Taxonomy & naming
Melanotaenia duboulayi was originally described in 1878 by the French naturalist François de Laporte, Comte de Castelnau, as Atherinichthys duboulayi, from a type specimen taken in a lagoon connected with the Richmond River in northern New South Wales. It was later transferred to Melanotaenia, the largest genus of the family Melanotaeniidae, the true rainbowfishes; because the species was moved from its original genus, the authority is written in parentheses — (Castelnau, 1878). Eschmeyer's Catalog of Fishes governs the valid name, and the Australian faunal reference Fishes of Australia (Gomon & Bray, Museums Victoria) carries it under CAAB code 37245004.
The genus name Melanotaenia combines the Greek 'melan' (black or dark) with the Latin 'taenia' (stripe or band), a reference to the diffuse dark midlateral stripe seen across many members of the genus. The specific epithet 'duboulayi' is a patronym honouring Francis Houssemayne du Boulay (1837–1914), a collector and entomologist who supplied specimens to museums, including the material on which Castelnau based the description. The English name 'crimsonspotted rainbowfish' comes from the red spots that adorn the fins of breeding males.
Morphology
Melanotaenia duboulayi is a moderately deep-bodied, laterally compressed rainbowfish reaching about 4.5 in standard length in males, with females noticeably smaller at around 2.5 in SL; common adult sizes are nearer 3 in in males and 2.5 in in females. Like all members of the genus it has the family's divided dorsal fin — a short first dorsal followed by a longer second dorsal — and, characteristically for rainbowfishes, it lacks a lateral line.
The ground colour is olive-brown above grading to silvery, greenish, or bluish flanks, white below, and often a yellow-to-orange wash toward the tail. A thin reddish line runs between the horizontal scale rows, and a diffuse dark midlateral stripe crosses the body; a red spot typically marks the upper edge of the gill cover. The species is markedly sexually dimorphic. Males grow deeper-bodied and more vividly coloured, especially when breeding, with a taller first dorsal fin, pointed dorsal and anal fin tips, dark fin margins, and scattered red or crimson spots on the caudal, dorsal, and anal fins. Females are slimmer with more rounded fins and plainer, clearer colour.
Habitat
The crimsonspotted rainbowfish is endemic to eastern Australia, where it occurs in coastal and near-coastal drainages from the Mary River in southeastern Queensland south to the Macleay River in New South Wales. Across this range it occupies a wide variety of still and slow-flowing fresh waters: rivers, creeks, drains, ponds, coastal dune lakes, and man-made reservoirs, in both sandy coastal lowlands and inland forested country.
The fish favours quiet or gently flowing water near the surface, congregating around aquatic vegetation, submerged log snags, and other debris that offer cover. It tolerates a wide span of water chemistry: coastal dune-lake populations live in very soft, acidic 'black' water, while inland populations occur in harder, alkaline conditions. FishBase reports a pH span of roughly 5.4 to 7.8 and a temperature range of about 16 to 28 degrees Celsius — the lower end well below what most tropical New Guinea rainbowfishes experience, reflecting this species' subtropical distribution. The entire life cycle is completed in fresh water.
Feeding
Melanotaenia duboulayi is an opportunistic omnivore, feeding through the upper and middle water column with a trophic level around 2.9. Studies of wild diet record a broad menu: terrestrial and aquatic insects, algae and charophytes, aquatic macrophytes, micro- and macro-crustaceans, molluscs, fish eggs, and terrestrial vegetation that falls or washes into the water. This generalist, surface-oriented feeding habit suits the vegetated, debris-rich margins the fish prefers.
In the aquarium it is undemanding and readily takes good-quality flake and small pellets alongside frozen and live foods such as Daphnia, brine shrimp, bloodworm, and Cyclops, plus some plant matter. A varied diet with a regular live or frozen component improves both colour and breeding condition. As an active, more or less constant grazer it does best given small amounts several times a day rather than one large feeding.
Mating
The crimsonspotted rainbowfish is an egg-scattering, plant-spawning species that pairs distinctly during courtship and offers no parental care. Fish reach sexual maturity at roughly 10 to 12 months of age. Spawning runs from late winter through summer, peaking in spring and summer, and is tied to beds of aquatic macrophytes and submerged vegetation.
As in other Melanotaenia, courtship is a visual affair driven by the males, which intensify their colours, flare their fins, and display to females and to rival males — the species is at its most colourful when several males are kept together and compete. A receptive female follows a displaying male into fine-leaved vegetation, where the pair release eggs a few at a time. Eggs are deposited within about 4 in of the surface, among the upper foliage of the spawning plants.
Breeding
Melanotaenia duboulayi is one of the easiest rainbowfishes to breed, and for decades it was a staple beginner's breeding project. Conditioned adults spawn over fine-leaved plants such as Java moss or over synthetic spawning mops, where the relatively large eggs — reported at 0.5 in in diameter — hang by fine adhesive filaments. The eggs are demersal and adhesive, and a female releases a small number each day over an extended spawning period rather than a single clutch.
Incubation depends on temperature: larvae hatch after roughly 5 to 10 days, emerging at about 0.5 in total length. The adults will eat their own eggs, so breeders typically transfer 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 graduating to newly hatched brine shrimp and finely crushed flake. Because the species tolerates cooler water, fry can be reared without the high temperatures some tropical rainbowfishes require, though growth and the development of adult colour are gradual.
In the aquarium
Crimsonspotted rainbowfish are active, peaceful, sociable schooling fish best kept in a group of six or more so that males display against one another and stay colourful. They are strong, restless swimmers that need a tank with a long, open swimming lane; a planted aquarium with open central space and some surface movement shows the fish to best effect. Their hardiness and tolerance of cooler conditions make them well suited to unheated or lightly heated tanks in temperate climates — an unusual virtue among rainbowfishes.
Water chemistry is not demanding: the species thrives anywhere across the wide range it occupies in the wild, from soft and slightly acidic to hard and alkaline, with temperatures comfortably in the high teens to mid-20s Celsius. Stable conditions and good filtration matter more than precise numbers. Tankmates should be similarly sized, peaceful fish — other rainbowfishes, larger tetras, peaceful barbs, and rasboras are natural companions, as are native Australian species in a biotope setting. Wild lifespan is around three to four years, and males need time and good conditions to colour up fully — expect to wait.
Conservation
Melanotaenia duboulayi is assessed as Least Concern on the IUCN Red List (Brown, Unmack & Hammer 2019, assessment published 15 February 2019). The species occupies a substantial range across coastal eastern Australia and a wide variety of habitats, from natural creeks and dune lakes to drains and reservoirs, and it is not currently considered to face range-wide threats sufficient to warrant a higher category.
As with many widespread Australian freshwater fishes, localised pressures still apply — habitat modification, water extraction, and the spread of introduced species can affect particular populations — but a broad distribution and tolerance of varied habitats buffer it against range-wide decline. Its long history as an easily bred aquarium fish also means it is widely maintained in captivity, and tank-bred stock supplies most of the hobby demand, so collection places little pressure on wild populations.