Taxonomy & naming
Melanotaenia etnaensis was described in 2016 by Gerald R. Allen, Peter J. Unmack and Renny K. Hadiaty in the journal aqua, International Journal of Ichthyology, as part of a revision titled 'The Goldiei group of rainbowfishes (Melanotaeniidae) from the Birds Neck Region of New Guinea.' The holotype, an adult male of 4 in standard length (MZB 21763), was collected by Renny Hadiaty from the Ambalanga River near Etna Bay, West Papua Province, Indonesia, on 9 December 2013. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia etnaensis Allen, Unmack & Hadiaty, 2016; because it was described in Melanotaenia, where it remains, no parenthetical authority is used.
The specific epithet etnaensis refers to the type locality, Etna Bay, the inlet on the southern coast of New Guinea into whose streams the species drains. The genus name Melanotaenia combines the Greek melanos, 'black', with taenia, 'stripe or band', a reference to the dark lateral band seen across many species of the genus and conspicuous in large males of this one. Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia.
Within that genus, M. etnaensis is placed in the 'Goldiei' species group, a cluster of closely related New Guinea rainbowfishes that share meristic characters and are distinguished from one another chiefly by genetics, coloration and geographic isolation rather than by clean morphological breaks. Among the group, M. etnaensis is diagnosed by the shortest average caudal-peduncle length (about 14.5% of standard length) and the highest average anal-fin ray count (about 23.4); those counts, plus its restricted Etna Bay range, set it apart from its relatives.
Morphology
Melanotaenia etnaensis is among the largest members of the Goldiei group, with the male holotype measuring 4 in standard length — close to the largest size recorded; the IUCN assessment cites a maximum of about 3.5 in SL. It has the deep, laterally compressed body characteristic of mature Melanotaenia, together with the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal. The original description distinguishes it within its group by a short caudal peduncle and a high anal-fin ray count.
In coloration, mature males show a bluish-grey side with yellowish to reddish dorsal and anal fins; the anal fin usually carries a yellow stripe along its base. Large males develop a strong dark mid-lateral stripe and yellow scales along the lower mid-side, the body deepening with age as in other rainbowfishes. Females are similarly patterned but less vivid, a normal expression of the sexual dimorphism that runs through the genus.
Colour is not uniform across the species' tiny range. The population from the Pama River (Sungai Pama) carries a different colour pattern from the Ambalanga type series and reportedly matures at a smaller size, yet it is genetically near-identical to the type population. Colour and locality are closely tied within the Goldiei group, and local variants arise readily among isolated stream systems.
Habitat
Melanotaenia etnaensis is endemic to streams flowing into Etna Bay, on the southern coast of New Guinea roughly 62 mi southeast of Kaimana in West Papua Province, Indonesia. Etna Bay is an inlet some 34 mi long, fringed by rainforest-covered mountains rising over 1,0 ft. The species is confined to streams entering the bay or its immediate vicinity, with confirmed records from the Ambalanga River (the type locality) and the Pama River.
The fish occupy rainforest streams running through primary and secondary forest, and within them favour deeper pools littered with fallen logs and branches — submerged wood being a defining feature of the habitat. Water measurements taken at the type locality and the Pama River by Heiko Bleher record temperatures of 78–82 °F and a pH ranging from 7.4 to 8.8, indicating warm, neutral-to-distinctly-alkaline water.
A small cluster of coastal rainforest streams around a single bay is the whole of the known range. There is no large outlying population to fall back on, which makes the integrity of the Etna Bay catchments the entirety of the species' world.
Feeding
No diet study specific to Melanotaenia etnaensis has been published, and the original description does not detail its feeding ecology, so its natural diet must be read from the genus. FishBase estimates a trophic level of about 2.9 for the species, consistent with the opportunistic omnivory typical of rainbowfishes.
Rainbowfishes of the genus Melanotaenia are generally upper- and mid-water omnivores that take small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects falling onto the surface — along with algae and plant material grazed from submerged surfaces. In a log-strewn rainforest stream of the kind M. etnaensis inhabits, drifting and surface-borne invertebrates and the biofilm on submerged wood are the most plausible food sources, though this is inference from habitat and relatives rather than direct observation.
Direct dietary data for this species are sparse. The safe statement is that it presumably feeds as a generalist omnivore, like its congeners; anything more specific runs ahead of the published record.
Mating
The reproductive behaviour of Melanotaenia etnaensis has not been described in detail; the 2016 description is a taxonomic and distributional work and does not report courtship or spawning for this species. What can be said draws on the well-documented breeding biology of the genus rather than on Etna Bay observations.
Melanotaenia rainbowfishes are egg-scattering, plant-spawning fishes with no parental care. Males are typically social and competitive, dominant individuals staking out display areas and courting females with intensified colour, flared fins and — in many species — a flickering pale 'courtship band' that flashes along the head and nape of a displaying male, most vividly in the early hours after first light. Females follow displaying males into fine vegetation to spawn.
Whether M. etnaensis follows that template in every particular is unrecorded; its cues, timing and display details have not been observed. The pronounced male coloration — the dark mid-lateral stripe and yellowed lower flank of large males — fits the visual, male-display mating system seen across the genus, but the specifics here remain data sparse.
Breeding
There is no published account of captive or wild breeding for Melanotaenia etnaensis. The species was described in 2016 and has not been the subject of the detailed aquarium spawning reports that exist for long-established hobby rainbowfishes, so its clutch size, egg development and fry rearing are unrecorded in the sources at hand.
By the general pattern of the genus, rainbowfishes deposit small, adhesive eggs that hang by fine filaments from fine-leaved plants or spawning mops, spawning over many days with a female releasing a few eggs at a time rather than a single clutch; the adults offer no care and will eat eggs and fry. Eggs of Melanotaenia species typically hatch after roughly a week or more, the incubation shortening with warmth, and the tiny fry require very small first foods before taking newly hatched brine shrimp. One field note suggests the Pama River population matures at a smaller size than the type population, hinting at population-level variation in life history within the species' tiny range.
Until someone documents a spawning of this species, read all of the above as expectations drawn from close relatives, not confirmed facts for M. etnaensis. Where the record is silent, better to mark the gap than fill it.
In the aquarium
Melanotaenia etnaensis is of potential ornamental interest — that prospect was the basis of an Australian Government import-risk assessment that catalogued the species — but it is not an established aquarium fish, and detailed husbandry experience with it is limited. As a large, active rainbowfish it would, on the model of its relatives, suit a long tank with open swimming space and be kept as a group so that males display and hold colour.
The wild water data give a useful starting point for anyone who does keep it: temperatures of roughly 77–82 °F and a neutral-to-alkaline pH between about 7.4 and 8.8, reflecting the warm, mildly hard streams of Etna Bay rather than the soft, acidic water often assumed for tropical fish. Stable, well-filtered conditions with some current, structure in the form of wood, and bright lighting to bring out the male's blue-grey body and yellow-to-red finnage would echo the natural habitat of deep, log-littered forest pools.
So little has been written about this species that the care notes here come from the genus and from its measured habitat, not from a body of keeping experience. Treat it as a robust, alkaline-water rainbowfish, and don't assume the detailed strain knowledge that exists for hobby staples like M. boesemani.
Conservation
Melanotaenia etnaensis is assessed as Least Concern (LC) on the IUCN Red List, assessed on 5 December 2019 and published in 2020. The LC rating was applied despite the species' limited and restricted distribution: no current major threats were identified to the Etna Bay streams at the time of assessment.
That verdict rests on the apparent intactness of the habitat rather than on a large range. The species is confined to a small set of rainforest streams draining a single bay, and a narrowly endemic fish in a few coastal catchments has little buffer should those streams be degraded — by logging in the surrounding rainforest, by changes to water quality, or by the introduction of non-native fishes that have harmed other narrow-range New Guinea rainbowfishes. FishBase rates the species' resilience as medium and its vulnerability to fishing as low, consistent with a fish under no current exploitation pressure.
So the fish is secure for now but exposed by its small range, and its status hangs on the continued health of the Etna Bay rainforest streams. Monitoring those catchments is the obvious safeguard for a species that lives, as far as anyone knows, nowhere else.