Taxonomy & naming
Gerald R. Allen and Norbert J. Cross described Melanotaenia ajamaruensis in 1980, in the same revision of New Guinea rainbowfishes (Records of the Western Australian Museum) that introduced its lake-mate Melanotaenia boesemani. The type material came from specimens the Dutch ichthyologist Marinus Boeseman collected during a 1954–1955 Papua expedition. The type locality is the Ajamaru (Ayamaru) Lakes near the centre of the Vogelkop Peninsula, in what is now the Indonesian province of West Papua. The species was described in Melanotaenia and stays there, so no parenthetical authority is used; Eschmeyer's Catalog of Fishes governs the valid name.
The genus name Melanotaenia combines the Greek melas/melanos, 'black' or 'dark', with taenia, a 'band' or 'stripe', after the dark lateral band carried by many members of the genus. The epithet ajamaruensis is toponymic, marking the Ajamaru Lakes to which the fish is endemic. Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes, which range across New Guinea and northern and eastern Australia; M. ajamaruensis sits among the Vogelkop-region species that share these limestone lake habitats.
It looks much like the sympatric M. boesemani but is the smaller and plainer of the two, with smaller scales. M. boesemani is larger and carries the famous blue-front, orange-rear split. The two were described together and remain the best-known endemics of the Ajamaru system.
Morphology
This is a small rainbowfish. FishBase records a maximum length of about 4 in SL; the reproduction study by Said and colleagues (2023) cites a body size of roughly 4.5 in and notes that laboratory broodstock matured at around 3 in at about 14 months of age. As in other Melanotaenia, the body is laterally compressed and carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal. Reported meristics run to 5–7 dorsal spines and 15–19 dorsal soft rays, with a single anal spine and 21–27 anal soft rays.
Coloration, described from captive and wild fish, is reddish-brown across the back and front half of the body, fading to yellow or tan toward the tail, with horizontal red-brown lines along the rear flanks. Some individuals show a metallic blue base shading to yellow-brass, or a greenish cast with yellow longitudinal stripes. The effect is more subdued than the two-tone of M. boesemani, and the easiest way to tell it from its lake-mate is its smaller size and smaller scales rather than any single bold marking. Males are reported to grow larger than females.
Beyond this the published morphological detail is thin, and finer characters are best taken from the original Allen & Cross description rather than inferred.
Habitat
The species is endemic to the Ajamaru (Ayamaru) Lakes near the centre of the Vogelkop Peninsula in West Papua, Indonesia — a small, interconnected lake system on a limestone plateau at roughly 820 ft elevation. FishBase classifies it as freshwater, benthopelagic and tropical, with a narrow latitudinal range of about 1–2° S. This is the same karstic basin occupied by M. boesemani, and the rainbowfish turns up nowhere else.
The Ajamaru system is a clear, still to slow-flowing lake-and-stream complex in a limestone catchment, which tends to give it harder, more alkaline water than many lowland New Guinea rainforest habitats. Citable physical and chemical measurements specific to M. ajamaruensis are sparse in the sources gathered here, so precise temperature, pH and hardness figures for its native water are noted as data-sparse rather than stated firmly; conditions are broadly those of the shared Ajamaru basin.
The single-basin distribution is the central fact of the fish's biology. Confined to one small lake system, it has no outlying population to buffer local losses — which is why introduced predators and competitors in those lakes translate so directly into extinction risk.
Feeding
Dietary studies of this species are limited in the sources at hand, but FishBase places it at a trophic level of about 2.9 — the omnivorous, slightly carnivore-leaning habit typical of rainbowfishes. Like its relatives it can be expected to feed in the upper and middle water column on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, surface-fallen insects — alongside algae and plant material.
In captivity it has been reared on prepared and supplemented diets. The reproduction study by Said and colleagues found that adding Spirulina (at 3–6% of the diet) improved fecundity, fertilisation, hatching and larval survival in broodstock — source-backed evidence that varied, nutritionally enriched feeding pays off here, as it does across the genus.
Beyond that the published feeding detail is thin, and claims about wild prey selection are best kept general.
Mating
M. ajamaruensis is a schooling, plant-spawning rainbowfish, and FishBase characterises its reproduction by distinct pairing during courtship. Mating turns on male display: across the genus, dominant males court females with intensified colour and fin-flaring, and spawning is concentrated in the hours after first light. Because the fish schools, the strongest displays — as in related rainbowfishes — tend to come out when several are kept together rather than as isolated pairs.
Under controlled conditions it spawns repeatedly rather than in a single seasonal burst. In a 40-day ex-situ trial at a 1:1 sex ratio, broodstock spawned around eleven to twelve times, depositing eggs onto an artificial plant-like substrate — the continuous, multi-batch pattern behind the 'distinct pairing' habit noted on FishBase.
Natural courtship in the wild Ajamaru population is not well documented in the sources gathered here, so the account above leans on the captive reproduction work and on the well-established behaviour of the genus.
Breeding
Breeding is the best-documented part of this fish's biology, thanks to the ex-situ reproduction study by Said, Mayasari, Triyanto and Kadarini (2023). M. ajamaruensis is a plant-spawner that attaches its eggs to substrate; in the laboratory a raffia-rope artificial substrate stood in for fine-leaved vegetation. Over a 40-day trial broodstock spawned about eleven to twelve times, and the eggs incubated roughly six to seven days before hatching, with high fertilisation and hatching rates achievable in captivity.
The study reported husbandry detail breeders can use: spawning tanks of about 45 × 45 × 15.5 in, smaller hatching tanks of about 30 × 20 × 8 in, and a Spirulina-supplemented diet (3–6%) that improved fecundity, fertilisation, hatching and larval survival. As in other rainbowfishes, the eggs are adhesive and hang from the spawning medium, and there is no parental care, so eggs or substrate are best moved to a separate rearing container away from the adults.
That captive-breeding capability is more than a hobby curiosity. It underpins an active ex-situ conservation effort — the fish has been domesticated and captive-bred precisely because its wild population is in trouble.
In the aquarium
M. ajamaruensis is rare in the general aquarium trade, turning up mainly through specialist rainbowfish keepers and conservation-minded breeding programmes rather than as a routine store fish. Keep it in a group: a small schooling rainbowfish shows its social behaviour and colour most fully in numbers, and the genus does best in active shoals with open swimming space.
Given the limestone-lake origins, neutral to alkaline, moderately hard water at warm tropical temperatures is the sensible starting point, matching what works for its better-known lake-mate M. boesemani. The captive reproduction work shows the fish thrives and breeds readily in the aquarium. Precise wild water chemistry is not firmly documented in the sources here, so exact numbers are kept general — stability and good water quality matter more than chasing a specific value.
For keepers, the point of this fish is conservation. Maintaining and breeding captive stock helps preserve a Critically Endangered species whose wild range is a single threatened lake system, and sourcing captive-bred fish and supporting association breeding programmes is the most useful thing an aquarist can do for it.
Conservation
The IUCN Red List assesses M. ajamaruensis as Critically Endangered (CR) under criteria B2ab(v), in an assessment dated 4 December 2019. This supersedes an older 1996 assessment that had listed the species as Data Deficient; some literature, including the 2023 reproduction paper, still cites that outdated Data Deficient status, but the current standing is CR. The species has not been evaluated under CITES or CMS.
The driver of the listing is the same thing that defines everything else about this fish: a range confined to the single Ajamaru lake system. Within those lakes it faces pressure from introduced species — the Toraja snakehead (a Channa) and introduced carp — which prey on and compete with the native fauna and are feared to be accelerating its decline. With no population outside this one basin, it has little capacity to absorb such losses.
The response has been ex-situ conservation: the fish has been domesticated and captive-bred, and the reproduction research described above is part of that effort. Captive breeding keeps the species alive outside its threatened native range and offers a path toward eventual restocking or trade supply that does not draw on the dwindling wild population — a lifeline for a fish whose entire wild world is one small cluster of lakes.