Taxonomy & naming
Chilatherina pagwiensis was described by Gerald R. Allen and Peter J. Unmack in 2012, in the journal aqua, International Journal of Ichthyology (vol. 18, no. 4), from material collected in the Sepik River system of Papua New Guinea. The description was based on a series of 31 specimens, roughly 1–3 in in standard length, taken from a Sepik tributary (Upper Kuins Creek, on the Maprik–Pagwi road, East Sepik Province) near the village of Pagwi — the type locality that gives the fish both its scientific epithet and its common name, the Pagwi rainbowfish. Eschmeyer's Catalog of Fishes records the valid name as Chilatherina pagwiensis Allen & Unmack, 2012; because it was described in Chilatherina, where it remains, no parenthetical authority is used.
The genus Chilatherina belongs to the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern Australia. The name pairs the Greek cheilos, lip, with atherina, the classical name for the silverside or smelt — a nod to the silverside-like build of these fishes. C. pagwiensis sits within a cluster of Chilatherina centred on northern New Guinea drainages. Its near-twin is the widely distributed Chilatherina fasciata, which shares its habitat; telling the two apart takes a close, even microscopic, look at the dentition. The molecular phylogeny of Unmack, Allen and Johnson (2013) helped sort out relationships within the group.
Morphology
This is a small rainbowfish. Allen and Unmack's type series reached about 3 in standard length, and the live-import literature cites a maximum of about 3 in (roughly 3 in) — smaller than the sympatric C. fasciata, which reaches around 4.5 in SL. The body follows the usual rainbowfish plan: laterally compressed, moderately elongate, with the family's divided dorsal fin — a short front section followed by a longer rear one.
Life colouration is thinly documented; most of what's on record comes from preserved type material, in which the ventral fins are semi-translucent whitish. The preserved male holotype was figured in the original description. Because the fish was split from C. fasciata on fine characters like dentition rather than on bold colour differences, picking it out in the field is genuinely hard. Living males would be expected to colour up more than females when displaying, as rainbowfishes do, but male nuptial colour for this species simply isn't well recorded — the data here are sparse.
Habitat
The Pagwi rainbowfish comes from the Sepik River system of northern Papua New Guinea, where the type series was collected from a tributary near Pagwi village. The type locality sits roughly 249 mi upstream from the sea at an elevation of about 98 ft — a lowland, slow-river setting deep in the Sepik basin. FishBase classes the species as a freshwater, benthopelagic fish of tropical waters.
It schools in the midwater-to-surface zone, keeping to the quieter parts of streams — the heads and tails of riffles and rapids rather than the fastest current — and tends to stay near aquatic and emergent vegetation or submerged snags that offer cover. C. fasciata occupies the same habitat, and the two co-occur there, which is part of why they're so easily confused. Physico-chemical measurements (temperature, pH, hardness) for the Pagwi habitat are absent from the available sources, so precise water parameters are data sparse.
Feeding
The diet of Chilatherina pagwiensis has not been recorded directly; the dietary studies of Allen (1982) and Unmack (2007) did not cover stomach contents for this species. Going by its body form, its midwater-to-surface schooling habit, and what's known of rainbowfishes generally, it is presumed to be a typical rainbowfish omnivore. FishBase estimates a trophic level of about 2.9 (± 0.3), in line with an omnivorous diet weighted toward small invertebrates and plant or algal material.
Rainbowfishes of this kind take small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, insects snapped at or near the surface — along with algae and fine plant matter grazed off submerged surfaces. No aquarium feeding observations specific to C. pagwiensis exist, but it would be expected to behave like its relatives. Past that general picture, feeding data for the Pagwi rainbowfish are sparse.
Mating
Chilatherina pagwiensis is an egg-scattering spawner with no parental care. Courtship follows the pattern documented in the live-import literature: a pair settles side by side near a chosen spot in the vegetation, quivering together, then releases eggs and sperm at the same instant before darting apart, scattering the fertilised eggs into the surrounding plants.
The eggs carry sticky adhesive filaments that hitch them to plants and submerged debris, anchoring them where they were released. This is the standard reproductive mode across the rainbowfishes; in related species, spawning runs most active in the early morning and is drawn out over many days rather than packed into a single clutch. Observations of display behaviour, seasonality, or spawning frequency in the wild for C. pagwiensis itself are not detailed in the available sources.
Breeding
No dedicated captive-breeding account for Chilatherina pagwiensis appears in the available sources, and the fish isn't established in the hobby, so the breeding picture has to be built from its known spawning mode and from analogy with related rainbowfishes. It is a plant-spawning egg-scatterer: the adhesive, filament-bearing eggs catch on fine-leaved vegetation or debris, the adults give no care, and they would eat the eggs given the chance.
In rainbowfishes that breed this way, females release small numbers of eggs over many days rather than one large clutch, the eggs hang by their filaments from the spawning medium, and incubation runs on the order of one to two weeks at warm tropical temperatures before tiny surface-oriented fry hatch. FishBase puts the species at medium resilience, with an estimated population doubling time of 1.4 to 4.4 years — consistent with the modest, steady output of small rainbowfishes. The concrete figures — clutch size, egg diameter, incubation period, fry-rearing details specific to this species — are not on record and remain data sparse.
In the aquarium
Chilatherina pagwiensis is all but absent from the aquarium trade and isn't a fish the average hobbyist will meet; it surfaced in an Australian live-import risk assessment rather than as a popular import. Any care guidance is therefore extrapolated from the husbandry of better-known Chilatherina and Melanotaenia species rather than drawn from direct experience.
On that footing, a Pagwi rainbowfish should behave like its relatives: active, peaceful, happiest in a group, in a roomy tank with open swimming room and some planted or structured cover near the margins to echo its vegetated stream. Warm tropical temperatures and stable, well-filtered water would suit it, and it would do best on a varied omnivorous diet of small flake or pellet supplemented with live or frozen invertebrates. Since the wild water parameters aren't recorded, any specific numbers would be guesswork; an aquarist would reasonably default to the conditions used for other northern New Guinea rainbowfishes. The species is harmless to humans.
Conservation
Chilatherina pagwiensis is assessed as Endangered (EN) on the IUCN Red List, under criterion B1ab(iii), with the assessment dated 3 December 2019. The criterion reflects a very restricted range plus a continuing decline in habitat quality — the classic vulnerability of a fish known from one small part of a single river system. It has not been evaluated under CITES or the CMS.
The main documented threat is the introduction of exotic fishes into the Sepik: the Australian live-import assessment notes at least four introduced species that prey on or compete with native rainbowfishes. A fish with so narrow a known range has little buffer against this, with no large outlying population to offset local declines. FishBase rates its fishing vulnerability as low (10 of 100), so harvest isn't the worry — the risk lies in habitat change and invasive species within its limited Sepik-tributary range. And because the fish is barely present in captivity, there is no tank-bred safety net of the kind that cushions some better-known endangered rainbowfishes.