Taxonomy & naming
Melanotaenia mairasi was described in 2011 by Gerald R. Allen and Renny K. Hadiaty in the Journal of the Australia New Guinea Fishes Association, on the basis of material collected from Lake Furnusu in the Indonesian province of West Papua. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia mairasi Allen & Hadiaty, 2011; the species was described in Melanotaenia and remains there, so the authority carries no parentheses.
The specific epithet honours the Mairasi people, the traditional landowners of the type-locality district in Indonesian New Guinea, tying the fish to the human community of its single home water. The genus name Melanotaenia combines the Greek melas, 'black', with the Latinised taenia, 'stripe', after the dark mid-lateral band seen across much of the genus and clearly expressed in this species.
Within the genus, M. mairasi is placed in the maccullochi species group, a cluster of generally small-bodied rainbowfishes. Like most Melanotaenia, it is separated from its relatives more by colour pattern and distribution than by hard meristic characters, which overlap broadly across the group. Reported counts include a divided dorsal fin (the spinous portion with four to six spines, the soft portion with one spine and usually eleven rays), an anal fin of one to two spines and eighteen to nineteen soft rays, twelve to fourteen pectoral rays, and usually thirty-four lateral scales.
Morphology
Melanotaenia mairasi is a small rainbowfish: FishBase records a maximum of about 2 in standard length, based on a female, making it one of the more modest-sized members of its genus. The body is laterally compressed in the typical rainbowfish manner, and the fish carries the family's characteristic divided dorsal fin — a short spinous section followed by a longer soft-rayed one.
The live colour pattern, as described from freshly collected fish, is understated rather than gaudy: brownish over the back, grading to white on the belly, with a blackish mid-lateral stripe running from the eye to the base of the caudal fin. Finer detail comes from a narrow red stripe between each row of scales and orange margins on the scales themselves, giving the flanks a delicately reticulated appearance in good light. There is no dramatic two-tone or block-colour pattern of the kind seen in showier rainbowfishes.
Reliable data on sexual dimorphism in this species are sparse, since it is essentially unknown in the aquarium trade. By analogy with the genus, males would be expected to grow larger and develop more elaborate, more intensely coloured fins than females, but published figures for M. mairasi are limited, and this account keeps to what the source material records.
Habitat
Melanotaenia mairasi is endemic to Lake Furnusu, a single lake in the Bird's Neck region of West Papua, Indonesia, within the Kaimana Regency. The lake sits at roughly 49 ft elevation and lies only about 1 mi from the sea, separated from it by a low ridge perhaps 1640 ft wide. The surrounding terrain is rugged limestone karst with subterranean drainage — a geological setting that helps explain why a freshwater fish so close to the coast remains isolated in fresh water.
Within the lake, the species was collected adjacent to rocky shoreline among aquatic vegetation and the submerged roots of Pandanus. Adults were observed among dense vegetation below depths of about two to three metres; the recorded depth range for the species is roughly zero to three metres. It shares the lake with a small assemblage of other fishes, including the catfish Neosilurus brevidorsalis, the hardyhead Craterocephalus fistularis, a Mogurnda gudgeon, and the gudgeon Oxyeleotris nullipora.
The single-lake distribution is the defining fact of the species' biology. With no outlying populations, the fish has no buffer against local change; its entire global range fits inside one small karst basin in the Bird's Neck.
Feeding
Direct dietary study of Melanotaenia mairasi in the wild is limited, but its assigned trophic level of about 2.9 places it as a low-level omnivore, consistent with rainbowfishes generally. Fish of this kind feed in the upper and middle water column on small invertebrates — insect larvae and small crustaceans — together with algae and plant material grazed from submerged surfaces and the dense vegetation in which the species lives.
The habitat description, with its emphasis on aquatic plants and submerged Pandanus roots, fits a fish that gleans small prey and grazes among structure rather than one that hunts in open water. Hobby sources, drawing on experience with related rainbowfishes, treat it as a straightforward omnivore that would accept a varied diet of prepared foods alongside small live and frozen invertebrates and some plant matter. Because the species is rare in captivity, these notes should be read as reasonable expectations for the group rather than as documented observations of M. mairasi specifically.
Mating
No detailed account of courtship in Melanotaenia mairasi appears in the source material, and data here are sparse. What can be said draws on the well-documented reproductive behaviour of the genus, to which this species is expected to conform.
Rainbowfishes of the genus Melanotaenia are egg-scattering plant spawners with no parental care. Males typically court females with colour displays, flaring their fins and intensifying their markings, and in many species a pale 'courtship band' flashes along the head and nape of a displaying male, most vividly in the early morning. Spawning is generally most active in the hours after first light, with a receptive female following a displaying male into fine-leaved vegetation to deposit eggs.
Whether M. mairasi expresses the full range of these behaviours has not been documented in print. The species' high biological resilience — FishBase estimates a population doubling time of under fifteen months — implies a productive, fast-maturing reproductive strategy of the kind seen across small rainbowfishes, but the specifics for this fish remain unrecorded.
Breeding
Captive breeding of Melanotaenia mairasi is essentially undocumented, as the species is not established in the aquarium hobby. The account below therefore describes the general rainbowfish pattern that this species would be expected to follow, and should be treated as expectation rather than confirmed record.
Like its relatives, M. mairasi is presumed to be a continual or near-continuous spawner that deposits small numbers of adhesive eggs at a time over fine-leaved plants or spawning mops, the eggs hanging by fine filaments from the medium. Hobby figures for the group suggest females release a few eggs daily across an extended spawning period rather than a single clutch, and that the adults offer no care and may eat the eggs, so breeders would harvest the medium to a separate rearing container.
Incubation in rainbowfishes typically runs from roughly six to twelve days depending on temperature, after which tiny fry hang near the surface and require very small first foods — infusoria and similar — before graduating to newly hatched brine shrimp and finely crushed flake. Again, these are genus-level expectations; the source material does not provide breeding data specific to M. mairasi.
In the aquarium
Melanotaenia mairasi is rarely if ever seen in the aquarium trade, and there is little husbandry experience specific to it. Hobby profiles that do exist extrapolate from the broader rainbowfish husbandry literature rather than from established keeping of this species, so the guidance here is general and provisional.
As a small, shoaling rainbowfish it would be expected to do best in a group of six to eight or more in a planted aquarium with open swimming space, kept with similarly peaceful, similarly sized tankmates. Hobby sources suggest middle-to-upper water levels, a tolerant range of conditions around pH 7.0 to 8.0, moderate hardness, and temperatures of roughly 21 to 26 degrees Celsius, broadly consistent with the warm, karst-influenced water of its home lake. Reported aquarium maximum sizes (around 3 in) run larger than the 2 in standard length recorded by FishBase, a common discrepancy between hobby and museum measurements.
Conservation context matters more here than care details: this is a Critically Endangered single-lake endemic, and any aquarium interest should rest on responsibly sourced, captive-bred stock rather than wild collection. For most aquarists it will stay a fish to read about rather than to keep.
Conservation
Melanotaenia mairasi is assessed as Critically Endangered (CR) on the IUCN Red List, under criteria B1ab(iii). The assessment reflects the species' entire global range being confined to a single lake — Lake Furnusu — giving it an extremely restricted extent of occurrence, combined with a continuing decline in the quality of that habitat.
A fish endemic to one small karst lake has no resilience to local disturbance: there is no second population anywhere to buffer a loss. Any degradation of the lake — through changes to water quality, vegetation, or hydrology, or through the introduction of non-native species — bears directly and immediately on the whole species. The lake's position barely 1 mi from the sea, behind a narrow ridge, adds a further note of fragility to an already tiny range.
The species is not Evaluated under CITES. Because M. mairasi is not maintained in the aquarium hobby in any meaningful way, there is no captive-breeding safety net of the sort that buffers better-known rainbowfishes such as M. boesemani. Its survival depends squarely on the integrity of its one home water, so protecting Lake Furnusu itself is the central conservation concern.