Taxonomy & naming
Melanotaenia ammeri was described in 2008 by Gerald R. Allen, Peter J. Unmack and Renny K. Hadiaty in the paper 'Two new species of rainbowfishes (Melanotaenia: Melanotaeniidae) from western New Guinea (Papua Barat Province, Indonesia)', published in aqua, International Journal of Ichthyology (14(4): 209–224). It is one of the many rainbowfishes that the Allen-led surveys of West Papua brought to light in the 2000s, when fieldwork on the Bird's Head (Vogelkop) and Bomberai peninsulas repeatedly turned up undescribed Melanotaenia in small, previously unsampled drainages. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia ammeri Allen, Unmack & Hadiaty, 2008; because the species was described in Melanotaenia, where it remains, the authority carries no parentheses.
The specific epithet honours Maximilian Johannes 'Max' Ammer of Sorong, West Papua — a Dutch diver and conservationist long associated with field exploration in the region — whose support helped enable the surveys that produced the species. The genus name Melanotaenia combines the Greek melas (black or dark) with the Latin/Greek taenia (band or stripe), a reference to the dark lateral stripe characteristic of many members of the genus. Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia.
Within the genus, M. ammeri belongs to a cluster of Bird's Head and Bomberai species. No single character pins it down; it is best separated from its relatives by colour pattern and by the combination of its meristic counts, since fin-ray and scale counts overlap broadly across the genus. Reported meristics include 5–7 dorsal spines and 12–15 (usually 13–14) dorsal soft rays, a single anal spine with 20–25 (usually 21–23) anal soft rays, 12–14 pectoral rays, 34–36 (usually 35) lateral scales, and 17–19 predorsal scales.
Morphology
Ammer's rainbowfish is a small, laterally compressed rainbowfish. Males reach about 4 in in standard length, while females are distinctly smaller, recorded to roughly 3 in SL. As in other Melanotaenia, the body carries the family's divided dorsal fin — a short anterior portion followed by a longer posterior one — and a long-based anal fin, giving the fish the symmetrical, deep-bodied silhouette typical of the group.
The live colour pattern is understated rather than gaudy. The back is bluish, and the flanks bear alternating narrow stripes — mauve to blue-grey alternating with pale yellow — with one stripe running along each scale row, producing a finely lined effect. A darker blue midlateral stripe stands out as the most prominent marking on the side. The median fins (dorsal, anal and caudal) are generally yellow, and this yellow is brighter and more saturated in adult males than in females or juveniles.
In life, that finely striped blue-and-yellow flank with yellow median fins is what most readily distinguishes the species. Preserved or stressed specimens lose much of the colour and are far harder to separate from congeners, which is why the original description leans on the living coloration alongside meristic and morphometric data.
Habitat
Melanotaenia ammeri is endemic to Indonesia, restricted to small freshwater systems on the Bird's Head Peninsula of West Papua, where it drains toward Arguni Bay on the southern side of the peninsula. The type locality is a small creek flowing into the northern part of Arguni Bay, with the species also recorded from a few nearby streams. This is a freshwater, shallow-water fish: FishBase lists it as pelagic in a depth range of roughly 0–3 ft in a tropical climate.
The type-locality habitat is described in detail in the original account: a narrow creek only 7–10 ft wide and shallow, reaching only about half a metre deep, with a gradual gradient and a course running through second-growth forest roughly one kilometre upstream from the sea. The fish were collected over sand and gravel substrate strewn with leaf litter and dead branches — the kind of clear, leaf-littered lowland forest creek that characterises much of the species' limited range. Hobby accounts similarly describe clear streams and small rivers with moderate flow, submerged vegetation, and sandy or rocky bottoms.
What stands out about the species' ecology is how little of it there is. Known only from a small cluster of creeks around a single bay, M. ammeri has an extremely restricted distribution — the narrow range behind both its conservation listing and the scarcity of detailed field data.
Feeding
No detailed dietary study of Melanotaenia ammeri has been published, and the species is best described as a small omnivore in the rainbowfish mould. FishBase places it at a trophic level of about 2.9, consistent with a diet drawn largely from small invertebrates supplemented by plant material — the general pattern across the genus, whose members feed in the upper and middle water column on insect larvae, small crustaceans, drifting terrestrial insects, and algae or fine plant matter grazed from submerged surfaces.
In the leaf-littered forest creeks it inhabits, prey would plausibly include aquatic insect larvae and small invertebrates associated with the leaf litter and submerged wood, along with terrestrial insects falling onto the water surface. Beyond the trophic-level estimate, however, the wild diet is data sparse and is inferred from the genus rather than measured for this species.
Hobby sources report that in captivity the fish behaves as a typical undemanding rainbowfish, accepting prepared flakes and pellets alongside frozen and live foods such as brine shrimp and bloodworms; these care notes are secondary and approximate rather than primary observations of natural feeding.
Mating
The reproductive behaviour of Melanotaenia ammeri has not been documented in the wild, and what can be said is drawn by analogy with better-studied rainbowfishes. Members of the genus are egg-scattering, plant-spawning fishes with no parental care, in which dominant males establish display territories and court females with intensified colour and fin-flaring, often most actively in the morning hours after first light. In M. ammeri the brighter yellow median fins of adult males are the visible expression of this sexual dimorphism.
As with its relatives, courtship and spawning would be expected to take place among fine vegetation or other submerged cover, with a receptive female following a displaying male to release small numbers of eggs at a time. Males of rainbowfishes generally display most strongly in the presence of rival males, so colour and courtship intensity tend to peak when several males are present.
Read these as genus-level expectations rather than confirmed observations for M. ammeri; species-level mating data are sparse.
Breeding
Direct breeding records for Melanotaenia ammeri are limited, and most of what is reported comes from hobby keeping rather than the scientific literature. Like rainbowfishes generally, it is understood to be an egg-scatterer that deposits adhesive eggs among fine-leaved plants or, in the aquarium, on synthetic spawning mops, spawning over a period of days rather than in a single clutch. Hobby accounts describe eggs hatching after roughly one to two weeks, with the tiny fry requiring very small first foods such as infusoria before progressing to newly hatched brine shrimp — the standard rearing approach for the genus.
Broader life-history context is available from an Australian Government conservation report, which notes that rainbowfishes generally begin breeding at around six months of age, live roughly four years in the wild, and can reach up to about eight years in captivity. These are family-level generalisations applied to the species rather than measured values specific to M. ammeri.
Because the fish is rarely kept and the wild population has not been studied for reproduction, species-specific breeding detail remains data sparse, and the captive figures above should be treated as approximate guidance rather than established facts for this particular rainbowfish.
In the aquarium
Melanotaenia ammeri is an uncommon fish in the aquarium hobby, and the husbandry guidance available for it is extrapolated from rainbowfish keeping generally rather than backed by long experience with this species. As an active, peaceful, schooling rainbowfish it is best kept in a group rather than singly, in a tank that offers a long, open swimming lane together with some planted cover and structure such as driftwood; hobby guides suggest a sizeable aquarium with open central space and a sand or fine-gravel substrate that echoes its natural creek bottoms.
The water conditions reported by hobby sources are mid-range tropical values: temperatures of roughly 75–82 °F, a pH spanning the mildly acidic to mildly alkaline range (around 6.5–7.8), and moderate hardness. These figures are hobby estimates rather than measurements from the wild habitat and should be treated as a starting point; in practice, stable, clean, well-filtered water matters more than hitting exact numbers, and the species' clear forest-creek origins suggest it will appreciate good water quality and gentle to moderate flow.
Like other rainbowfishes it is a constant, restless swimmer that does best with similarly sized, peaceful tankmates and shows its best colour — the brighter yellow fins of mature males — when kept in a settled group with a varied diet. Given its restricted wild range and rarity in the trade, anyone keeping it should prioritise responsibly sourced, captive-bred stock where available.
Conservation
Melanotaenia ammeri is assessed as Near Threatened (NT) on the IUCN Red List, under criterion B1a, with the assessment dated 3 December 2019. The listing is driven primarily by the species' extremely restricted range: it is known only from a small cluster of creeks draining into Arguni Bay on the Bird's Head Peninsula of West Papua, a distribution so narrow that even localised habitat change could affect a large share of the global population.
A narrowly endemic stream fish of this kind has little natural buffer against habitat change — no large outlying population elsewhere to absorb local declines. The Red List assessment gives the species an extent of occurrence of about 388 mi² and treats it as a single location, and the one identified threat is an oil and gas concession in north-western Arguni Bay (active in 2014, its current status unknown); the population is presumed stable and there is no documented continuing decline, which is why the species sits at Near Threatened rather than in a threatened category. Elsewhere in its range no major threats are recorded.
Because the fish is little studied and only sparingly kept, conservation attention is correspondingly limited. The most useful safeguards are protection of the small Arguni Bay drainages it depends on and, in the hobby, reliance on captive-bred rather than wild-collected stock so that aquarium interest does not add collecting pressure to an already restricted wild population.