Taxonomy & naming
Melanotaenia eachamensis was described by Gerald R. Allen and Norbert J. Cross in 1982, in their monograph Rainbowfishes of Australia and Papua New Guinea, from a type locality of Lake Eacham, a circular crater lake on the Atherton Tableland of northern Queensland, Australia. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia eachamensis Allen & Cross, 1982; because the species was described in Melanotaenia, where it remains, no parenthetical authority is used. The specific epithet eachamensis is a toponym, simply meaning 'from Lake Eacham'.
The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia. M. eachamensis belongs to a complex of closely related, often difficult-to-separate Tableland and wet-tropics rainbowfishes; it overlaps broadly with relatives in hard meristic characters and is distinguished more by colour, body proportions, and geography than by any single diagnostic count. The taxonomy of the original Lake Eacham population has itself drawn recent genomic attention: Tims and colleagues (2024) examined museum specimens of the extinct crater-lake fish. This is a genetically tangled group, and the work shows it.
Morphology
Melanotaenia eachamensis is a small rainbowfish, reaching roughly 2.5 in in standard length, with most individuals smaller. It has the laterally compressed, moderately deep body typical of the genus — greatest body depth around a quarter of the standard length — together with the family's characteristic two-part dorsal fin, a short anterior dorsal followed by a longer posterior one. Published meristics give the dorsal fin as V–VI spines followed by I,9–13 rays, the anal fin I,15–21, the pectoral fins 11–14 rays, with about 10–12 horizontal and 33–38 vertical scale rows.
In colour it is a comparatively understated rainbowfish: silvery to bluish on the body, marked by a dark mid-lateral stripe with two or three fainter, thinner dark bands below it, and reddish fins. Breeding males intensify, turning bronze to orange and developing a black margin along the dorsal and anal fins; their fins also grow longer, more pointed, and more brightly coloured than those of females. Brighter, longer-finned males against plainer females is the usual pattern across the genus. As in other rainbowfishes the male's colour is mood-dependent, flushing brightest during morning display.
Habitat
Although named for Lake Eacham, the species is today understood as an upland endemic of the Atherton Tableland and adjacent wet tropics. It occurs in the upper reaches of the Barron, North Johnstone, and South Johnstone River catchments in Queensland, generally at elevations above about 1509 ft, and is also recorded from Koombooloomba Dam on the Tully River; FishBase notes records spread across Tableland localities between the Barron and South Johnstone systems. These are cool, clear, higher-elevation freshwaters rather than lowland rainforest streams.
The fish inhabits small creeks and crater lakes, where it schools along shallow edges among aquatic vegetation, fallen logs, and submerged branches. FishBase characterises the water as tropical, around 73–81 °F, with a pH of roughly 7.0–7.4 — slightly cooler and closer to neutral than many lowland rainbowfish habitats, reflecting the upland Tableland setting. Its patchy occurrence across separate headwater catchments is central to its conservation story: populations are small and isolated, with little buffering against local declines.
Feeding
The Lake Eacham rainbowfish is an opportunistic omnivore. Fishes of Australia reports its diet as aquatic and terrestrial invertebrates together with algae — the broad feeding strategy shared across rainbowfishes, which take insect larvae and small crustaceans from the water column, snap insects from the surface, and graze algae and biofilm from submerged surfaces. FishBase places it at a low trophic level of about 2.9, consistent with a small, largely invertebrate-and-algae feeder.
In captivity the species is undemanding and accepts standard rainbowfish fare: good-quality flake and small pellets, supplemented with frozen and live foods such as Daphnia, brine shrimp, bloodworm, and Cyclops, plus some vegetable or algal content. As with its relatives, a varied diet with a meaningful live or frozen component improves both colour and breeding condition, and the fish does best on small amounts fed several times a day rather than a single large feeding.
Mating
Melanotaenia eachamensis follows the rainbowfish reproductive pattern of distinct pairing during spawning, with no parental care. FishBase records it as a species with distinct pairing; in practice a displaying male courts a receptive female with intensified colour — the bronze-to-orange flush and dark-margined fins of breeding condition — before the pair move together into fine vegetation or root masses to release eggs. As in the genus generally, male display is strongest in the morning hours and is heightened in the presence of rival males.
Spawning in the wild is seasonal but protracted, running from August to April with a peak from August to November according to Fishes of Australia. Rather than a single large clutch, the female releases small numbers of eggs over an extended period, the eggs being deposited onto fine substrates in well-oxygenated water. Recorded fecundity is modest — on the order of two hundred eggs (FishBase cites a figure around 206) — fitting the species' small size and the small-clutch, repeat-spawning strategy typical of rainbowfishes.
Breeding
Breeding biology is reasonably well documented for this species. Eggs are small, demersal, and adhesive, attaching by fine filaments to root masses in well-oxygenated areas — a placement that keeps the developing eggs in flowing, oxygen-rich water. Spawning extends across the warmer months (peak August–November), with females depositing eggs a few at a time over a long period rather than in one batch.
Larvae are small at hatching and, unusually, lack a well-developed yolk sac, so the newly hatched young must begin feeding quickly; Fishes of Australia reports larval development complete at about 0.5 in total length. Tiny, free-feeding larvae mean captive rearing demands very small first foods — infusoria, rotifers, or commercial liquid fry food — before fry can take newly hatched brine shrimp. As in other rainbowfishes, the adults offer no care and will consume eggs and fry, so breeders harvest spawning media or eggs to a separate rearing container. FishBase rates the species' resilience as medium, with a population doubling time of roughly 1.4 to 4.4 years.
In the aquarium
The Lake Eacham rainbowfish owes its survival to the aquarium hobby. When it vanished from Lake Eacham in the late 1980s, it persisted in private aquarist collections, from which it was effectively rediscovered. It is a peaceful, active, sociable schooling fish that — like its relatives — is best kept in a group of six or more, ideally with several males so that they display against one another and hold their breeding colour.
Because it comes from cooler, higher-elevation Tableland waters, it suits slightly cooler, near-neutral conditions than lowland tropical rainbowfishes: aquarium temperatures in the low-to-mid 20s Celsius and a pH near neutral (around 7.0–7.4) reflect its wild habitat. A long tank with open central swimming space, some surface movement, and planted margins shows the fish off and encourages morning display. Tankmates should be similarly sized and peaceful. One conservation caveat carries straight into the hobby: the species hybridises readily with the eastern rainbowfish (Melanotaenia splendida), so keepers wanting pure M. eachamensis lines must never house the two together — the same precaution that mirrors the threat erasing the fish in the wild.
Conservation
Melanotaenia eachamensis is assessed as Endangered (EN) on the IUCN Red List under criteria B1ab(ii,iii,v)+2ab(ii,iii,v), in an assessment by Unmack and Brown dated 15 February 2019 (IUCN 2019: e.T13054A123378257). It is the textbook Australian example of how an introduced species can erase an endemic: the original Lake Eacham population disappeared after non-native fishes entered the lake, and the species was thought extinct in 1987 before being recovered from aquarist stock and later found to persist more widely in upper Barron and Johnstone catchments. A 1980s attempt to reintroduce some 300 fish to Lake Eacham failed because the invasive predators were not removed first, and the eastern rainbowfish is now established in the lake.
The principal ongoing threat is genetic rather than purely demographic. As the widespread eastern rainbowfish (Melanotaenia splendida) expands into its range, it hybridises and introgresses with M. eachamensis, eroding the species from within; invasive mosquitofish (Gambusia) and tilapias add further pressure, alongside habitat degradation from siltation linked to grazing and sugar-cane cultivation, riparian clearing, and weeds. Where it survives the species can be locally common but its distribution is patchy, and observers have recorded a decline in mature individuals attributable to introgression. Conservation therefore hinges on protecting genetically intact populations in upland refuges and preventing further mixing with M. splendida — a task complicated by the fact that the threat is another rainbowfish rather than an obvious alien predator.