Taxonomy & naming
Melanotaenia lacunosa was described in 2016 by Gerald R. Allen, Peter J. Unmack and Renny K. Hadiaty in a study of the goldiei group of rainbowfishes from the Bird's Neck (Birds Neck) region of New Guinea, published in the journal aqua: International Journal of Ichthyology. It was one of two new taxa named in that paper from the vicinity of Etna Bay, the other being Melanotaenia etnaensis, both occupying southern drainages of the western New Guinea isthmus. Eschmeyer's Catalog of Fishes records the valid name and authority as Allen, Unmack & Hadiaty, 2016; because the species was described in Melanotaenia, where it remains, no parenthetical author is used.
The species sits in the 'goldiei group', a cluster of at least six closely related Melanotaenia that share morphological and meristic characters and are told apart chiefly by genetics and by their allopatric, non-overlapping ranges rather than by anything you can see. Its identity rests heavily on collection locality and molecular data. FishBase lists the authorship as 'Allen, Unmack & Renny, 2016' — a misparsing of Hadiaty's given name, Renny — but the Catalog of Fishes form, Allen, Unmack & Hadiaty, 2016, is the one followed here.
Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes and blue eyes, which radiate across New Guinea and northern and eastern Australia. Beyond the description and Red List assessment, published detail on M. lacunosa is thin — the full description is paywalled, and most of what can be said with confidence comes from the conservation listing.
Morphology
Detailed morphological data for Melanotaenia lacunosa are sparse in publicly available sources. FishBase records no maximum-length value and lists maturity as unknown, but the IUCN assessment, drawing on the original description, gives a maximum size of at least 3.5 in SL — a modest, typically rainbowfish size. The full description in aqua, which would carry the meristic counts and detailed measurements, is not openly accessible.
What can be said is general to its placement. As a goldiei-group member, M. lacunosa shares the body form and meristics of its close relatives and is separated from them mainly by genetic and distributional evidence rather than by anything striking on the outside. Like other Melanotaenia it is a laterally compressed fish carrying the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior one — though species-specific colour and proportion details are not documented in the sources consulted.
The goldiei-group species are cryptic and best told apart by genetics, so any account of this fish's appearance is provisional. For now its morphometric detail stays largely locked in the primary description.
Habitat
Melanotaenia lacunosa is endemic to Indonesia, known only from streams in the vicinity of the Mbuta (Mbutu) Basin near Etna Bay, in West Papua Province, on the southern side of the Bird's Neck region of New Guinea. FishBase characterises the environment simply as freshwater, demersal and tropical, with a depth range to about 7 ft. The basin itself is an oval feature roughly 6 mi across that is often mapped as a lake but is in practice seasonal and swamp-like, with large natural fluctuations in water level.
The IUCN assessment gives an extent of occurrence of only about 85 km2 (a range note also cites 95 km2) and treats the species as occurring at a single threat-defined location. That pinpoint distribution — one small basin and its streams — is the central fact of the species' ecology. There is essentially nowhere else for it to live, and the habitat runs on seasonal drying and refilling rather than stable lake conditions.
The only specimens were collected from a 7–10 ft wide creek above its confluence with a small turbid river, where the water was very clear but darkly tea-stained, flowed relatively fast under an almost-closed forest canopy, and ran over a mostly muddy bottom with occasional rocks and log debris. A single field reading there gave a water temperature of 78 °F and a pH of 6.4 (IUCN assessment); hardness was not recorded. The system is lowland, tropical and subject to strong wet–dry seasonality; beyond that, the published record is thin.
Feeding
No diet study specific to Melanotaenia lacunosa was located in the sources consulted, so its feeding ecology must be inferred cautiously from the genus. FishBase places the species at a trophic level of roughly 2.9, consistent with an omnivore that takes a mix of small animal and plant material rather than a specialist predator.
Melanotaenia rainbowfishes are opportunistic omnivores that feed in the upper and middle water column on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, insects taken at the surface — along with algae and plant matter. Living in a vegetated, seasonally fluctuating basin, M. lacunosa most likely does the same, but there are no direct dietary observations for it.
So the feeding account here is generic, not species-confirmed. The FishBase trophic level is the only dietary number tied directly to the fish, and it points to a broad omnivore.
Mating
There is no published account of the reproductive behaviour of Melanotaenia lacunosa specifically. The original description and the Red List assessment do not detail courtship or spawning for this species, and no hobbyist breeding record exists because the fish is not kept in the aquarium trade.
What can be said is general to the genus. Melanotaenia rainbowfishes are egg-scattering plant spawners: males court females with colour displays, usually most intense in the morning, and pairs lay adhesive eggs among fine-leaved vegetation. Whether M. lacunosa departs from that is unknown.
Given a basin that swings hard between wet and dry, reproduction may well track the seasonal cycle, but nothing documents it and it is not asserted here. The mating biology of this species is simply unstudied.
Breeding
No captive- or wild-breeding data are available for Melanotaenia lacunosa. The species is a 2016-described, Critically Endangered, range-restricted endemic that is not established in the aquarium hobby, and no dedicated breeding profile from any rainbowfish association or hobby site could be located.
FishBase's length–weight parameters (a = 0.01122, b = 3.04) are derived figures that exist for many rainbowfishes whether or not their reproduction has been studied; they say nothing about fecundity, egg size or development. Members of the genus lay small numbers of adhesive eggs daily over long periods, the eggs hanging by fine filaments from vegetation with no parental care — but applying that template here is inference, not observation.
The breeding biology of this species is undocumented. Any future captive programme would have to be built from congeneric husbandry, not a species-specific record.
In the aquarium
Melanotaenia lacunosa is not established in the aquarium trade. FishBase notes only a theoretical aquarium potential for the species, and no care, water-parameter or stocking guidance specific to it was found on rainbowfish hobby resources such as ANGFA, rainbowfish.net or Home of the Rainbowfish.
Any husbandry notes would have to be borrowed wholesale from congeners — soft to slightly hard, neutral to alkaline water at roughly 75–82 °F, the fish kept as an active school in long, well-planted tanks. That guidance is here only to flag that it is generic; read it as a placeholder, not established practice for M. lacunosa.
Given the Critically Endangered status and pinpoint wild range, the real aquarium point is conservational rather than practical. This is a fish to be aware of and, if it ever enters managed care, to handle inside a coordinated programme rather than the general trade. For now there is no keeper experience to draw on.
Conservation
Melanotaenia lacunosa is assessed by the IUCN Red List as Critically Endangered under criterion B1ab(iii) (ver. 3.1). The assessment was completed on 5 December 2019 and published in 2020 (assessment 161099045/161099077). The listing rests on the species' extremely small extent of occurrence — about 85 km2 — and its occurrence at a single threat-defined location, the Mbuta Basin near Etna Bay in West Papua.
The population trend at assessment was reported as stable, with the species noted as common in surveys by G. R. Allen in 2010, 2014 and 2016. The threat that drives the Critically Endangered listing is not collapse already observed but vulnerability: the habitat experiences large natural fluctuations in water level, including droughts, and these are expected to worsen under prolonged dry seasons associated with climate change. The assessment also flags concerns in the categories of problematic species, genes and diseases. With its entire world range packed into one small, seasonally drying basin, the species has no outlying population to buffer a bad year.
The Red List recommends further research on population size and distribution as the priority conservation action. The formal citation is: Melanotaenia lacunosa, The IUCN Red List of Threatened Species 2020: e.T161099045A161099077.