Taxonomy & naming
Melanotaenia parkinsoni was described by the Australian ichthyologist Gerald R. Allen in 1980. It sits in the family Melanotaeniidae, the true rainbowfishes, within the order Atheriniformes. Melanotaenia is by far the largest genus in the family, radiating across New Guinea and northern and eastern Australia. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia parkinsoni Allen, 1980; because the species was described in Melanotaenia, where it remains, the authority carries no parentheses.
The genus name combines the Greek melan, 'black' or 'dark', with the Latin taenia, 'stripe' or 'band', a reference to the dark lateral band that runs along the body of many species in the genus. The specific epithet is a patronym honouring Brian J. Parkinson, a New Zealand naturalist, shell dealer, and collector who worked in New Guinea. Within the genus it is told apart more by colour and its robust, deep-bodied build than by hard meristic characters, which overlap broadly across Melanotaenia; reported fin counts are 6–7 dorsal spines, 9–12 dorsal soft rays, a single anal spine, and 18–23 anal soft rays.
Morphology
Parkinson's rainbowfish is among the larger and more deep-bodied members of the genus. Males reach about 4.5 in in standard length, while females stay smaller, to roughly 3.5 in SL, so total length in a fully grown male approaches 4.5–5.5 in. The body is laterally compressed and deepens with age into the high-backed, almost diamond-shaped profile of a dominant male, and carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal.
Colour is the species' signature. The ground is silvery to pale blue-grey on the upper body; the lower flank and rear half are marked by rows of orange to copper-red dots and dashes that strengthen toward the caudal peduncle and into the unpaired fins, and the head and shoulders often carry a yellow to coppery wash. As in other rainbowfishes the colour tracks mood and condition — a displaying male flushes far brighter than a stressed or subordinate one — and shifts with diet and lighting in captivity. Females and juveniles are plainer, silvery-gold with subdued markings, the same sexual dimorphism that runs throughout the genus.
Habitat
Melanotaenia parkinsoni is endemic to south-eastern Papua New Guinea, where it occurs in the coastal lowlands between the Kemp Welch River and Milne Bay, including the Collingwood and Kemp Welch river systems. It is a freshwater, benthopelagic fish of rainforest streams in lowland areas, recorded across a narrow band of low latitudes, roughly 9–11° S.
It favours clear, well-oxygenated rivers and streams with moderate current and plenty of aquatic vegetation. FishBase reports warm, hard-to-alkaline wild water: temperatures of about 79–86 °F, pH around 7.5–7.8, and moderate hardness. That single-region range — a strip of lowland drainages in one corner of New Guinea — is the central fact of the species' biology and the reason its conservation status is watched. There is little outlying habitat to buffer local pressures.
Feeding
Like rainbowfishes generally, Parkinson's rainbowfish is an opportunistic omnivore that feeds through the upper and middle water column, with an estimated trophic level near 2.9. In the wild the diet comprises small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects taken at the surface — together with algae and plant material grazed from submerged surfaces and vegetation.
In the tank it is undemanding and eats almost anything: good-quality flake and micro-pellets as a staple, with frozen and live foods such as brine shrimp, Daphnia, bloodworm, and tubifex, plus a vegetable or spirulina component. A varied diet with a real live or frozen element improves both colour and breeding condition; colour-enhancing prepared foods are widely used to hold the orange and copper tones, which fade on a flake-only regime. Being an active, constant grazer, it does best on small amounts several times a day rather than one large feeding.
Mating
Parkinson's rainbowfish scatters eggs among plants, pairs off distinctly during courtship, and gives no parental care. Given good conditions it spawns more or less continuously rather than to a fixed season. Males are social and competitive: a dominant male stakes out a display area and courts females with everything he has, flaring his fins and flashing his flank until the orange and copper hit full saturation.
Many rainbowfishes signal with a flickering pale stripe along the head and nape, and it shows brightest in the first hours after light, when spawning peaks. A receptive female follows the male into fine-leaved vegetation, where the pair release a few eggs at a time. Since males show off hardest with rivals present, the fish look their best when several males share a tank, and breeders tend to condition groups rather than isolated pairs to bring them into form.
Breeding
As rainbowfishes go, this one is fairly easy to breed. Conditioned adults spawn over fine-leaved plants such as Java moss or over synthetic mops, leaving small adhesive eggs that hang by fine filaments from the medium. A female releases a few eggs daily over many days rather than one large clutch.
The eggs hatch after roughly 7–10 days, sooner in warmer water. Since adults eat their own eggs and tend none, breeders move the mops or eggs to a separate rearing container. The tiny fry hang near the surface at first and need very small first foods — infusoria, rotifers, or commercial liquid fry food — before moving on to newly hatched brine shrimp and finely crushed flake. Growth is slow, and full colour comes in gradually; males in particular need time and good conditions before they reach their best.
In the aquarium
Keep Parkinson's rainbowfish in a group of six or more — it is an active, peaceful schooler, and a balanced or male-skewed mix keeps the males displaying against one another and holding colour. It is a strong, restless mid-water swimmer that wants a long, open swimming lane; at around 4.5 in SL, give it a tank with room for a proper school. It is peaceful toward its own kind and toward other fish, an easy community member with similarly sized, non-timid tankmates.
Water chemistry should echo its well-oxygenated, neutral-to-alkaline origins. Hobbyists keep it across pH about 7–8 and hardness of roughly 8–18 dGH; tanks usually run cooler than the wild figures, commonly around 70–77 °F, though the fish tolerates the warm end of its natural 79–86 °F range. Stable conditions, good filtration, and some surface movement matter more than the exact numbers. A planted tank with open central space and bright lighting shows the colours off and draws out the morning display. It is hardy and long-lived for its size, around five to seven years, but males take time to colour up — patience is part of keeping them well.
Conservation
Melanotaenia parkinsoni is assessed as Near Threatened (NT) on the IUCN Red List, under criterion B1b(iii); the assessment was carried out on 6 December 2019 and published in 2020. The listing reflects a tight distribution — one coastal region of south-eastern Papua New Guinea between the Kemp Welch River and Milne Bay, an extent of occurrence of about 13,124 mi² — and the inferred decline in habitat quality from logging across that range.
A fish confined to one cluster of lowland drainages has little slack: no large outlying population stands ready to refill a local decline. As with other popular Melanotaenia, captive breeding takes the edge off — tank-bred stock is widely available in the hobby, which cuts collecting pressure on wild fish and keeps the species secure in captivity. Aquarists can help the wild population simply by buying captive-bred fish.