Taxonomy & naming
Melanotaenia affinis was described by the Dutch zoologist Max Weber in 1907, on the basis of material from northern New Guinea. Because the species was originally placed in a different genus and later moved to Melanotaenia, the authority is conventionally written in parentheses: (Weber, 1907). Eschmeyer's Catalog of Fishes is the governing authority for the valid name, and recognises it as a valid species of Melanotaenia, the largest genus in the family Melanotaeniidae — the true rainbowfishes of the order Atheriniformes.
The family Melanotaeniidae radiates across New Guinea and northern and eastern Australia, and within it M. affinis belongs to the assemblage of species found on the northern, seaward side of New Guinea's central cordillera. This biogeographic position — north of the central dividing range — separates it from the southern lowland rainbowfishes and is the defining fact of its distribution. As is typical across the genus, the meristic and morphometric characters of M. affinis overlap broadly with those of its congeners, and field identification leans heavily on locality and colour rather than on hard counts.
The widespread range has produced a number of distinguishable local populations, several of which circulate in the aquarium hobby under place-names. The strain labelled 'Pagwi', after a locality on the Sepik River, is the best known of these and is frequently kept and bred under that tag. Such locality forms are colour variants of a single valid species rather than separate taxa, and the Catalog of Fishes recognises no subspecies.
Morphology
Melanotaenia affinis is a fusiform, laterally compressed rainbowfish of moderate size. FishBase records a maximum length of about 4.5 in SL in males, with females somewhat smaller at around 4 in SL; aquarium specimens are commonly in the 4–5 in range overall. The body carries the family's characteristic divided dorsal fin — a short, spiny anterior dorsal followed by a longer soft-rayed posterior dorsal — and a long-based anal fin. Reported fin counts are 5–7 dorsal spines and 13–20 dorsal soft rays, with a single anal spine and 18–24 anal soft rays.
Colour is variable across the species' wide range, which is part of its appeal. The base tends toward a silvery to greenish or bluish body, often with a darker mid-lateral region and a pattern of fine longitudinal lines following the scale rows; fins may carry yellow, orange, or reddish tones that intensify in displaying males. Because populations differ, no single description fits every fish: the Sepik-region 'Pagwi' form, for instance, is noted for warmer reddish and yellow tones, while other populations are cooler and plainer.
As in rainbowfishes generally, mature males are larger and deeper-bodied than females, with more vivid colour and extended median fins, and they flush brightest when displaying. Females and juveniles are rounder-bellied and more subdued — a clear but unexceptional sexual dimorphism shared across the genus.
Habitat
Melanotaenia affinis is widely distributed through the freshwater drainages of northern New Guinea, on the northern side of the island's central dividing range. An Australian government assessment of the species describes a range extending from the Oomsis River near Lae northward and westward across the northern New Guinea watershed, taking in the Sepik and other northern river systems. That broad, well-watered range contrasts with the single-lake endemism of some of its showier relatives, and it is the reason the species is secure where narrow-range rainbowfishes are not.
FishBase characterises the species as a freshwater, benthopelagic fish of tropical latitudes (about 1°S to 7°S), occurring in a temperature range of roughly 68–86 °F. It occupies rivers, streams, and associated still waters, typically among aquatic vegetation and marginal cover in the clear to slightly turbid conditions favoured by rainbowfishes.
Detailed in-situ water chemistry across this large range is data sparse, but hobby sources working from collected stock report the species doing well in slightly acidic to neutral, soft-to-moderately-hard water — broadly the conditions of northern New Guinea's rainforest and floodplain streams, and distinct from the hard, alkaline limestone lakes that some Vogelkop species inhabit.
Feeding
Like rainbowfishes in general, the North New Guinea rainbowfish is an opportunistic omnivore. FishBase places it at a low trophic level (around 2.9), consistent with a diet built mainly on small invertebrates, insects taken at or near the surface, and plant and algal material grazed from submerged surfaces. It feeds actively in the upper and middle water column, where its shoaling, restless habit suits a fish that forages on the move.
In the aquarium it is undemanding and accepts a wide range of foods. Hobby care notes report that it readily takes good-quality flake alongside live and frozen items such as bloodworm, brine shrimp, and Daphnia, and that it will also accept blanched vegetable matter such as lettuce or spinach. A varied diet with a meaningful live or frozen component improves both colour and breeding condition.
As a constant grazer it does best with small amounts offered several times a day rather than a single large feeding, which keeps the fish active and well-coloured without fouling the water.
Mating
Melanotaenia affinis is an egg-scattering, plant-spawning species with no parental care. FishBase notes reproduction by distinct pairing, and in the rainbowfish manner a male courts a female with intense colour displays, flaring his fins and flashing the body so that his warm tones reach maximum saturation. As in many rainbowfishes, a displaying male of this group commonly flashes a pale 'courtship band' along the head and nape, a signal that is most vivid in the early morning.
Courtship and spawning are concentrated in the hours after first light. A receptive female follows a displaying male into fine-leaved vegetation, where the pair release small numbers of adhesive eggs at a time. Because males display most strongly against rivals, the fish are at their most colourful and most reproductively active when several are kept together, and breeders generally condition groups rather than isolated pairs to bring fish into peak form.
The species is a near-continuous spawner under good conditions rather than a strictly seasonal one, returning to the spawning medium day after day as long as it is well fed and warm.
Breeding
The North New Guinea rainbowfish is a straightforward egg-scatterer to breed in captivity, following the standard rainbowfish pattern. Conditioned adults spawn over fine-leaved plants such as Java moss or over synthetic spawning mops, depositing small, adhesive eggs that hang from the medium by fine filaments.
Spawning typically plays out over many days, with a female releasing only a few eggs daily rather than a single large clutch. The adults offer no care and will eat their own eggs, so breeders move the mops or eggs to a separate rearing container. Hobby accounts report that the eggs hatch after roughly a week, the exact interval shortening with warmth within the species' tolerated range.
The tiny fry hang near the surface at first and require very small first foods — infusoria or a comparable liquid fry food — before graduating to newly hatched brine shrimp and finely crushed flake. Growth is gradual and adult colour develops slowly, with males needing time and good conditions to come into full form. The species is reported to be long-lived for its size, on the order of five to eight years.
In the aquarium
Melanotaenia affinis is an active, peaceful, sociable schooling fish best kept in a group of six or more, ideally with several males present so that they display against one another and stay colourful. It is a strong, restless swimmer that occupies the upper levels of the tank and needs a long, open swimming lane; a planted aquarium with open central space and some surface movement suits it well and shows the colours to best effect.
Water requirements are forgiving, reflecting the species' wide natural range. Hobby care guides report success across roughly 70–81 °F, with a pH of about 6.1–7.2 and hardness in the range of 3–16 dGH — that is, slightly acidic to neutral and soft-to-moderately-hard water. Stable conditions and good filtration matter more than precise numbers, and a moderate current is appreciated by this active shoaler.
Tankmates should be similarly sized, peaceful, and not so timid as to be outcompeted at feeding — other rainbowfishes, larger tetras, peaceful barbs, and rasboras are natural companions. It's hardy and long-lived, and the locality forms — the 'Pagwi' strain above all — give aquarists a reason to track down and keep specific populations rather than generic stock.
Conservation
Melanotaenia affinis is assessed as Least Concern (LC) on the IUCN Red List. The assessment reflects the species' wide distribution across the northern New Guinea watershed and the absence of evidence for any major range-wide decline — unlike the narrowly endemic, threatened rainbowfishes of single lake basins such as the Vogelkop species.
A broad range confers resilience: local pressures in one drainage are buffered by populations across many others, so the species as a whole is not at risk from the habitat degradation or over-collection that endanger range-restricted relatives. FishBase rates its population resilience as medium, with a population doubling time of roughly 1.4 to 4.4 years. The species supports both commercial and aquarium fisheries within its range and is considered harmless.
As with all wild-collected rainbowfishes, sourcing tank-bred stock where available reduces pressure on natural populations, and the ease with which M. affinis breeds in captivity means hobby demand can largely be met without continued heavy collecting.