Taxonomy & naming
Melanotaenia arguni was described in 2012 by Kadarusman, Renny K. Hadiaty, Gilles Segura, Setiawibawa, Domenico Caruso and Laurent Pouyaud, in a paper titled 'Four new species of Rainbowfishes (Melanotaeniidae) from Arguni Bay, West Papua, Indonesia', published in Cybium 36(2): 369–382. The species was one of four rainbowfishes named in that single work, all drawn from the Arguni region of the Bomberai Peninsula in far western New Guinea. Because it was described in the genus Melanotaenia, where it remains, no parenthetical authority is used; Eschmeyer's Catalog of Fishes governs the valid name.
The holotype is a male of 3 in standard length, catalogued as MZB 17715 in the collection of the Museum Zoologicum Bogoriense, collected on 1 November 2010 during the Lengguru–Kaimana Expedition. The type locality is a karst spring emerging from the Berari Anticline at the altitudinal level of Sewiki Lake, roughly 4 mi southeast of Urisa village and about 27 mi north of Kaimana, in the District of Arguni Bawah, Papua Barat. The species is allied to other rainbowfishes of the Bomberai Peninsula and the Bird's Neck and Bird's Head region of western New Guinea.
Within that regional group, no single trait pins M. arguni down; it is diagnosed by a combination of characters: a short snout (8.3–9.1% of standard length), a small eye (6.6–7.2% of standard length), and a slender caudal peduncle. The describing authors separated it from the closely similar M. ammeri by its colour pattern, smaller eye and slimmer caudal peduncle, and from M. irianjaya by its shorter snout, smaller eye and lower gillraker count.
Morphology
Melanotaenia arguni is a small, fusiform rainbowfish reaching about 3 in standard length in the type series; range-based assessments elsewhere cite a maximum of roughly 3.5 in SL. It carries the family's divided dorsal fin and the laterally compressed shape typical of the genus, but it runs slimmer than most of its relatives. Reported meristics include 4–5 dorsal spines, 12–16 dorsal soft rays, a single anal spine with 20–24 anal soft rays, 34–37 lateral scales, and 17–18 gillrakers.
The coloration is restrained. The body is generally brown dorsally, each scale outlined with a narrow dark margin, while the lower half of the side and the belly are whitish; narrow yellowish stripes run between the horizontal scale rows on the lower body. A diffuse dark blotch sits behind the eye on the upper operculum, and a dark midlateral stripe roughly one scale wide extends from the preopercle to the base of the caudal fin. The pectoral fin is translucent and the other fins are grey-tinted toward their tips.
Unusually for Melanotaenia, males of M. arguni are not strongly deeper-bodied than females and lack the pronounced elongation of the posterior dorsal and anal rays seen in most congeners. Adult male body depth is reported at 32–37% of standard length. Add the subdued palette, and the result is a plainer fish than the brightly bicoloured rainbowfishes more familiar to aquarists.
Habitat
The species is freshwater and benthopelagic, recorded from a tropical lowland setting on the Bomberai Peninsula. It is known only from the type locality region: Jasu Creek, a stream less than five metres wide that flows into the western side of Arguni Bay, West Papua Province, Indonesia. Near Egerwara village the habitat is described as shallow, slow-flowing water over a sandy substrate, set within secondary forest and cocoa plantations.
The type material came from a karst spring emerging from the Berari Anticline at the altitudinal level of Sewiki Lake, which points to a limestone-influenced catchment. A karst spring feeding a small, shallow forest creek puts the species in clear, gently flowing inland water rather than the larger lake systems some better-known rainbowfishes occupy.
The sources record no captive or in-situ measurements of water temperature, pH or hardness for M. arguni, so its water-chemistry tolerances remain data-sparse. What is clear is how little water it has: essentially a single small creek. That fact dominates its ecology and underlies its conservation listing.
Feeding
No diet study specific to Melanotaenia arguni is available, and the published sources do not detail its natural feeding habits. FishBase estimates a trophic level of about 2.9 from a model rather than from gut-content analysis, placing it among the lower-order consumers as expected for a small rainbowfish.
With no species-specific data, the safest inference comes from the genus as a whole: rainbowfishes are opportunistic omnivores that take small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects from the surface — along with algae and plant material. A small creek-dwelling Melanotaenia such as M. arguni would forage the same way in the shallow, slow-flowing water of its habitat.
In the tank the species is regarded as harmless and of aquarium potential, and would take the standard rainbowfish fare of quality flake, small pellets, and frozen or live foods, though, again, dedicated husbandry notes for this fish are thin.
Mating
No detailed courtship observations have been published for Melanotaenia arguni; this part of its biology is data-sparse. The species reaches sexual maturity at around 1 in standard length at roughly six months of age, a figure recorded from aquarium observation.
Because males of M. arguni are not strongly dimorphic — they lack the deep body and extended fins, and females are merely paler — the showy courtship of the most colourful rainbowfishes is presumably toned down here. Across the genus, displaying males court females with colour and fin signalling, usually most intense in the early morning, and pairs spawn into fine vegetation; M. arguni most likely follows suit.
The available material does not describe how pairs form, display, or time their spawning in this species, so treat any account of its mating behaviour as provisional.
Breeding
No breeding study dedicated to Melanotaenia arguni has been published, and species-specific egg and larval measurements are not available. The species has reportedly been farm-raised in Indonesia, which indicates it can be bred in captivity, but the published record does not give its clutch sizes, incubation times, or rearing details.
What can be stated is drawn from the genus generally and is not specific to M. arguni: rainbowfishes are egg-scatterers whose eggs are clear to light amber, mostly around 0.5 in in diameter, and bear sticky adhesive filaments that anchor them to fine-leaved plants or spawning mops. Larvae hatch at roughly 0.5 in, and the fry are difficult to identify to species until they reach about 1 in. Adults offer no parental care.
Given those genus-level patterns and the reported sexual maturity at about 1 in SL at around six months, M. arguni is most likely a continuous egg-scattering plant-spawner like its relatives. With no direct breeding report for the fish itself, though, that stays an inference, not a documented account.
In the aquarium
Melanotaenia arguni is described as harmless and of aquarium potential, and it has reportedly been farm-raised in Indonesia, but it is not an established hobby fish and detailed husbandry guidance for the species is genuinely sparse. No specific captive water temperature, pH or hardness values are recorded for it in the available sources.
Without species-specific care notes, a keeper extrapolates from general rainbowfish husbandry: an active, peaceful schooling fish best kept in a group, given a long open swimming lane, clean well-filtered water, and a varied omnivorous diet. Its smaller size — around 3–3.5 in — and plainer, slimmer build make it a quieter fish to look at than the brightly bicoloured rainbowfishes that dominate the trade.
Since precise water parameters for M. arguni are not documented, an honest aquarium account flags that gap rather than inventing numbers. This is a little-kept species whose captive requirements are inferred from the genus, not earned through keeping it.
Conservation
Melanotaenia arguni was assessed by Kadarusman and G.R. Allen and published on the IUCN Red List in 2020 (assessment dated 3 December 2019) as Endangered (EN) under criterion B1ab(iii), version 3.1; the assessment carries the identifier e.T161082638A161082652. The listing rests on the species' extremely restricted distribution: an extent of occurrence of about 137 mi², a single known location, and a continuing decline in habitat quality.
The species is known only from one small creek flowing into the western side of Arguni Bay. At the type locality the population is reported as common, with the trend assumed stable, but a single-location species in a habitat of shallow, slow-flowing creek water amid secondary forest and cocoa plantations has essentially no buffer against local change. The assessment notes that some individuals show signs of malnutrition possibly linked to the nearby plantation, and it flags gas drilling in the region as a further pressure.
No conservation actions are in place, and the assessment recommends research on the species' population and distribution. The central conservation fact is the same one that defines its biology: it is known from a single creek, and there is essentially nowhere else for it to be.