Taxonomy & naming
Melanotaenia nigrans was described by the Scottish naturalist and surgeon John Richardson in 1843, making it the earliest-named species of the genus and the type species of the family Melanotaeniidae — the true rainbowfishes and blue-eyes of the order Atheriniformes. Because Richardson originally placed it in a different genus, Eschmeyer's Catalog of Fishes records the valid authority in parentheses: (Richardson, 1843). It anchors both genus and family in name — even though, in the flesh, it's one of the plainer members of the group.
The genus name Melanotaenia derives from the Greek melas/melanos, 'black' or 'dark', and the Latinised taenia (from Greek tainia), 'stripe' or 'band' — literally 'black stripe', a direct reference to the dark lateral band that runs along the flanks of this and many related species. The specific epithet nigrans, from the Latin for 'becoming black' or 'darkening', names the same feature again. The standard taxonomic and distributional treatment is Gerald Allen's Freshwater Fishes of Australia (1989), which built on Allen & Cross's earlier revision of the family.
Morphology
Melanotaenia nigrans is a small, slender rainbowfish, reaching about 4 in total length at most, with a common size closer to 2.5 in standard length; many wild and aquarium specimens are smaller still, and some sources cite a typical adult size of around 3 in. The body is laterally compressed and only moderately deep — less diamond-shaped than the high-bodied species such as M. boesemani — and carries the family's characteristic divided dorsal fin, a short anterior portion followed by a longer posterior one set against the anal fin.
The defining feature, and the source of both names, is a dark mid-lateral stripe running from snout to caudal base. Against a base colour of silvery to olive or bluish-grey on the back, the black band reads clearly, often bordered above by paler or reddish tones. Fins may carry yellow, orange, or reddish margins. Colour intensity varies with population, mood, and condition — a displaying fish is much brighter than a stressed or subordinate one. Sexual dimorphism is slight by the standards of the showier Melanotaenia: males run a little deeper-bodied and more intensely coloured than the plainer females.
Habitat
Melanotaenia nigrans is endemic to Australia, where it has a broad but strongly discontinuous distribution across the tropical north, occurring in three widely separated areas: the eastern Kimberley of Western Australia (around Napier Broome Bay), the northernmost Northern Territory including Groote Eylandt, and the northern tip of Queensland's Cape York Peninsula including Prince of Wales Island in Torres Strait. It usually stays within about 25 mi of the coast but has been recorded up to 81 mi inland. The result is a patchwork of disjunct populations rather than a continuous range — familiar across its native country, with wide gaps between the occupied drainages.
The habitats it uses are unusually varied. It turns up in rainforest streams, lily-covered lagoons and small creeks, and in the backwaters and shorelines of larger streams where flow is minimal and grassy or other marginal vegetation is thick. Unusually for a rainbowfish, it also pushes into brackish coastal waters and tolerates some salinity. Water chemistry skews soft and acidic in its rainforest-stream habitats — FishBase notes a pH range around 5.2 to 5.6 in one reference — while broader biological data give roughly pH 5.5 to 7 across its range. Temperatures are tropical, with reported figures spanning about 64–82 °F depending on locality and season. It commonly co-occurs with other rainbowfishes and with blue-eyes (Pseudomugilidae).
Feeding
In the wild, Melanotaenia nigrans is an omnivore feeding mainly on aquatic and terrestrial insects and their larvae, supplemented by filamentous green algae. Its reported trophic level of about 2.9 marks it as a small predator-grazer that takes a fair share of plant and algal material alongside invertebrate prey. A benthopelagic fish of slow and still water, it works the middle and upper column and the vegetated margins, picking insects off the surface and grazing algae from submerged surfaces.
In the aquarium it's undemanding and takes most prepared and natural foods. Good flake and small pellets make a suitable staple; add frozen and live foods such as Daphnia, brine shrimp, bloodworm, and Cyclops, plus some vegetable or algal matter to echo the natural diet. As with rainbowfishes generally, a varied diet with a regular live or frozen component improves colour and brings fish into breeding condition. Several small feedings a day suit this constantly active grazer better than one large meal.
Mating
Reproduction follows the genus pattern: an egg-depositing, plant-spawning fish that pairs off during courtship and spawns year-round rather than in a single seasonal pulse. Males court with heightened colour and active display, and as in other rainbowfishes the spawning runs in the first hours after light.
The pair forms at spawning, the male leading a receptive female into fine-leaved vegetation where eggs go out a few at a time. Rainbowfish males display hardest with rivals present and an audience of females, so colour and breeding behaviour are most consistent in a group, not an isolated pair — breeders condition small shoals to bring fish into form before spawning. (Specific courtship signals for this species are poorly documented; the account here reflects the well-known genus-wide pattern.)
Breeding
Breeding follows the standard rainbowfish template. The eggs are adhesive and thread-bearing, hanging by fine filaments from aquatic plants or, in the tank, from synthetic spawning mops; spawning runs through the year under good conditions. There's no single large clutch — a female releases a few eggs at a time over a long stretch, so a conditioned pair or group will spawn repeatedly across many days.
Conditioned adults spawn over fine-leaved plants such as Java moss or over mops, which you then lift to a separate container to hatch, since the adults give no parental care and will eat eggs and fry. The adhesive eggs hatch in roughly a week to ten days at typical breeding temperatures in the mid-to-upper 20s Celsius, sooner in warmer water. The fry are tiny and need very small first foods — infusoria, rotifers, or commercial liquid fry food — before they graduate to newly hatched brine shrimp and finely crushed flake. Growth is slow, and full adult colour fills in over the following months. Its adaptability and year-round spawning put it among the more manageable rainbowfishes to breed at home.
In the aquarium
A hardy, peaceful, active schooling fish that has been in the hobby a long time and settles in well. Like all rainbowfishes it wants a group — six or more — in a tank with open swimming space, where the shoal stays busy and the fish colour up against one another. Its modest adult size means it needs less tank length than the larger rainbowfishes, though it still wants room to swim and some surface movement.
The wide natural range and brackish tolerance translate into an easy fish for a range of conditions. It does well in soft to neutral water around pH 5.5 to 7, matching its rainforest-stream origins, at tropical temperatures in the low-to-mid 20s Celsius — but it takes a wider band than most rainbowfishes and will handle slightly brackish water if you need it to. Stable, clean water and good filtration count for more than precise numbers. A planted tank with open central room and bright lighting shows the dark lateral band and fin colours best. Peaceful, similarly sized tankmates — other rainbowfishes, larger tetras, peaceful barbs, rasboras — suit it well.
Conservation
Melanotaenia nigrans is assessed as Least Concern (LC) on the IUCN Red List, evaluated on 15 February 2019. The status rests on a wide, if discontinuous, distribution across multiple drainage systems of tropical and subtropical northern Australia, tolerance of everything from rainforest streams to brackish coastal backwaters, and the absence of any range-wide threat. With medium resilience and low vulnerability to fishing, it sits among the more secure rainbowfishes — the opposite end of the spectrum from narrowly endemic, threatened relatives like Melanotaenia boesemani.
That adaptability shows up in its history abroad. The species was recorded once in the United States, in the Lower Kaskaskia system of Illinois in 1930, near St. Louis — a likely failed aquarium-trade release, and one of the earliest records of an introduced ornamental fish in open US waters, with specimens identified by the ichthyologist Carl Hubbs. None have turned up since; the introduction never took hold. At home it faces no documented conservation concern, and how easily it breeds in captivity keeps collecting pressure off wild stocks.