Taxonomy & naming
Melanotaenia caerulea was described by the Australian ichthyologist Gerald R. Allen in 1996, in a paper treating two new rainbowfishes from the Kikori River system of Papua New Guinea (Revue française d'Aquariologie 23(1-2):9-16). Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia caerulea Allen, 1996; 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 of the order Atheriniformes, which radiate across New Guinea and northern and eastern Australia. M. caerulea is one of a cluster of Melanotaenia endemic to the rivers draining the southern flank of New Guinea's central highlands. The specific epithet, from the Latin caerulea meaning 'blue', refers to the characteristic colour of the fish — the same naming logic that gives it the English common name blue rainbowfish. Within the genus it is separated more by colour and geography than by hard meristic characters, which overlap broadly across Melanotaenia.
Morphology
Melanotaenia caerulea is a small rainbowfish, recorded to about 2.5 in standard length in the original description and to at least 2.5 in SL by later field assessment — modest even by the standards of a genus whose larger members reach 4.5–6 in. It carries the family's characteristic laterally compressed body and divided dorsal fin, a short anterior dorsal followed by a longer posterior dorsal.
The defining feature, as the name promises, is colour: a predominantly blue body that sets it apart from its more red- or gold-toned congeners. Detailed published descriptions of mating dress and fin colour for this species are sparse, and the fish is not widely photographed — FishBase, for instance, carries no image of it. As with rainbowfishes generally, the blue is likely to be brightest in displaying, well-conditioned males and more subdued in females and stressed fish, but precise dimorphism notes for M. caerulea are not well documented and should not be assumed beyond the genus-typical pattern.
Habitat
Melanotaenia caerulea is endemic to the middle and lower Kikori River system in south-central Papua New Guinea, where it spans roughly 78 mi between about 20 and 1312 ft elevation. Its known extent of occurrence is small — on the order of 5,62 mi² — and it is treated as occupying effectively a single location. Curiously, it appears to be absent from the main Kikori River itself, living instead in the smaller waters around it.
Two habitat types are recorded. In the Lower Kikori it occupies flood-plain ponds and small creeks; in the Middle Kikori it lives in small streams that run through closed-canopy forest over limestone hills. The latter setting — clear, limestone-influenced forest creeks — points to mineral-rich, hard-to-alkaline water typical of karstic catchments, shaded by intact forest. The species is benthopelagic and non-migratory. That confined range — a handful of waters within one river basin — is the central fact of its biology, and the reason its conservation status is precarious.
Feeding
No diet study specific to Melanotaenia caerulea is available, so its feeding ecology is inferred from the genus. FishBase places it at a trophic level of about 2.9, consistent with an opportunistic omnivore rather than a specialist. Rainbowfishes of this size typically feed in the upper and middle water column on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and insects taken at or near the surface — supplemented by algae and plant material.
In the forest-creek and flood-plain habitats it occupies, allochthonous input — insects and other matter falling from the overhanging canopy — is likely an important food source, as it is for many shaded-stream fishes. Because the species is essentially absent from the aquarium trade, there is no body of captive feeding observation to draw on; any husbandry would reasonably follow standard Melanotaenia practice of a varied omnivorous diet.
Mating
Reproductive behaviour specific to Melanotaenia caerulea is not documented in detail. FishBase records its reproduction simply as involving distinct pairing, which is consistent with the courtship pattern seen across the genus, where a male pairs with and displays to a female before spawning.
In rainbowfishes generally, mating peaks in the first hours after light, with dominant males staking out display areas and courting females through hard colour displays — flared fins, a flashing body, and in many species a flickering pale 'courtship band' along the head and nape. Whether M. caerulea shows the same signalling has not been published, so these details are genus-level context, not species-confirmed observation.
Breeding
There is no published account of captive or wild breeding for Melanotaenia caerulea specifically, and the species is not in aquarium culture, so direct breeding data is absent. What can be said comes from the genus and from the species' demography. FishBase rates its resilience as high, with an estimated minimum population doubling time of under 15 months, which implies a fairly productive reproductive strategy when conditions allow.
Melanotaenia rainbowfishes are typically egg-scattering plant-spawners with no parental care: conditioned adults release small adhesive eggs a few at a time over many days, the eggs hanging by fine filaments from fine-leaved plants, and the tiny fry needing very small first foods. M. caerulea most likely follows this pattern, but without species-specific records that remains inference rather than established fact.
In the aquarium
Melanotaenia caerulea is essentially unknown in the aquarium hobby. The IUCN assessment notes it has potential as an aquarium species but is not currently in the trade, and targeted searches of the main rainbowfish hobby resources turn up no dedicated care page for it — the closest material concerns congeners from the same Kikori and Lake Kutubu region rather than this species itself.
Any keeping recommendations are therefore extrapolated from genus-level husbandry and from the wild habitat data, not from hobby experience with this fish. Its native waters — limestone-influenced forest creeks and flood-plain ponds in tropical Papua New Guinea — suggest warm, hard-to-alkaline water would suit it, and as a small, active schooling rainbowfish it would do best in a group with open swimming space, in line with standard Melanotaenia practice. With no captive track record, these notes are general rather than tested. Treat the fish as a conservation-sensitive rarity, not a routine aquarium species.
Conservation
Melanotaenia caerulea is assessed as Vulnerable (VU) on the IUCN Red List under criterion B1ab(v), in an assessment dated 6 December 2019 and published in 2020 (Allen, G.R. & Unmack, P. 2020). The listing rests on an extremely restricted range — an extent of occurrence of about 5,62 mi² confined to a single location in the Kikori basin — combined with a documented continuing decline in the number of mature individuals. The fish was reported as common in the 1990s, but no current abundance data exists and the population trend is assessed as decreasing.
The principal threat is the spread of invasive fishes through the Kikori system. Introduced tilapia (Oreochromis) prey on and compete with the native rainbowfish, and common carp (Cyprinus carpio) and invasive water hyacinth (Eichhornia crassipes) add further pressure through habitat degradation and disease risk (Imbun & Smith 2018). For a narrowly endemic fish occupying effectively one location, such pressures carry no buffer — there is no large outlying population to absorb local losses. No conservation actions are currently in place; the assessors recommend research into the species' population and distribution. The species' potential as an aquarium fish is noted by the IUCN but, unlike some threatened rainbowfishes, it is not currently sustained by captive breeding.