Taxonomy & naming
Melanotaenia maccullochi was described by the Australian ichthyologist James Douglas Ogilby in 1915. Because it was described in Melanotaenia, where it remains today, no parenthetical authority is used. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia maccullochi Ogilby, 1915. The genus Melanotaenia is the largest in the family Melanotaeniidae — the true rainbowfishes — which radiate across New Guinea and northern and eastern Australia within the order Atheriniformes.
The specific epithet honours Allan Riverstone McCulloch (1885–1925), the prolific Australian Museum ichthyologist whose work documented much of Australia's fish fauna in the early twentieth century. The genus name combines the Greek 'melas' (black or dark) with the Latin-via-Greek 'taenia' (a band or stripe), a reference to the dark lateral striping seen across many species of the genus and clearly expressed in this one.
The species was the subject of a modern redescription by Hammer, Allen, Martin, Adams and Unmack in 2019 (Zootaxa 4701(3): 201–234), which clarified its characters and distribution within a group of small north-eastern Australian rainbowfishes. Diagnostic meristics include 6–7 dorsal spines, 8–10 dorsal soft rays, a single anal spine with 13–17 anal soft rays, 30–33 lateral-line scales, and 32–35 vertebrae.
Morphology
A small, slim, laterally compressed fish, Macculloch's rainbowfish reaches about 3 in standard length — one of the smaller species in the genus, and mature fish in the hobby run around 3 inches. Like all Melanotaenia it carries the family's divided dorsal fin, a short anterior dorsal followed by a longer posterior one, but its body is more slender and less deep than large-growing relatives such as M. boesemani.
The base colour is bronze to brown, lighter on the belly, overlaid with dark brown lateral stripes that run along the flanks. The fins are the draw: orange to deep red, intensifying in nuptial males — the source of the 'redfin' dwarf-rainbow name in the trade. As across the genus, colour tracks mood and condition closely; a displaying male flushes far brighter than a stressed or subordinate one. Females and juveniles are plainer, with the silvery-bronze ground colour and more subdued finnage.
Habitat
Melanotaenia maccullochi is native to tropical north-eastern Queensland, Australia, where it occurs in coastal streams from the Cape Flattery / Cooktown area south to the Cardwell region (roughly 15–18.5 degrees south), including small creeks around Cairns and the Daintree. It inhabits lowland swamps and small streams, typically the clear, acidic waters of small creeks and Pandanus swamps with ample cover from log debris and aquatic vegetation. It favours quiet, slow-moving or still water rather than fast-flowing rivers.
Field measurements across its range run between 24 and 86 °F, with pH spanning roughly 5.5 to 7 — slightly acidic to neutral, soft-water conditions that reflect the tannin-rich, vegetated swamps it occupies. That contrasts with the hard, alkaline lakes some West Papuan congeners prefer, a useful corrective for aquarists who assume a single 'rainbowfish' water profile.
In its native waters the species co-occurs with the larger Melanotaenia splendida and shares the swamps with other small specialists: the blue-eye Pseudomugil gertrudae and the sleeper goby Oxyeleotris nullipora. The company tells you what kind of fish it is — a vegetated soft-water community species, not an open-river swimmer.
Feeding
Like rainbowfishes generally, Macculloch's rainbowfish is an opportunistic omnivore, with a trophic level (estimated around 2.9 from its relatives) consistent with a small-invertebrate and plant diet. In the wild it takes small aquatic and terrestrial invertebrates — insect larvae, tiny crustaceans, and insects from the surface — together with algae and plant material grazed from submerged vegetation and debris.
In the aquarium it is an unfussy feeder that readily accepts good-quality flake and small pellets, frozen and live foods such as brine shrimp, Daphnia, and bloodworm, and small live foods generally. A varied diet with a meaningful live or frozen component improves both colour and breeding condition; as a small, active fish it does best with small amounts offered several times a day rather than a single large feeding.
Mating
Macculloch's rainbowfish is an egg-scattering, plant-spawning species with no parental care, forming distinct pairs at spawning time. Males court females with colour displays, the orange-red finnage reaching its fullest saturation during courtship; as in many rainbowfishes, display and spawning activity are most intense in the early morning after first light.
A receptive female follows a displaying male into fine-leaved vegetation, where the pair release small numbers of eggs over an extended period. FishBase reports a fecundity on the order of 150–200 eggs. Because males display most strongly in the presence of company, the species shows its best colour when kept in groups, and breeders typically condition shoals rather than isolated pairs to bring fish into peak form.
Breeding
This is one of the more straightforward rainbowfishes to spawn in captivity, which — together with its small size and long history in the hobby — accounts for the prevalence of tank-bred stock. Conditioned adults deposit small adhesive eggs among fine-leaved plants such as Java moss or over synthetic spawning mops, the eggs hanging by fine filaments from the spawning medium.
Spawning typically occurs over many days, a female releasing a few eggs at a time rather than a single large clutch, often in the early morning. The adults offer no parental care and will eat eggs, so breeders harvest the mops or eggs to a separate rearing container. The tiny fry require very small first foods — infusoria or commercial liquid fry food — before graduating to newly hatched brine shrimp and finely crushed flake. As with rainbowfishes generally, growth is gradual and full adult finnage colour develops slowly over months.
In the aquarium
Macculloch's rainbowfish is a peaceful, active, shoaling fish best kept in groups of six or more, where males display against one another and stay colourful. Its small size makes it one of the more apartment-friendly rainbowfishes: a tank of around 30 gallons (a longer footprint preferred over a tall one) suits a group, with open swimming lanes flanked by planting, some driftwood, and gentle-to-moderate flow.
Stability matters more than chasing precise numbers. Hobby experience places it comfortably at roughly 73–82 °F in neutral to slightly alkaline, soft-to-moderately-hard water, though its wild range runs cooler and more acidic; the species tolerates a fairly wide band provided conditions are stable. It makes an excellent peaceful community fish, mixing well with tetras, rasboras, Corydoras, peaceful dwarf cichlids, and other peaceful rainbowfishes. A planted tank with open central swimming space and good lighting shows the red finnage to best effect and encourages the morning display behaviour.
Conservation
Melanotaenia maccullochi is assessed as Least Concern (LC) on the IUCN Red List, assessed on 5 December 2019. The species is widespread across suitable lowland swamp and creek habitat in tropical north-eastern Australia and is not considered to face range-wide threats of the kind that imperil narrowly endemic rainbowfishes elsewhere.
The species is harvested for the aquarium trade and is recorded in commercial fisheries use, but it is harmless to humans and its broad distribution and ready captive breeding buffer it against collection pressure. As with all rainbowfishes, the principal long-term risks are local: habitat degradation and the introduction of non-native fishes into the soft-water swamp systems it depends on. A medium resilience and an estimated population doubling time of 1.4–4.4 years give it reasonable capacity to recover from local declines.