Taxonomy & naming
Melanotaenia herbertaxelrodi was described by the Australian ichthyologist Gerald R. Allen in 1981. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia herbertaxelrodi Allen, 1981; because the species was described in Melanotaenia, where it remains, the authority carries no parentheses. It belongs to the order Atheriniformes and the family Melanotaeniidae, the true rainbowfishes, of which Melanotaenia is by far the largest and most widely radiated genus across New Guinea and Australia.
The genus name combines the Greek melan (black, dark) with the Latinised taenia (stripe or band), a reference to the dark mid-lateral stripe characteristic of many species in the group — a stripe this species wears plainly. The specific epithet honours Dr Herbert Richard Axelrod (1927–2017), the American tropical-fish publisher and aquarium-trade figure. Within the genus the species is placed among the New Guinea highland rainbowfishes; as is typical of Melanotaenia, it is distinguished from relatives more by its live colour pattern than by hard meristic characters, which overlap broadly across the genus. Reported fin counts are 5–7 dorsal spines, 10–16 dorsal soft rays, a single anal spine, and 17–25 anal soft rays on a fusiform body.
Morphology
Melanotaenia herbertaxelrodi is a moderately deep-bodied, laterally compressed rainbowfish. FishBase gives a maximum size of about 6 in total length, though fish in the aquarium more typically reach around 4 in, so the larger figure likely reflects exceptional wild specimens. Like other members of the genus it carries the family's divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — and a body that deepens with age, especially in dominant males.
The colour pattern is the species' signature. The base body colour is a bright yellow, frequently with a greenish tinge along the back, crossed by a distinct bluish-to-black mid-lateral stripe running from in front of the eye to the base of the tail. The unpaired fins — dorsal, anal and caudal — are washed with red or yellow. Males are deeper-bodied, larger and more intensely coloured than females, and develop longer dorsal and anal fins. As with rainbowfishes generally, colour intensity is strongly mood- and condition-dependent: a displaying male flushes far brighter than a stressed or subordinate one, and diet and lighting markedly affect the saturation seen in captive fish.
Habitat
The species is endemic to the highland lake-and-stream system of the Lake Tebera basin in the Southern Highlands of Papua New Guinea, at roughly 6–7 degrees south. Lake Tebera sits at about 2602 ft elevation, ringed by steep mountains rising to 1,200–1,1640 ft. The fish is essentially restricted to this one basin; a single specimen has been recorded from Karamui, around 24 mi north of the lake, which is the extent of the known natural range.
Within the basin it inhabits clear, slow-flowing, densely vegetated waters — the lake itself together with surrounding streams, ponds, swamps, springs and marshes set in rainforest-covered terrain. It is a benthopelagic, freshwater fish of the upper and middle water column. Reported water chemistry leans hard and alkaline relative to many lowland New Guinea habitats: FishBase gives a pH of roughly 7.5–7.8 and a tropical temperature range of about 68–79 °F, with hardness on the higher side. The lake is known to fluctuate in size and has experienced observed drying — a fact central both to the species' biology and to its conservation outlook, since the entire wild population depends on this one impermanent water body.
Feeding
Like rainbowfishes in general, the Lake Tebera rainbowfish is an opportunistic omnivore, reflected in a low trophic level of around 2.9. In the wild it feeds in the upper and middle water column on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans and surface-taken insects — supplemented by algae and plant material grazed from submerged vegetation. It is reported to be fond of vegetable matter and surface plants such as duckweed.
In the aquarium it is undemanding and accepts a wide range of foods: good-quality flake, granules and micropellets, algae or green flake, and frozen or live foods including bloodworm, brine shrimp, mosquito larvae and Daphnia. A varied diet with a meaningful live or frozen component, plus some vegetable matter, improves both colour and breeding condition; colour can fade on a monotonous flake-only regime. As an active grazer it does best fed small amounts several times a day rather than one large feeding.
Mating
The Lake Tebera rainbowfish forms distinct pairs at spawning, in the manner typical of the genus: an egg-scattering, plant-spawning species with no parental care that breeds more or less continuously under good conditions rather than in a single seasonal pulse. Males are social and competitive, staking out display areas and courting females with intense colour displays — flaring the fins and flashing the body so the yellow and red reach maximum saturation.
A spawning male changes almost beyond recognition: a white-to-blue stripe runs from the first dorsal forward across the neck to the tip of the snout, the head darkens to blackish, and the body glows brilliant yellow against red-flushed fins. As in many rainbowfishes this courtship signalling is most vivid in the early morning. Because males display most strongly in the presence of rivals, the fish are at their most colourful when several are kept together, and a receptive female follows a displaying male into fine-leaved vegetation to spawn.
Breeding
Melanotaenia herbertaxelrodi is a straightforward rainbowfish to breed in captivity, which together with its colour explains its hobby popularity and the prevalence of tank-bred stock. It is an egg-scatterer: spawning fish attach small adhesive eggs by fine threads to fine-leaved plants such as Java moss or to synthetic spawning mops, releasing a few eggs at a time over an extended period rather than a single large clutch.
The eggs hatch in roughly 7 to 13 days, the incubation shortening with warmth. The adults offer no care and will eat their own eggs, so breeders harvest the mops or eggs to a separate rearing container. The tiny fry hang near the surface at first and require very small first foods — infusoria — before graduating to microworms and newly hatched brine shrimp. Growth is gradual and the full adult colours develop slowly, so patience and good conditioning are part of producing well-coloured fish.
In the aquarium
The Lake Tebera rainbowfish is an active, peaceful, sociable shoaling fish best kept in a group of six or more, ideally with several males so they display against one another and stay colourful. It occupies the middle and upper levels of the tank and is a restless swimmer that benefits from a long, open swimming lane. It is widely regarded as a good beginner's rainbowfish — hardy and adaptable, with a lifespan of around five to eight years.
Water chemistry should reflect the species' hard, alkaline highland origins rather than the soft-water defaults many aquarists assume for tropical fish: it prefers moderately hard, neutral-to-alkaline water and does not thrive in soft, acidic conditions. Hobby sources recommend pH around 7.5–8.0 with hardness of roughly 10–15 dGH and temperatures of about 68–79 °F, though it acclimatises to a fairly wide range. A planted tank with open central swimming space and bright lighting shows the colours best and encourages the morning display. It is a peaceful community fish that mixes well with other rainbowfishes, barbs, danios, characins and peaceful Corydoras, and is robust enough to share space with larger, well-behaved tankmates.
Conservation
Melanotaenia herbertaxelrodi is assessed as Vulnerable (VU) on the IUCN Red List under criterion D2, in an assessment dated 6 December 2019. The D2 criterion applies to species with a very restricted range that face a plausible threat capable of pushing them to Critically Endangered or Extinct in a short time — exactly the situation of a fish confined to a single lake basin.
The basis for the listing is the species' extreme endemism: it is restricted to the Lake Tebera basin, with an extent of occurrence on the order of 124 mi². Lake Tebera is known to fluctuate in size and has shown observed drying, so the population effectively sits at a single location with no large outlying refuge to buffer a local decline. As with several other narrowly endemic New Guinea rainbowfishes, ease of captive breeding softens the picture somewhat: much of the fish in the aquarium trade is tank-bred, which reduces collecting pressure on the wild population and maintains the species securely in captivity even as its single natural home remains precarious.