Taxonomy & naming
Ian S. R. Munro described this fish in 1964 as Nematocentris sexlineatus; it was later transferred to Melanotaenia, the largest genus of the rainbowfish family Melanotaeniidae. The move out of its original genus is why the authority is written in parentheses: (Munro, 1964). Eschmeyer's Catalog of Fishes governs the valid name and records it as Melanotaenia sexlineata.
Melanotaeniidae, the true rainbowfishes, range across New Guinea and northern and eastern Australia, with Melanotaenia as the dominant lineage. M. sexlineata is a New Guinea member of the genus, restricted to the upper Fly River drainage. Like most rainbowfishes it is told apart from its congeners by colour, body proportions, and geography more reliably than by meristic counts, which overlap broadly within the genus.
The counts that are recorded fit the genus closely: a divided dorsal fin of roughly 6 to 8 spines and 10 to 12 soft rays, the anal fin a single spine with 16 to 19 soft rays, all on a fusiform, laterally compressed body. The epithet sexlineata — 'six-lined' — points to the longitudinal striping along the flanks.
Morphology
This is a small rainbowfish, reaching about 3 in standard length; reports note it may occasionally approach 3 in but usually stays under 3 in. The body is fusiform and laterally compressed — more slender and less deep-bodied than show species such as Boeseman's or the lake rainbowfishes — and carries the family's two-part dorsal fin: a short anterior dorsal followed by a longer posterior one.
The colour is understated. Field observers describe an iridescent turquoise and golden sheen over the flanks, with the 'six lines' of the name running longitudinally along the body. A colour variety with a blue back and orange fins has been reported from the upper Fly catchment along the Kiunga–Tabubil Road, which hints at some local variation across the species' small range.
Colour intensity here is mood- and condition-dependent, as in the rest of the genus: a displaying or dominant fish flushes far brighter than a stressed or subordinate one. Published data on sexual dimorphism in this species are sparse, but the general rainbowfish pattern applies — males tend to be the more deeply coloured and slightly larger sex.
Habitat
The Fly River rainbowfish is known only from the upper Fly River system of western Papua New Guinea, from a handful of small creeks between the towns of Kiunga and Rumginae. Many collection records come from streams immediately north of Kiunga along the Kiunga–Tabubil Road. This is a lowland and low-foothill species, found in rainforest streams below about 656 ft elevation, at a latitude of roughly 6 to 8 degrees south.
The habitat is tannin-rich rainforest country. The streams are mostly small, shallow, slow-flowing and clear, though often darkly tea-stained with tannins leached from the surrounding forest. Water at collection sites is soft and gently acidic to neutral — pH readings from about 6 upward, with FishBase citing a range near 7.0 to 7.5, and temperatures around 24 to 26 degrees Celsius. The fish keeps to cover in these small creeks rather than open water.
Within its limited range it can be locally common, but it is reported to be becoming rarer, particularly around Kiunga, as human settlement expands. That tiny natural distribution — one small cluster of streams in a single drainage — is the central fact of its biology and the reason for its conservation concern.
Feeding
Like other rainbowfishes, M. sexlineata is an omnivore that takes a mix of small invertebrates and plant material. In its rainforest creeks the natural diet runs to aquatic insect larvae, small crustaceans, terrestrial insects fallen onto the surface, and algae and other plant matter grazed off submerged surfaces. Its low trophic level, recorded around 2.9, reflects that broad, partly herbivorous feeding.
In the aquarium it is undemanding at the table and takes the standard rainbowfish fare: good-quality flake and small pellets, supplemented with frozen and live foods such as Daphnia, brine shrimp, and bloodworm, plus some vegetable matter. A varied diet with a real live or frozen component improves both colour and breeding condition. Small feedings spread through the day suit a constant, active grazer better than one large meal.
Mating
Courtship details specific to M. sexlineata are sparse, but it follows the well-documented reproductive pattern of the genus. Rainbowfishes are egg-scattering, plant-spawning fishes that pair to spawn: a male holds a display area and courts a female with intensified colour, flared fins, and the flickering pale 'courtship band' that many Melanotaenia males flash along the head and nape.
Display and spawning peak in the early morning after first light, and colour is most vivid then. Males display hardest in the presence of rivals, so this fish is probably at its most colourful and reproductively active when several are kept together — the standard observation across the genus. FishBase records it as a peaceful community fish that pairs distinctly to spawn.
Breeding
Breeding follows the genus standard: a plant-spawning egg-scatterer with no parental care. Adults deposit small, adhesive eggs that hang by fine filaments from fine-leaved vegetation or, in a tank, from synthetic spawning mops, with a female releasing a few eggs at a time over an extended period rather than one clutch. Import-risk documentation notes that the eggs and larvae resemble those of most Melanotaenia.
Incubation and rearing follow the same template. At warm temperatures the adhesive eggs hatch after roughly one to two weeks; the adults give no care and will eat eggs and fry, so breeders move the mops to a separate rearing container. The tiny fry need very small first foods — infusoria, rotifers, or a commercial liquid fry food — before graduating to newly hatched brine shrimp. The fish reached the Australian aquarium hobby from wild specimens collected in the upper Fly River and was then bred and distributed, so it clearly can be propagated in captivity, even if detailed published spawning data for this species are limited.
In the aquarium
This is a peaceful community fish, and it wants the same handling as other small rainbowfishes: a group rather than a pair or single specimen, an open swimming lane, and gentle, stable water. FishBase notes preferred aquarium conditions of about pH 7.1, hardness around 8, and a temperature near 27 degrees Celsius, with vegetative cover — in line with the planted, gently flowing creeks it comes from.
At an adult size of around 3 in it does not need the very large tanks the bigger lake rainbowfishes demand, but as an active, social shoaler it is happiest in a group with room to swim and some planted cover at the margins. Keep tankmates peaceful and not big enough to bully or outcompete it. A diet varied with live and frozen foods brings out the turquoise-and-gold sheen that is its main draw.
It is uncommon in the hobby, mostly because of its remote, restricted range. Anyone keeping it should remember that the wild population is threatened, and captive-bred stock is the responsible choice where available.
Conservation
The IUCN Red List assesses M. sexlineata as Endangered (EN) under criterion B1ab(iii), in an assessment dated 6 December 2019 (assessment T13076A147688118). The listing rests on a very small known range — currently only the upper Fly system of Papua New Guinea, a few small creeks between Kiunga and Rumginae — combined with documented decline in the quality of that habitat.
The fish can be locally common in lowland and low-foothill rainforest streams below about 656 ft, yet the population is reported to be becoming rarer, especially around Kiunga, where expanding human settlement is the main pressure. Identified threats include settlement growth, habitat change, and the introduction of non-native fishes into the catchment. A species confined to a single small drainage has little resilience to any of this: there is no large outlying population to buffer local losses.
That it has been collected, bred, and distributed in the hobby is a modest hedge against extinction, since captive stock reduces reliance on wild collection and keeps the fish in cultivation. A 2023 Australian import-risk assessment classified it as not high-risk for import. Even so, the wild future of the Fly River rainbowfish hangs on the condition of a very small number of Papua New Guinean streams.