Taxonomy & naming
Melanotaenia picta was formally described in 2015 by Gerald R. Allen, Renny K. Hadiaty, Peter J. Unmack, and Mark V. Erdmann, in a paper in aqua, International Journal of Ichthyology (21(2): 66–108) that revised the rainbowfishes of the Aru Islands and named five new species. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia picta Allen, Hadiaty, Unmack & Erdmann, 2015; because the species was described in Melanotaenia, where it remains, no parenthetical authority is used. The type locality is the Mareremar River drainage on Kobroor Island in the Aru Islands of Indonesia.
Before its formal description, the fish circulated among specialist hobbyists as Melanotaenia sp. 'Aru II'. It belongs to a small Aru-endemic group, closely related to the New Guinea M. goldiei group, that also includes M. kolaensis, M. senckenbergianus, and M. wokamensis. These Aru species look much alike and are hard for non-specialists to tell apart — which matters in practice, since aquarists are advised to keep them in separate tanks to avoid accidental hybridisation. The Aru Islands are a land-bridge remnant of the Sahul shelf that once joined New Guinea and Australia, which explains the close affinity between the Aru rainbowfishes and their New Guinea relatives.
The genus name Melanotaenia combines the Greek melan, 'black', with the Latin taenia, 'stripe', after the dark midlateral band common across the genus; the epithet picta is Latin for 'painted', for the fish's colour pattern.
Morphology
Melanotaenia picta is a moderately deep-bodied, laterally compressed rainbowfish. FishBase records a maximum length of about 3.5 in SL for males and unsexed fish and 3 in SL for females, while the hobby trade reports growth to around 4.5 in in total length; dominant males are the largest and deepest-bodied individuals, with the greatest body depth of an adult male reaching roughly 34% of standard length. Like other members of the genus it carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — set on a deep body that becomes more so with age.
Meristic counts from the original description are: dorsal V–VI followed by I,11–14; anal I,19–23; pectoral 13–16 rays (usually 14); 35–36 lateral-line scales; 16–19 predorsal scales; and 15–19 total gill rakers. These overlap broadly with related Aru and New Guinea species, which is why colour pattern, not meristics, is the better field character within this group.
In life the body shows shades of blue-green to blue with metallic reflections. A darker blue midlateral stripe is most prominent over the caudal peduncle, edged below by a broad white stripe, and the flanks carry brilliant golden reflections. The dorsal, anal, and caudal fins are picked out with reddish-to-yellow highlights. As with rainbowfishes generally, this colour is mood- and condition-dependent and intensifies with age, reaching its fullest saturation in a displaying adult male.
Habitat
Melanotaenia picta is endemic to the Mareremar River system on Kobroor Island in the Aru Islands of Indonesia — a single drainage, and nothing more. It is a freshwater, benthopelagic, tropical species occupying what the describing authors called some of the most scenic freshwater environments in the Aru Islands.
These are small rainforest brooks: quiet pools broken by cascades three to four metres high, running through smooth limestone channels with excellent water clarity. Aquatic vegetation is minimal, but log snags and submerged wood provide plenty of shelter. The limestone setting points to relatively hard, neutral-to-alkaline water, in line with how the species is kept in captivity.
Precise physical and chemical parameters of the wild habitat are sparse beyond these qualitative descriptions, and no long-term water-chemistry profile of the Mareremar system has been published in the sources at hand. What is clear is that this is a clear-water, structurally complex stream fish of one small island drainage — the fact that shapes both its biology and its conservation outlook.
Feeding
Specific dietary studies of Melanotaenia picta in the wild are not available, but its trophic level is estimated at about 2.9, placing it among the omnivores that take a mixed diet of small animal and plant material. As with rainbowfishes generally, the natural diet can be expected to consist of small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects taken at or near the surface — supplemented by algae and other plant matter, fed in the upper and middle water column.
In captivity it is undemanding. Trade and specialist sources report that it takes any standard ornamental food of suitable size; a varied diet — live and frozen Daphnia, brine shrimp, and bloodworm alongside good flake and small pellets — keeps it in the best colour and condition. Colour deepens with age, and a varied diet helps the metallic blues and golden flanks reach their full development.
Mating
Courtship in Melanotaenia picta itself is little observed, but should follow the well-documented rainbowfish pattern. Males are social and competitive, staking out display areas and courting females with deepened colour, flared fins, and body-flashing that brings the blue-green sheen and golden flanks to their peak. In many Melanotaenia a flickering pale 'courtship band' runs along the head and nape of a displaying male, most vivid in the morning.
Because males display most strongly in the presence of rivals, the species is at its most colourful when several males are kept together, and breeders generally condition groups rather than isolated pairs. A receptive female follows a displaying male into fine cover to spawn. As is typical of the genus, spawning is most active in the early hours after first light.
Breeding
Melanotaenia picta has been bred in captivity, though mainly by rainbowfish specialists rather than at commercial scale, reflecting its rarity in the trade. Specialist sources describe its care and breeding as comparable to most rainbowfishes, which are egg-scattering plant spawners with no parental care.
Following the genus pattern, conditioned adults deposit small, adhesive eggs that hang by fine filaments from fine-leaved plants such as Java moss or from synthetic spawning mops. Females release a few eggs a day over a drawn-out spawning period rather than one large clutch. The adults will eat the eggs, so breeders move the mops or eggs to a separate rearing container. The tiny fry need very small first foods — infusoria, rotifers, or liquid fry food — before graduating to newly hatched brine shrimp and crushed flake, and the full adult colours come in only slowly as the fish mature. Precise figures for clutch size and incubation period specific to M. picta are not documented in the available sources.
In the aquarium
Rare in the hobby and bred mainly by specialists, M. picta is not a fish the average aquarist will find for sale. Those who keep it treat it as a typical active, peaceful, schooling rainbowfish: best in a group, so males display against one another and stay coloured up, and given a tank with a long, open swimming lane to suit its restless habit.
Reported husbandry follows the species' limestone-stream origins — water that is not too soft, a roughly neutral pH, a temperature of about 72–79 °F, and the clear, well-oxygenated conditions and plentiful cover (wood and structure rather than dense planting) that mirror its native brooks. It accepts any standard ornamental food of appropriate size, and colour deepens with age under good conditions.
The one real caution is hybridisation. M. picta belongs to a cluster of near-identical Aru rainbowfishes (M. kolaensis, M. senckenbergianus, M. wokamensis) that non-specialists can barely separate, so it should be kept in single-species tanks to keep the line clean. Given its scarcity, holding genetically clean stock is itself a worthwhile contribution from the hobbyists who keep it.
Conservation
Melanotaenia picta is assessed as Least Concern (LC) on the IUCN Red List (assessment dated 3 December 2019; species ID 161110162). Despite a very restricted distribution confined to the Aru Islands, the assessment found no major threats to the species, a conclusion consistent with regional reporting on the Aru freshwater fauna.
The narrow range is still the central fact: a fish endemic to a single river system on one island has little buffer against future habitat change, even where current pressures are low. For now the remote, relatively undisturbed limestone brooks work in its favour, and it escapes the heavy ornamental-trade collection that has pressured some of its better-known relatives. Because it is rare in the hobby and bred chiefly by specialists, captive populations are small but keep the species going outside its wild range; aquarists holding clean, non-hybridised lines add to that reserve.