Taxonomy & naming
Melanotaenia sikuensis was described in 2015 as one of eight new rainbowfishes from the Bird's Head and Bird's Neck region of West Papua, Indonesia, in a paper by Nugraha and a team of collaborators published in the journal Cybium (volume 39, issue 2). Eschmeyer's Catalog of Fishes — the authority for valid fish names — records the species' authorship as Kadarusman, Sudarto & Pouyaud, 2015, with the type locality given as Siku Creek on the Bird's Neck Isthmus. The species was placed in Melanotaenia, the largest genus of the family Melanotaeniidae, and remains there, so its name carries no parenthetical authority.
The genus name Melanotaenia combines Greek and Latin roots meaning 'black stripe', a reference to the dark lateral band typical of many members of the genus. The specific epithet sikuensis is a toponym formed from the type locality, the Siku Creek, with the Latin suffix -ensis ('belonging to'). The 2015 description set M. sikuensis against its regional congeners on a combination of fin-ray and scale counts and, most usefully in the field, on colour pattern; the authors placed it within the 'Southern Birds Head' species group and the 'Birds Neck' species cluster, genetically allied to M. angfa, M. parva, M. naramasae and M. rumberponensis.
The Bird's Head and Bird's Neck region is a centre of rainbowfish diversification, with many short, isolated drainages each tending to hold their own forms. M. sikuensis is part of that radiation. Like most of its neighbours, it is distinguished from close relatives more reliably by live colour pattern and locality than by hard meristic characters, which overlap broadly across the genus.
Morphology
Melanotaenia sikuensis is a small rainbowfish; the original description records a maximum size of about 3.5 in standard length, based on the type specimen. (FishBase flags this figure as resting on the type material and likely to be refined by future collections, so it is best read as an approximate upper bound rather than a firm maximum.) In overall build it follows the Melanotaenia plan — a laterally compressed, moderately deep body bearing the family's divided dorsal fin, a short anterior dorsal followed by a longer posterior one, with the head making up roughly a quarter of the standard length.
Meristically, the original description gives the divided dorsal fin as four to five spines in the first part plus one spine and fourteen to fifteen rays in the second, an anal fin of one spine and twenty to twenty-three rays, thirty-four to thirty-six lateral scales, and thirteen to fifteen gill rakers. Head length falls in the range of about 24.7 to 26.1 percent of standard length.
The most useful diagnostic feature is colour pattern. M. sikuensis carries a continuous blue midlateral stripe, two to three scale rows wide, running from the pectoral region to about mid-body and widening to three or four scale rows over the caudal section. Unlike several of its Bird's Head congeners, it lacks dark stripes along the margins of the anal and dorsal fins — the clean fin margins combined with the unbroken, rearward-broadening blue stripe set it apart. As is typical of the genus, full colour is expected to be most intense in displaying males, though species-specific dimorphism data for this recently described fish are sparse.
Habitat
Melanotaenia sikuensis is known from Siku Creek, a small stream on the Bird's Neck Isthmus of West Papua, Indonesia (type locality at roughly 1°46.8'S, 134°04.8'E), described by FishBase as a typical karstic-environment aquatic system. The type material was collected from this clear tropical freshwater creek, and the species has not been reliably reported from elsewhere — its known distribution is essentially that single drainage.
Karstic streams of this kind, draining limestone country, tend toward clear, mineral-rich water, and the catchment geology is the dominant ecological fact about the species' home. The fish is benthopelagic, occupying the water column over the stream bed in the manner usual for the genus. Beyond the type-locality description, detailed in-situ measurements of temperature, pH, and hardness specific to Siku Creek are not published in the sources at hand, so precise water-chemistry figures are best treated as data sparse and inferred from the karstic setting and genus-typical conditions.
That narrow range — a single small creek — underpins the conservation listing. A fish confined to one stream has essentially nowhere to fall back on.
Feeding
No species-specific feeding study has been published for Melanotaenia sikuensis, so its diet is inferred from the genus. FishBase places it at an estimated trophic level of about 2.9, consistent with an omnivore, based on its relatives. Rainbowfishes of the genus Melanotaenia are opportunistic feeders that forage in the upper and middle water column, taking small aquatic and terrestrial invertebrates — insect larvae, tiny crustaceans, and surface-fallen insects — alongside algae and plant material grazed from submerged surfaces.
In a small, clear karstic creek, drifting and surface-borne invertebrates together with periphyton grazed from rock and vegetation would be the expected food base. Nothing in the available record suggests M. sikuensis departs from this generalist pattern.
In captivity — though the species is rarely kept — it would be expected, like its congeners, to accept a varied omnivorous diet of good-quality flake and small pellets supplemented with frozen and live foods such as Daphnia, brine shrimp, and small bloodworm, plus some vegetable matter. This husbandry is genus-typical and not specifically documented for M. sikuensis.
Mating
Reproductive behaviour has not been documented specifically for Melanotaenia sikuensis, and what follows is genus-typical rather than species-confirmed. Rainbowfishes of the genus Melanotaenia are egg-scattering, plant-spawning fishes that, given good conditions, breed more or less continuously rather than in a single seasonal burst. Males are social and competitive, staking out small display areas and courting females with heightened colour.
A hallmark of courtship in many rainbowfishes is a flickering pale 'courtship band' that flashes along the head and nape of a displaying male, brightest in the early hours after dawn. Display intensity rises in the presence of rival males, so the genus tends to be at its most colourful in groups rather than as isolated pairs.
Whether M. sikuensis follows this template in every detail is unrecorded. Given its placement within Melanotaenia and the absence of any contrary observation, the morning-peaked, group-driven display-and-spawn pattern of the genus is the reasonable expectation, but it should be read as inference rather than as a documented account of this species.
Breeding
There is no published captive-breeding account for Melanotaenia sikuensis — it is an obscure, recently described Bird's Neck endemic with no notable presence in the aquarium trade — so this section describes the genus-typical reproductive mode rather than confirmed observations of this fish.
Melanotaenia rainbowfishes are egg-scattering plant-spawners with no parental care. Conditioned adults deposit small, adhesive eggs that hang by fine filaments from fine-leaved plants or synthetic spawning mops, a female releasing a few eggs at a time over many days rather than a single clutch. At typical rainbowfish breeding temperatures the eggs hatch after roughly a week to a week and a half. Adults take no care of eggs or fry and will eat both, so breeders of related species harvest the spawning medium to a separate rearing container; the tiny fry hang near the surface at first and need very small first foods such as infusoria or rotifers before graduating to newly hatched brine shrimp.
Until M. sikuensis is bred and documented in captivity, these particulars stand as expectations drawn from its relatives. The species' own clutch sizes, incubation times, and fry development remain unrecorded in the sources at hand.
In the aquarium
Melanotaenia sikuensis is essentially absent from the aquarium hobby. Described only in 2015 and confined to a single karstic creek on the Bird's Neck Isthmus, it has no established trade presence, and no species-specific care or breeding pages exist for it; hobby searches turn up only genus-level rainbowfish guidance and other, better-known Melanotaenia. FishBase notes only a 'potential' for aquarium and fisheries use, not an actual fishery or trade.
Were it kept, the husbandry would follow general Melanotaenia practice: an active, peaceful schooling fish, best held in a group with a long, open swimming lane, fed a varied omnivorous diet and given stable, clean water. Its small adult size — under about 3.5 in — makes it a modest rainbowfish by the standards of the genus. Because the Siku River is a karstic, limestone-influenced system, neutral to alkaline, mineral-rich water is the most plausible starting point, but published in-situ chemistry for this species is lacking, so any captive parameters would be inferred rather than documented.
Given its Vulnerable conservation status and narrow wild range, M. sikuensis is best regarded as a fish for documented conservation or specialist breeding contexts rather than the general trade, and any keeping should rest on responsibly sourced stock.
Conservation
Melanotaenia sikuensis is assessed as Vulnerable (VU) on the IUCN Red List, listed under criterion D2 — the category applied to species whose population is acutely restricted in its area of occupancy or number of locations, leaving it liable to be pushed to a far worse status by a single plausible threat. The listing reflects the species' very narrow known range: essentially Siku Creek and its immediate karstic system on the Bird's Neck Isthmus of West Papua.
A fish confined to one small creek has little buffering capacity. Localised disturbance of a single karstic catchment — through land-use change, water abstraction, pollution, or the introduction of non-native fishes, pressures that have damaged other West Papuan rainbowfish habitats — could affect a large share of the entire global population at once. The D2 listing formalises exactly this fragility of a single-location endemic.
Because the species is recently described and barely present in the aquarium trade, collecting pressure is not the central concern; the dominant risk is the integrity of its one known habitat. Continued survey work to establish whether M. sikuensis occurs in additional drainages, and protection of the Siku system itself, are the measures most relevant to its long-term security.