Taxonomy & naming
Melanotaenia aruensis was described in 2015 by Gerald R. Allen, Renny K. Hadiaty, Peter J. Unmack and Mark V. Erdmann, in a paper in the journal aqua, International Journal of Ichthyology (volume 21, issue 2, pages 66–108) that treated the rainbowfishes of the Aru Islands. That study described five new Melanotaenia species from the archipelago and redescribed two earlier-named forms, M. patoti and M. senckenbergianus, both originally described by Weber. The type series of M. aruensis comprised nineteen specimens ranging from 1.5–3 in in standard length, collected from streams on Trangan and Kobroor islands; the type locality is Sungai Sin on Trangan Island. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia aruensis Allen, Hadiaty, Unmack & Erdmann, 2015; because the species was described in Melanotaenia, where it remains, no parenthetical authority is used.
The genus name Melanotaenia combines the Greek melan, 'black' or 'dark', with the Latin taenia (from Greek tainia), 'stripe' or 'band', a reference to the dark lateral band conspicuous in many members of the genus. The specific epithet aruensis is a toponym formed from the Aru Islands, the species' restricted home range, plus the Latin suffix -ensis, 'belonging to'. The Aru Islands sit on the Sahul Shelf, the shallow continental platform that links New Guinea to Australia, and their freshwater fish fauna is dominated by three families — Melanotaeniidae, Eleotridae and Gobiidae — which leaves the rainbowfishes carrying much of the region's stream ecology.
Among its congeners, M. aruensis is reported to be superficially similar to M. albimarginata, sharing a comparable body form and the whitish margin on the second dorsal fin, and the two are best separated on a combination of colour and meristic details rather than any single diagnostic character.
Morphology
Melanotaenia aruensis is a small, slender, fusiform rainbowfish. Males reach about 3 in and females about 2.5 in in standard length, making it a modestly sized member of the genus. Like all rainbowfishes it carries the family's divided dorsal fin — a short anterior first dorsal followed by a longer second dorsal — set on a laterally compressed body that is only moderately deep; the greatest body depth in an adult male is around 35 percent of standard length, slimmer than the deep, diamond-bodied appearance of species such as M. boesemani.
Meristic counts from the original description are diagnostic in combination: the dorsal fin bears 5 to 7 spines and 10 to 13 soft rays, the anal fin a single spine and 20 to 23 soft rays, and the pectoral fins 14 to 15 rays. There are 34 to 36 lateral-line scales, 15 to 18 predorsal scales, 13 to 19 cheek scales, and 17 to 18 gill rakers on the first arch.
In life the fish is grey-brown over the head, nape and anterodorsal body, while the posterior two-thirds of the upper flank is a soft bluish-grey, with each scale finely outlined in bronze and a bronze stripe running between the horizontal scale rows. The dorsal and caudal fins are semi-translucent grey, and the second dorsal fin carries a relatively broad whitish margin — the feature that recalls M. albimarginata. The overall effect is understated: an elegant, metallic-toned fish rather than a brightly two-tone one. As is usual in the genus, displaying males intensify their colours, but the published description does not detail a high-contrast nuptial pattern, so finer points of breeding colour are data-sparse for this species.
Habitat
Melanotaenia aruensis is endemic to the Aru Islands of Maluku province, Indonesia, and is known only from Trangan Island and Kobroor Island. It is a freshwater, benthopelagic fish of clear, slow-flowing forest streams. The type locality on Trangan Island lies in dense rainforest, immediately downstream of a 75-metre-long limestone tunnel through which the stream emerges; here the watercourse is roughly 10 to 12 metres wide and 0.5 to 2.0 metres deep, with relatively clear, slow-moving water over a rocky limestone bottom and abundant cover from submerged fallen tree limbs.
The Kobroor Island site is broadly similar — a forest-lined stream some 15 to 20 metres wide and 1 to 3 metres deep, again clear and slow-flowing, with the margins fringed by partially submerged terrestrial plants that provide shelter for small fishes. Both habitats are shaded, structurally complex, limestone-influenced streams rather than open lakes.
FishBase reports the species from warm, alkaline water, with a minimum recorded temperature of about 81 °F and a pH on the basic side — consistent with the limestone catchment, which mineralises and hardens the water. Precise pH and hardness figures across the species' range are not well documented, so aquarium-relevant water chemistry should be read as approximate. The narrow, two-island distribution is the defining fact of the species' ecology, typical of the fine-scale endemism that characterises Melanotaenia across New Guinea and its satellite islands.
Feeding
No detailed dietary study has been published for Melanotaenia aruensis specifically, and its feeding ecology is inferred from the genus and from a modelled trophic level of about 2.9, which places it among the small omnivores. Rainbowfishes of this kind are opportunistic feeders of the upper and middle water column, taking small aquatic and terrestrial invertebrates — insect larvae, tiny crustaceans, and insects fallen onto the surface — along with algae and organic detritus grazed from submerged surfaces.
The limestone streams it inhabits are full of submerged timber and overhanging marginal vegetation, so the species has ready access to drifting and surface-borne invertebrate prey as well as biofilm grazed from rock and wood. The abundant cover noted at both collection sites suggests a fish that forages around structure rather than in open water.
In captivity, where it is rarely kept, the species would be expected to behave like other small rainbowfishes: an undemanding omnivore accepting good-quality flake and micro-pellets supplemented with small frozen and live foods such as Daphnia, Cyclops and brine shrimp, with some vegetable content. Because direct husbandry records are sparse, these expectations are extrapolated from close relatives rather than documented for M. aruensis itself.
Mating
The reproductive behaviour of Melanotaenia aruensis has not been described in detail, and what can be said is inferred from the well-studied breeding biology of the genus. Rainbowfishes are egg-scattering, plant-spawning fishes with no parental care, and most are continuous or near-continuous spawners under favourable conditions rather than strict seasonal breeders.
In related species, dominant males establish display areas and court females with intensified colour and flared fins, often flashing a pale 'courtship band' along the head and nape; this signalling and the resulting spawning activity peak in the early hours after first light. Males display most vigorously in the presence of rivals, so the genus is generally at its most colourful and reproductively active when several males are kept together.
Whether M. aruensis follows this pattern in every particular is unrecorded. Given its modest, bronze-and-bluish colouration, its nuptial display is likely to be subtler than that of the flashier rainbowfishes, but the underlying mechanics — male courtship, female choice, and spawning into fine vegetation — should be the same. This section is therefore best read as the genus pattern applied to a species whose own mating behaviour remains undocumented.
Breeding
There are no published captive-breeding accounts specific to Melanotaenia aruensis, so its breeding is described here by analogy with congeners and flagged as data-sparse. Rainbowfishes in general are straightforward egg-layers: conditioned adults spawn over fine-leaved plants or synthetic spawning mops, depositing small, adhesive eggs that hang from the spawning medium by fine filaments.
In the genus, spawning typically extends over many days, with a female releasing a few eggs daily rather than a single large clutch, and the eggs hatching after roughly a week to twelve days at warm temperatures, the incubation time shortening with warmth. Adults will eat their own eggs and offer no care, so aquarium breeders harvest the mops or eggs to a separate rearing container. The tiny fry hang near the surface at first and need very small first foods — infusoria, rotifers, or commercial liquid fry food — before they can take newly hatched brine shrimp.
FishBase rates the species' resilience as medium, with an estimated population doubling time of 1.4 to 4.4 years, which is consistent with the moderate fecundity and steady, drawn-out spawning seen across small rainbowfishes. Until someone publishes a dedicated account, however, the specifics of egg number, incubation period and fry development for M. aruensis remain unconfirmed.
In the aquarium
Melanotaenia aruensis is only rarely, if ever, encountered in the aquarium trade, and there is little first-hand husbandry experience to draw on. FishBase notes it as a species of potential aquarium interest, but it has nothing like the commercial profile of its famous relatives. The notes that follow are therefore extrapolated from the care of comparable small rainbowfishes and from the species' known wild habitat.
As an active, peaceful schooling fish of around 3 in, it would be best kept in a group of six or more in a tank offering a long, open swimming lane together with structure — driftwood, rockwork and marginal planting — echoing the timber-strewn, vegetation-fringed limestone streams it comes from. Its wild water is warm and alkaline: temperatures from the mid-20s into the high-20s Celsius and neutral-to-basic, moderately hard water would suit it better than the soft, acidic conditions often assumed for tropical fish. As with rainbowfishes generally, stable conditions and good filtration matter more than chasing exact numbers.
Its colours are subtle rather than spectacular — a metallic interplay of bluish-grey, bronze and a pale-edged dorsal fin — so it rewards good lighting and a darker substrate that let the bronze scale margins show. Peaceful, similarly sized tankmates such as other rainbowfishes and robust small characins or rasboras would be appropriate. Because so little is documented for this species specifically, an aquarist keeping it should treat the above as a starting point and observe the fish closely.
Conservation
Melanotaenia aruensis is assessed as Least Concern (LC) on the IUCN Red List. Despite its narrow distribution — known only from Trangan and Kobroor islands in the Aru group — the assessment concluded that the species does not currently face a level of threat that would warrant a higher-risk category, reflecting habitats that remain relatively intact within forested, limestone-stream catchments.
FishBase rates the species' fishing vulnerability as low and its resilience as medium, and it is classed as harmless. There is no indication of significant commercial exploitation: although the description notes potential interest for fisheries and the aquarium trade, no large-scale collection of M. aruensis is documented, and the species is essentially absent from the ornamental market.
That said, the same fine-scale, two-island endemism that defines its ecology is also its principal long-term vulnerability. A species confined to a handful of streams on two islands has no outlying populations to buffer it should those particular catchments be degraded — by logging of the surrounding rainforest, alteration of stream flow, or the introduction of non-native fishes, the pressures that have harmed other narrowly endemic New Guinea rainbowfishes. For now the wild population appears secure, but its restricted range warrants continued monitoring.