Taxonomy & naming
Melanotaenia wilsoni was formally described in 2019 by Michael Hammer, Gerald Allen, Helen Martin, Mark Adams and Peter Unmack, in a paper in Zootaxa (4701(3): 201–234) describing two new dwarf rainbowfishes from northern Australia and southern New Guinea. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia wilsoni Hammer, Allen, Martin, Adams & Unmack, 2019; because the species was described in Melanotaenia, where it remains, the authority carries no parentheses.
For most of its documented history the fish was hidden in plain sight. Pre-2019 publications, including aquarium literature, almost universally identified the small isolated Northern Territory population as Melanotaenia maccullochi, and the new species was recognised as part of the broader M. maccullochi species complex only once molecular and morphological data were brought to bear. During the assessment process it circulated under the provisional name Melanotaenia sp. nov. 'Darwin', the name under which its IUCN listing was first prepared.
The specific epithet honours David Wilson of the Territory Wildlife Park at Berry Springs, who first documented the small, isolated population that proved to be a distinct species. Melanotaenia combines the Greek melas, 'black', with taenia (Latinised from Greek tainia), 'stripe' or 'band' — the dark lateral striping that runs through so many members of the genus, and which in this dwarf is unusually conspicuous.
Morphology
This is the smallest rainbowfish on record. Mature individuals reach only about 1.5 in in standard length — under 1.5 in SL, equivalent to less than roughly 1.5 in in total length — making M. wilsoni a true dwarf even by the modest standards of the maccullochi complex. The body is deep for so small a fish, its depth contained about 3.1 to 3.6 times (mean around 3.3) in the standard length, giving the laterally compressed, slightly diamond-shaped profile characteristic of the genus, together with the divided dorsal fin of two separate sections.
Meristic counts recorded in the original description are tightly bounded: dorsal-fin spines 5 to 7, dorsal soft rays 7 to 9, a single anal spine with 13 to 16 anal soft rays, 31 to 33 vertebrae, and roughly 30 to 34 lateral scales (modally 31). These overlapping counts are typical of the genus and do little on their own to separate the species, which is why it went unrecognised for so long.
The colour pattern is the most useful field character. The live fish is brownish to yellowish above, fading to a silvery or whitish belly, and is crossed lengthwise by seven or eight contrasting dark stripes in black, dark brown or purple. The midlateral stripe is the boldest, often carrying a dark purple hue, with a second prominent stripe lower on the flank. A small orange-red spot marks the upper edge of the gill cover, the iris ranges from silver to orange, and the dorsal and anal fins bear a broad greyish-black diagonal band edged with pale yellow above and a whitish to pale-blue margin. As in many rainbowfishes the colours and stripe contrast are most pronounced in nuptial males, which also show distinct dark markings on the second dorsal fin; otherwise the sexes are similar in fin shape and overall form.
Habitat
Melanotaenia wilsoni is endemic to a small area below the Tabletop Range escarpment in Litchfield National Park, in Australia's Northern Territory. Its known range is a handful of small spring-fed sandy streams in two adjacent catchments: Burton Creek in the Finniss River system, and Tolmer and Sandy creeks in the Reynolds River system. The type locality is Sandy Creek, in the Reynolds River system within Litchfield National Park (approximately 13°15.21′S, 130°44.42′E), collected at about half a metre depth.
The habitat is shallow and confined. The fish lives in clear, spring-fed creeks — typically less than 19.5 in deep — that run over sandy soils and are lined with Pandanus, with abundant aquatic vegetation, woody debris, and connections to densely vegetated peat-swamp side channels. These small systems can be seasonal, and in the dry season the fish retracts into refuge pools and holes as flows shrink.
The water is soft, acidic and cool relative to many tropical lowland habitats, reflecting its spring-fed, peaty origins. Dry-season measurements give a pH of about 6.0 to 6.5 (with values as low as 5.0 noted), temperatures of roughly 73–75 °F, very low conductivity of 15 to 30 µS/cm, and general and carbonate hardness both below about 10 ppm. M. wilsoni shares these creeks with a distinctive assemblage of small fishes, including Melanotaenia nigrans, the purple-spotted gudgeon Mogurnda mogurnda, the spotted blue-eye Pseudomugil gertrudae, Oxyeleotris nullipora, the obbes' catfish Porochilus obbesi, Denariusa bandata, and the swamp eel Ophisternon gutturale.
Feeding
Detailed dietary studies of this dwarf species are sparse, but its ecology and a recorded trophic level of about 3.0 place it among the small, opportunistic omnivores typical of the genus. In its shallow, vegetated creeks the fish would take small aquatic and terrestrial invertebrates — insect larvae, micro-crustaceans, and small insects falling onto or drifting at the surface — along with some algae and organic matter grazed from plants, sand and submerged wood.
In captivity it is reported to accept standard rainbowfish fare scaled to its tiny mouth: fine flake, micro-pellets, and small live or frozen foods such as Daphnia, baby brine shrimp, Cyclops and finely chopped bloodworm. Because of its very small size, foods must be correspondingly small, and frequent modest feedings suit a fish that grazes more or less continuously in the wild. A varied diet with a live or frozen component supports both colour and breeding condition, as it does across the rainbowfishes.
Mating
Direct observations of courtship in M. wilsoni are limited, and much of what can be said is inferred from its placement in the M. maccullochi complex, whose members are egg-scattering plant-spawners without parental care. As in those relatives, nuptial males develop their strongest colour and stripe contrast during display, and the distinct dark markings on the second dorsal fin of breeding males suggest the same fin-flaring, body-flashing courtship used by rainbowfishes generally.
In the genus as a whole, dominant males establish small display areas and court females most intensely in the first hours after dawn, often signalling with a flickering pale 'courtship stripe' along the head and nape. Spawning follows when a receptive female joins a displaying male in fine vegetation. Given the confined, shallow, heavily vegetated creeks this species inhabits, that fine plant cover doubles as both courtship arena and spawning substrate. The species is extremely shy — an adaptation to its very shallow, exposed habitat — which makes natural breeding behaviour difficult to observe even in the aquarium.
Breeding
No species-specific breeding account for M. wilsoni is well documented in the literature, so the following reflects the general pattern of the M. maccullochi complex to which it belongs rather than confirmed observations of this fish; data here are genuinely sparse. Members of the complex are continuous, egg-scattering plant-spawners: conditioned adults deposit small numbers of adhesive eggs over many days into fine-leaved vegetation or spawning mops, the eggs hanging by fine filaments from the substrate. There is no parental care, and adults will consume their own eggs and fry.
A point of conservation relevance is that the species is bred in aquaculture. Fishes of Australia notes that M. wilsoni is produced in aquaculture facilities, with only limited wild harvest of broodstock — meaning the fish reaching the trade are largely captive-bred rather than removed from its tiny natural range. Its high biological resilience, with an estimated population doubling time of under fifteen months, is consistent with the prolific, small-clutch-but-continuous spawning style of its relatives and helps explain why captive production is feasible despite the species' diminutive size.
In the aquarium
Melanotaenia wilsoni is a niche fish, kept by rainbowfish specialists for its rarity and its status as the smallest of all rainbowfishes rather than for colour. The keeping note that matters most is temperament: it is reported to be extremely shy in captivity, a hangover from life in very shallow, exposed creeks where caution is survival. Keep it as a group in a quiet, well-planted tank with subdued light, plenty of fine vegetation and cover, and calm tankmates that will not outcompete so small and timid a fish at feeding time.
Water should reflect its soft, acidic, cool origins more than the hard-water defaults used for some other Melanotaenia. Conditions in the wild run to a pH of about 6.0 to 6.5, very low hardness (GH and KH each below roughly 10 ppm), low conductivity, and temperatures around 73–75 °F — cooler than many tropical community tanks. Stable, clean, gently filtered soft water in that range, with the heavy planting the species favours, gives the best chance of seeing it relax and colour up. Given a maximum size of barely over three centimetres, it is suited only to small, peaceful, species-appropriate setups, not a busy general community.
Conservation
Melanotaenia wilsoni is assessed as Vulnerable on the IUCN Red List under criterion D2, the category for species with a very restricted range at few locations. The assessment was prepared under the provisional name Melanotaenia sp. nov. 'Darwin' (Hammer 2019), and the listing turns squarely on the fish's biology: it is endemic to a tiny cluster of spring-fed creeks below the Tabletop Range escarpment in Litchfield National Park, occupying only a few locations across the Finniss and Reynolds River systems.
A range that small carries inherent fragility. There is no large outlying population to absorb a local shock, so any degradation of the spring-fed creeks — altered flows, drying, fire effects on the catchment, or the arrival of introduced fishes — could affect a meaningful fraction of the entire species at once. The seasonal nature of some of these creeks, with the fish retreating to refuge holes as flows fall, adds to that exposure.
Two factors moderate the picture. The species' whole known range lies within Litchfield National Park, a protected area, and the fish in the aquarium trade are largely produced in aquaculture facilities with only limited harvest of wild broodstock, which keeps direct collecting pressure off the small natural population. As with other narrowly endemic rainbowfishes, supporting captive-bred stock is the most direct way hobbyists can help keep that pressure low.