Taxonomy & naming
Melanotaenia boesemani was described by Gerald R. Allen and Norbert J. Cross in 1980, in a revision of the rainbowfishes of the genus Melanotaenia published in the Records of the Western Australian Museum. The type locality is the Ajamaru (Ayamaru) Lakes region of the Vogelkop (Doberai) Peninsula in what is now the Indonesian province of West Papua. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia boesemani Allen & Cross, 1980; because it was described in Melanotaenia, where it remains, no parenthetical authority is used.
The specific epithet honours the Dutch ichthyologist Marinus Boeseman (1916–2006) of the Rijksmuseum van Natuurlijke Historie in Leiden, who collected extensively in New Guinea and whose material underpinned much subsequent rainbowfish work. The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia. M. boesemani belongs to a group of Vogelkop-region species and is most readily distinguished from its relatives by the adult male's two-tone colour pattern rather than by hard meristic characters, which overlap broadly within the genus.
Morphology
Boeseman's rainbowfish is a moderately deep-bodied, laterally compressed rainbowfish reaching around 4.5 in standard length in males, with females and most aquarium specimens somewhat smaller. Like other Melanotaenia it carries the family's characteristic divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — set against a deep, almost diamond-shaped body that becomes deeper with age, particularly in dominant males.
The colour pattern is the species' signature. Mature males show a striking front-to-back gradient: the head and anterior body are bluish-grey to purplish, often with a steel or greenish sheen, while the posterior body and tail region glow orange to brick-red, with the two regions divided by a roughly vertical boundary near mid-body. A series of narrow dark and pale vertical bars may show through this base colour. Intensity depends on mood and condition — a displaying male flushes far brighter than a stressed or subordinate one — and on diet and lighting in captivity, where line-bred strains can be especially vivid. Females and juveniles are far plainer: a silvery to yellowish fish with subdued markings, the usual sexual dimorphism of the genus.
Habitat
Melanotaenia boesemani is endemic to the Ayamaru (Ajamaru) Lakes and associated waters of the Vogelkop Peninsula in West Papua, Indonesia, at an elevation of roughly 328 ft. This is a small, shallow, interconnected lake-and-stream system on a karstic plateau. The species occupies clear to slightly turbid still and slow-flowing water among aquatic vegetation, submerged roots, and marginal cover.
Reported water conditions for the Ayamaru system are warm and hard-to-alkaline relative to many lowland rainforest habitats: temperatures typically in the mid-20s Celsius, and unusually high pH and conductivity for a rainbowfish, with pH reported around 7 to 9 in parts of the lakes. The alkaline, mineral-rich water reflects the limestone catchment. That restricted range — a handful of lakes and their feeder streams — is the central fact of the species' biology and the reason its conservation status is so precarious: there is essentially nowhere else for it to be.
Feeding
Like rainbowfishes generally, Boeseman's rainbowfish is an opportunistic omnivore that feeds in the upper and middle water column. The natural diet comprises small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects taken from the surface — together with algae and plant material grazed from submerged surfaces and vegetation.
In the aquarium it is undemanding and accepts a wide range of foods: good-quality flake and small pellets, frozen and live foods such as Daphnia, brine shrimp, bloodworm, and Cyclops, and some vegetable matter. A varied diet with a real live or frozen component improves both colour and breeding condition; many hobbyists lean on colour-enhancing prepared foods to hold the red and blue, which fade on a flake-only diet. As a constant, active grazer it does best fed small amounts several times a day rather than one large meal.
Mating
Boeseman's rainbowfish is an egg-scattering, plant-spawning species with no parental care, and it is a continuous or near-continuous spawner under good conditions rather than a seasonal one. Males are highly social and competitive: dominant males stake out display areas and court females with intense colour displays, flaring fins and flashing the body so that the red and blue zones reach maximum saturation. Many rainbowfishes signal with a flickering pale stripe — a 'courtship band' — that flashes along the head and nape of a displaying male; it is most vivid in the morning.
Spawning is most active in the early hours after first light. A receptive female follows a displaying male into fine-leaved vegetation or spawning mops, where the pair release small numbers of eggs at a time. Because males display most strongly in the presence of rivals, the species is at its most colourful when several males are kept together, and breeders often condition groups rather than isolated pairs to bring fish into peak form.
Breeding
Melanotaenia boesemani is one of the more straightforward rainbowfishes to breed in captivity, which — together with its colour — explains its hobby popularity and the prevalence of tank-bred stock. Conditioned adults spawn readily over fine-leaved plants such as Java moss or over synthetic spawning mops, depositing small, adhesive eggs that hang by fine filaments from the spawning medium.
Spawning typically occurs over many days, with a female releasing a few eggs daily rather than a single large clutch; a productive female may yield several dozen to a few hundred eggs over a spawning period. The adhesive eggs are relatively large and, at typical breeding temperatures in the mid-to-upper 20s Celsius, hatch after roughly 7 to 12 days, the incubation time shortening with warmth. The adults will eat their own eggs and offer no care, so breeders harvest the mops or eggs to a separate rearing container. The tiny fry hang near the surface at first and need very small first foods — infusoria, rotifers, or commercial liquid fry food — before graduating to newly hatched brine shrimp and finely crushed flake. Growth is slow, and the adult colours come in slower still: males do not reach full two-tone until well into their second year.
In the aquarium
Boeseman's rainbowfish is an active, peaceful, sociable schooling fish best kept in a group of six or more, ideally with a balanced or male-skewed mix so that males display against one another and stay colourful. It is a strong, restless swimmer and needs a tank with a long, open swimming lane; an aquarium of around 47 in or more in length suits an adult group, given the fish's size, activity, and the desirability of keeping a proper school.
Water chemistry should follow the species' hard, alkaline origins rather than the soft-water default many aquarists assume for tropical fish: it does best in neutral to distinctly alkaline, moderately hard water, in the low-to-mid 20s Celsius. Stable conditions and good filtration matter more than precise numbers, and the fish tolerates a fairly wide range. A planted tank with open central swimming space, some surface movement, and bright lighting shows the colours off and brings on the morning display. Tankmates should be similarly sized, peaceful, and not so timid that they get outcompeted at feeding — other rainbowfishes, larger tetras, peaceful barbs, and rasboras all work. Hardy and long-lived for its size, often five years or more; but males need time and good conditions to colour up fully, so patience is part of keeping them well.
Conservation
Melanotaenia boesemani is assessed as Endangered (EN) on the IUCN Red List. The listing reflects the species' extremely restricted natural range — essentially the Ayamaru Lakes system and adjacent waters of the Vogelkop Peninsula — combined with documented pressures on that habitat. Heavy collection for the international ornamental-fish trade during the 1980s and 1990s removed very large numbers of wild fish, and the population was further stressed by habitat degradation and by the introduction of non-native fishes into the lake system, which prey on or compete with the rainbowfish.
A narrowly endemic species in a single small lake basin has little to fall back on: there is no large outlying population to buffer local declines. The one bright spot is how easily it breeds. Most Boeseman's rainbowfish in the trade today is tank-bred rather than wild-caught, which eases collecting pressure on the wild population and keeps the species widely held in captivity. Sourcing captive-bred stock is the simplest thing an aquarist can do for it, and its threatened status is exactly why hobbyist and association breeding programmes take the species on.