Taxonomy & naming
Melanotaenia lacustris was described by Ian S. R. Munro in 1964. The type locality is Lake Kutubu, a deep highland lake in the Kikori River drainage of the Southern Highlands Province of Papua New Guinea. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia lacustris Munro, 1964; because the species was described in Melanotaenia, where it remains, no parenthetical authority is used.
The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia in the order Atheriniformes. The specific epithet lacustris is Latin for 'of lakes', a nod to the species' lake-dwelling habit — it is one of the few rainbowfishes tied to still lacustrine water rather than rivers and streams. Within the genus, M. lacustris is told apart from its relatives mainly by the adult male's turquoise to blue-green coloration rather than by hard meristic characters, which overlap broadly across Melanotaenia.
Morphology
Melanotaenia lacustris is a moderately deep-bodied, laterally compressed rainbowfish reaching about 4 in in standard length, making it one of the larger members of the genus and a good deal bigger than many of the small stream rainbowfishes in the trade. The body is fusiform and, as in all Melanotaenia, deepens with age, particularly in dominant males. It carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal. Fin counts run to roughly 5–7 dorsal spines and 11–14 dorsal soft rays, with a single anal spine and around 17–21 anal soft rays.
Colour is the species' defining feature. Mature males run turquoise to blue-green, the body seeming to glow with a metallic sheen that shifts with angle, mood and lighting; a darker mid-lateral region and a paler belly complete the pattern, and the colour flushes much brighter when a male is displaying. Females and juveniles are far plainer — a subdued silvery to blue-grey — the marked sexual dimorphism typical of rainbowfishes. How intense the male's turquoise gets depends heavily on condition and mood, and in captivity on diet and lighting.
Habitat
The Lake Kutubu rainbowfish is known only from Lake Kutubu and its outlet, the Soro River, within the Kikori River drainage of the Southern Highlands Province of Papua New Guinea. Lake Kutubu sits in the highlands at roughly 2625 ft elevation, around 137 mi upstream from the Gulf of Papua, and is one of the largest and deepest lakes in the country — a clear-water highland lake with rocky margins and submerged structure.
Water conditions reported for the habitat are warm but cooler than many lowland rainbowfish waters, with temperatures around 68–75 °F, a neutral to alkaline pH of roughly 7.0–8.0, and moderate hardness on the order of 9–12 dH. The species is freshwater and benthopelagic, ranging through the water column near rocky and vegetated cover. Its tiny range — essentially a single lake and its short outflow — is the central fact of its biology and the reason its conservation status is precarious. There is almost nowhere else for it to occur.
Feeding
Like rainbowfishes generally, Melanotaenia lacustris is an opportunistic omnivore, occupying a low-to-middle trophic position (trophic level around 2.9). In the wild it takes a mix of small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and surface-fallen insects — together with algae and plant material grazed from submerged surfaces.
In the aquarium it is undemanding and accepts a wide range of foods: good-quality flake and small pellets, frozen and live foods such as Daphnia, brine shrimp, bloodworm and Cyclops, and some vegetable matter. A varied diet with a real live or frozen component improves both colour and breeding condition, and many hobbyists lean on colour-enhancing prepared foods to hold the turquoise, which fades on a flake-only diet. Being an active, constant grazer, it does best on small amounts several times a day rather than one large feeding.
Mating
Melanotaenia lacustris is an egg-scattering, plant-spawning species with no parental care, and under good conditions it spawns more or less continuously rather than in a single seasonal burst. Males are social and competitive. A dominant male stakes out a display area and courts females by flaring his fins and flashing his body until the turquoise hits maximum saturation. As in many rainbowfishes, a displaying male may show a flickering pale 'courtship band' along the head and nape, a signal most vivid in the morning.
Spawning is most active in the early hours after first light. A receptive female follows a displaying male into fine-leaved vegetation or spawning mops, where the pair release small numbers of eggs at a time. Because males display most strongly in the presence of rivals, the species is at its most colourful when several are kept together, and breeders often condition groups rather than isolated pairs to bring fish into peak form.
Breeding
As rainbowfishes go, the Lake Kutubu is fairly easy to breed, and a good share of trade stock is tank-bred. Conditioned adults spawn readily over fine-leaved plants such as Java moss or over synthetic spawning mops, depositing small, adhesive eggs that hang by fine filaments from the spawning medium.
Spawning typically occurs over many days, with a female releasing a few eggs daily rather than a single large clutch. The adhesive eggs hatch after roughly one to one-and-a-half weeks at typical breeding temperatures, the incubation time shortening with warmth. The adults offer no care and will eat their own eggs, so breeders move 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. Growth is slow, and the adult turquoise develops slower still: males don't reach full colour until well into their second year.
In the aquarium
Melanotaenia lacustris is an active, peaceful, sociable schooling fish best kept in a group of six or more so that males display against one another and stay colourful. It is a strong, restless swimmer and — because it grows larger than many rainbowfishes — needs a long tank with open swimming lanes; an aquarium of around 47 in or more in length suits an adult group, with planted margins and some rockwork or driftwood for cover.
Water chemistry should follow the species' neutral-to-alkaline highland origins, not the soft-water defaults many aquarists reach for with tropical fish. It does best in neutral to slightly alkaline, moderately hard water (around pH 7.0–8.0, hardness on the order of 9–12 dH) at temperatures in the low-to-mid 20s Celsius, a touch cooler than many lowland rainbowfishes want. Stable conditions and good filtration matter more than hitting exact numbers. A planted tank with open central swimming space, some surface movement, and bright lighting shows the turquoise off and brings 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, and peaceful barbs or rasboras all work. The fish is hardy but rewards patience: males need time and good conditions to colour up fully.
Conservation
Melanotaenia lacustris is assessed as Endangered (EN) on the IUCN Red List under criterion B1ab(iii). The listing reflects the species' extremely restricted natural range — essentially Lake Kutubu and its short outflow, the Soro River — which leaves the fish little resilience to any decline in that single basin. With no large outlying population to buffer local losses, habitat change, pollution, or the introduction of non-native fishes to the lake could all have outsized effects.
Ease of captive breeding softens the picture. A substantial share of the Lake Kutubu rainbowfish in the trade is tank-bred rather than wild-caught, which both eases collecting pressure on the wild population and keeps the species widely held in captivity. Aquarists can help simply by sourcing captive-bred stock, and the fish is a frequent subject of hobbyist and association breeding programmes precisely because its wild status is threatened.