Taxonomy & naming
Melanotaenia splendida was originally described by Wilhelm Peters in 1866. Because it was placed in a different genus on description and later moved to Melanotaenia, the authority is conventionally written in parentheses: (Peters, 1866). It is one of the most widespread members of the genus Melanotaenia, the largest genus in the family Melanotaeniidae — the true rainbowfishes that radiate across New Guinea and northern and eastern Australia. The species sits within the order Atheriniformes.
The taxonomy of M. splendida has a long and tangled history. The species was for many years divided into several subspecies spread across its range, but Eschmeyer's Catalog of Fishes (as reflected in its March 2011 working version) synonymised these former subspecies under Melanotaenia splendida, treating it as a single broadly distributed and morphologically variable species. That synonymy explains why the fish varies so much from one population to the next: what were once treated as distinct geographic forms are now read as regional variation within one species. The most recent comprehensive treatment of the group is Allen, Hadiaty, Unmack and Erdmann (2015), published in the journal aqua.
The genus name combines the Greek melas/melanos (black or dark) with taenia (a band or stripe), referring to the dark lateral band characteristic of many species in the genus; the specific epithet splendida is Latin for splendid or shining, a nod to the fish's bright, reflective flanks.
Morphology
Eastern rainbowfish is a moderately deep-bodied, laterally compressed, fusiform fish that reaches a maximum of about 8 in standard length, though a common adult length is closer to 4.5 in and most aquarium and wild specimens are well short of the maximum. Like other Melanotaenia it carries the family's characteristic divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — together with a long-based anal fin. Recorded meristics include 6 to 8 dorsal spines, 10 to 12 dorsal soft rays, a single anal spine, and 18 to 20 anal soft rays.
Colour varies widely across the species' large range — part of why it was once divided into subspecies. The ground colour runs silvery to olive or bluish on the back, paler below, often with rows of fine reddish or orange spots or dashes along the scale rows and a darker mid-lateral stripe. Fins are frequently suffused with red, orange, or yellow, and many populations show reddish margins on the median fins. As with rainbowfishes generally, colour intensity tracks condition and mood: a displaying or well-fed fish flushes far brighter than a stressed one, and dominant males are the most intense in a school. Body depth increases with age, particularly in older males, which become noticeably deeper-bodied than females.
Habitat
Melanotaenia splendida is a freshwater, benthopelagic fish native to Oceania — widespread across much of eastern and northern Australia, from the Adelaide River in the Northern Territory east and south through Queensland and into the Lake Eyre and Murray-Darling basins — making it one of the most widely distributed of all Australian rainbowfishes. It turns up in streams, rivers, and lakes across a wide spread of habitat types, from clear, flowing upland creeks to still, turbid lowland pools and billabongs, and forms large surface-schools, often in considerable numbers.
That breadth of habitat goes with a wide tolerance of water conditions, which is where the fish's reputation for hardiness comes from. Field measurements across its range span temperatures of roughly 20 to 29 degrees Celsius and pH values from about 5.3 to 8.5, in water from clear to turbid. FishBase gives a typical aquarium-relevant range of pH 6.0 to 8.0, hardness of about 10 to 30 dH, and a tropical band of 20 to 30 degrees Celsius. A fish equally at home in soft acidic blackwater and harder neutral-to-alkaline water is one that can occupy an enormous range in the wild and forgive a lot in captivity.
Feeding
Eastern rainbowfish is an opportunistic omnivore, sitting at a relatively low trophic level (around 2.7) that reflects a mixed diet rather than dedicated predation. In the wild it feeds in the upper and middle water column on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects taken at or near the surface — together with algae and plant material. The surface-schooling habit suits a fish that picks off whatever drifts or floats past.
In the aquarium it is undemanding and takes most things: good-quality flake and small pellets, frozen and live foods such as Daphnia, brine shrimp, bloodworm, and Cyclops, and some vegetable matter. As with other rainbowfishes, a varied diet with a real live or frozen component improves both colour and breeding condition; a flake-only regime leaves the fish looking duller. It grazes more or less constantly, so small amounts several times a day suit it better than one large feeding.
Mating
Melanotaenia splendida is an egg-laying, plant-spawning species that forms distinct pairs at spawning even though it lives in schools. Males display to females with intensified colour, and as in rainbowfishes generally, courtship and spawning concentrate in the early morning after first light, when displaying males are at their brightest. Many rainbowfishes flicker a pale 'courtship stripe' along the head and nape during display, and the species shows its best colour when several males compete for females.
Reproductive output is high. The species is rated as having high resilience and a fecundity on the order of several hundred eggs, with maturity reached at a small size — length at maturity around 1.5 in, in the range of roughly 1–1.5 in. Spawning is drawn out rather than a single event: a female releases eggs over an extended period rather than all at once. Since males display hardest with rivals around, conditioning groups rather than isolated pairs brings the fish into the best spawning form.
Breeding
Eastern rainbowfish is a straightforward egg-layer in captivity, regarded by hobbyists as relatively easy to breed. Females attach adhesive eggs by fine filaments to aquatic vegetation and submerged material — in the wild this includes things such as the submerged roots of Melaleuca (paperbark) along river banks and margins, and in the aquarium fine-leaved plants such as Java moss or synthetic spawning mops serve the same purpose.
The standard hobby method follows the one used for other rainbowfishes: a separate breeding tank fitted with spawning mops or fine-leaved plants, into which conditioned adults drop small numbers of eggs over many days rather than in one clutch. The adhesive eggs hang from the spawning medium and can be harvested to a rearing container, since the adults give no parental care and will eat eggs and fry given the chance. After hatching, the tiny fry hang near the surface 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 adult colour develops slowly too, as it does throughout the genus.
In the aquarium
Active, peaceful, and sociable, Melanotaenia splendida is one of the most forgiving rainbowfishes for the aquarium, thanks to its wide tolerance of water conditions in the wild. Keep it in a group of six or more so the males display against one another and hold their colour. It is a strong, restless swimmer that wants a long, open swimming lane, and given its potential size, a long tank beats a tall one.
Water chemistry is undemanding. The species does well anywhere from soft and slightly acidic to moderately hard and slightly alkaline — roughly pH 6.0 to 8.0, hardness around 10 to 30 dH — at tropical temperatures in the low-to-mid 20s Celsius. Stability and good filtration matter more than hitting exact numbers. A planted tank with open central space, some surface movement, and bright lighting shows the colours best and draws out the morning display. Tankmates should be similarly sized, peaceful, and not so timid that they lose out at feeding time: other rainbowfishes, larger tetras, peaceful barbs, and rasboras all work. The fish is hardy and reasonably long-lived, with a maximum reported age of around 3 years.
Conservation
Melanotaenia splendida is assessed as Least Concern (LC) on the IUCN Red List. The listing reflects an enormous range across eastern and northern Australia, abundance, and a broad tolerance of habitat and water conditions — the opposite of the narrow-endemic vulnerability that threatens some of its relatives. It is not on CITES and is not evaluated under the Convention on Migratory Species.
The species is harmless to humans and of minor commercial value, taken in local fisheries and supplied to the public-aquarium and ornamental trade. High reproductive output, early maturity, and adaptability give it strong resilience to localised pressure. Regional populations can still be hit by habitat alteration, introduced species, and changed flow regimes, as with any widespread freshwater fish, but at the species level M. splendida is secure and not of conservation concern.