Taxonomy & naming
Melanotaenia albimarginata was described in 2015 by Gerald R. Allen, Renny K. Hadiaty, Peter J. Unmack and Mark V. Erdmann, in a substantial revision of New Guinea and adjacent rainbowfishes published in the journal aqua (International Journal of Ichthyology, vol. 21(2): 66–108). Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia albimarginata Allen, Hadiaty, Unmack & Erdmann, 2015; because the species was described in Melanotaenia, where it remains, no parenthetical authority is used.
The specific epithet albimarginata is Latin for 'white-margined', a direct reference to the prominent white markings at the tips of the upper and lower caudal-fin lobes — the feature that most readily separates living fish from their congeners in the field. The genus name Melanotaenia combines Greek melas (black or dark) with Latin taenia (a stripe or band), describing the dark midlateral stripe carried by many members of the genus. M. albimarginata belongs to the largest genus in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia.
The species was collected in the Aru Islands group, at the southern margin of the rainbowfish range, and is one of several Aru endemics formally recognised in the 2015 revision. It occurs syntopically with another small rainbowfish, Melanotaenia senckenbergiana, from which it differs in coloration and in the diagnostic white caudal tips.
Morphology
Melanotaenia albimarginata is a small, fusiform rainbowfish. Males reach about 3.5 in standard length and females about 3 in SL, making it a modestly sized member of the genus. The greatest body depth recorded for an adult male is roughly 36 percent of standard length, giving a body that is moderately deep but not strongly compressed into the diamond profile of larger Melanotaenia.
Meristic counts from the original description place the dorsal fin at 5–7 spines and 11–12 soft rays, the anal fin at 1 spine and 19–22 soft rays, and the pectoral fins at 14–15 rays. There are 35–36 lateral-line scales, 16–18 predorsal scales, and 16–18 gill rakers. These counts overlap broadly with related species, so coloration rather than hard meristics is the practical means of identification.
In life, the upper side is a dappled grey scattered, in males, with pale bluish scales and thin yellow stripes running between the scale rows. A dark grey midlateral stripe one to two scale rows wide runs from behind the eye to the caudal base, with an equally broad yellow zone immediately below it; the lower body is pale grey. The first dorsal fin bears a small black spot at the base of its posterior portion, and the dorsal and anal fins show greyish membranes with whitish to pale-yellow rays. The caudal fin is semitranslucent grey with the white tips that name the species. Females lack the dark midlateral stripe, show more obvious yellow inter-row striping, and have more diffuse white caudal tips. Freshly collected fish photographed in an aquarium turned almost entirely bluish, with reddish scale margins on the upper side and two to three rows of dark spots on the second dorsal fin — a captive colour state quite unlike the in-situ pattern, sharing with it only the white caudal tips and whitish anal rays.
Habitat
The whitetip rainbowfish is known only from Alur Warnafa, a small tributary of the Sungai Unmar, itself a minor stream near the village of Algadang on Kobroor Island in the Aru Islands of Indonesia's Maluku province. The Aru group is a low, forested archipelago dissected by 'marine rivers' — tidal channels that bisect the islands — and the type locality is reached by travelling roughly 24 mi inland from the west coast along one such channel, then about 4 mi upstream into fresh water. The lowermost kilometre of the system is brackish and mangrove-lined; the rainbowfish habitat itself is purely fresh, though its water level is influenced by the tides.
When the type series was collected in January 2013, the fish lived in very clear, slow-flowing water 3–10 ft deep over a smooth limestone-pavement bottom. Aquatic plants were sparse, limited mainly to Cryptocoryne ciliata, and the channel held abundant leaf litter and fallen logs; pursued fish took refuge among the dead branches of submerged trees. A return visit in December 2013, at the onset of the monsoon, found higher water, stronger current and much reduced visibility, indicating a habitat that swings seasonally between clear low-water and turbid high-water states.
Water chemistry at the type locality reflects the limestone catchment: pH measured 8.04–8.64 at high tide, conductivity ranged from 144 to 319 µS/cm, and water temperature was about 81 °F. This is a warm, hard, distinctly alkaline freshwater environment. The fish shares its stream with Melanotaenia senckenbergiana (in smaller numbers), the silverside Craterocephalus cf. randi, the glassfish Ambassis interrupta, the mouth-almighty Glossamia aprion, and assorted gobies and eleotrids.
Feeding
No dietary study has been published for Melanotaenia albimarginata, and its feeding ecology is best described as data sparse. FishBase estimates a trophic level of about 2.9, consistent with an omnivore that leans toward animal prey. As with rainbowfishes generally, the species can be expected to feed in the upper and middle water column on small aquatic and terrestrial invertebrates — insect larvae, micro-crustaceans, and insects taken at or near the surface — supplemented by algae and organic detritus grazed from submerged surfaces.
The clear, debris-rich limestone stream it inhabits would supply drifting invertebrates among the leaf litter and fallen wood, and the presence of syntopic insectivorous fishes such as glassfish and gobies suggests an invertebrate-rich prey base. Without direct evidence, anything more specific about its natural diet is guesswork; the genus-level pattern of opportunistic omnivory is the safest thing to say.
Mating
Reproductive behaviour has not been documented in the wild or in captivity for this species, which was described only in 2015 and is rarely if ever kept. What can be said is drawn by analogy with the well-studied members of the genus, in which spawning behaviour is broadly conservative.
Melanotaenia rainbowfishes are egg-scattering, plant-associated spawners with no parental care. Males establish display areas and court females with intense colour signalling, typically flaring the fins and flashing the body, and in many species a pale 'courtship band' flickers along the head and nape of a displaying male. Spawning activity in the genus is usually concentrated in the early hours after first light. Whether M. albimarginata follows this pattern in detail is unknown, but the strong sexual dichromatism evident in preserved and freshly caught material — males with the dark stripe and yellow band, females plainer — points to the male visual display typical of the group.
Breeding
There are no published breeding records for the whitetip rainbowfish. The original description and the available field notes give no information on spawning, egg characteristics, incubation, or fry development, and the species does not appear to be established in the aquarium trade.
By reference to the genus, captive breeding would be expected to follow the standard rainbowfish pattern: adhesive eggs deposited a few at a time over many days onto fine-leaved plants or spawning mops, hanging by fine filaments; incubation of roughly one to two weeks at warm temperatures; and tiny surface-oriented fry that require very small first foods before taking newly hatched brine shrimp. This is offered only as the genus-level expectation. Until the species is bred and the results recorded, its specific reproductive parameters remain unknown.
In the aquarium
As an aquarium fish, Melanotaenia albimarginata is a blank. Described in 2015 from a single remote locality, with no care or breeding notes in the original account and no place in the ornamental trade, it has left behind no body of husbandry experience to draw on.
What guidance there is comes from the wild habitat and from the genus. The fish lives in warm, clear, hard, alkaline freshwater — near 81 °F and pH above 8 at the type locality — so captive maintenance should aim for warm, neutral-to-alkaline, moderately hard water rather than the soft acidic conditions often assumed for tropical fish. Like other rainbowfishes it would be an active, schooling, open-water swimmer, best kept in a group with a long swimming lane and stable, well-filtered conditions. Treat any of this as provisional. A keeper who does manage to acquire the species should document what they see, because reliable care recommendations do not yet exist.
Conservation
Melanotaenia albimarginata is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 3 December 2019 (species ID 161081159). The Least Concern listing indicates that, on current information, the species is not considered to face an elevated extinction risk, and it has not been evaluated under CITES or the CMS.
That status sits alongside a very narrow known range. The species has been recorded only from a single small tributary stream in the Aru Islands, reached by a long inland transit from the coast, and the available collections come from just two visits in 2013. A point-endemic distribution of this kind offers little buffer against localised habitat change, so the Least Concern assessment is best read as reflecting an absence of documented threats rather than a demonstrated wide and robust range. The remoteness of the locality and the absence of any commercial collection pressure currently work in the species' favour. Continued monitoring and any further survey of the Aru freshwater fauna would help confirm whether the known range reflects the true distribution or simply limited sampling.