaldi, treat exact figures as provisional and put stable, clean water ahead of precise numbers.
Conservation
The IUCN Red List assesses Melanotaenia grunwaldi as Vulnerable (VU) under criterion B1ab(iii), an assessment dated 5 December 2019. A B1 listing turns on a restricted extent of occurrence combined with evidence of decline — here, a continuing decline in habitat quality — and reflects the narrow range typical of western New Guinea rainbowfishes, which leaves them little buffer against local pressures.
A narrow-range endemic confined to a single stream system has limited resilience to habitat damage, so the Vulnerable listing signals real concern even though M. grunwaldi is far less traded than long-established hobby fish like Boeseman's rainbowfish. It is among the Melanotaenia taxa reviewed in Australian biosecurity import assessments, which noted its maximum body size and diagnostic features. As with other threatened rainbowfishes, buying captive-bred stock where it turns up, rather than wild-caught fish, is the most direct way the hobby keeps off a restricted wild population.