Taxonomy & naming
Pseudomugil pellucidus was described in 1998 by Gerald R. Allen, Walter Ivantsoff, M.A. Shepherd and S.J. Renyaan, in a paper published in aqua, Journal of Ichthyology and Aquatic Biology (3(1):1–8), under the title 'Pseudomugil pellucidus (Pisces: Pseudomugilidae), a newly discovered blue-eye from Timika-Tembagapura region, Irian Jaya.' The type material came from the Timika–Tembagapura area of what is now the Indonesian province of Papua (formerly Irian Jaya), on the southern side of New Guinea. Eschmeyer's Catalog of Fishes records the valid name as Pseudomugil pellucidus Allen & Ivantsoff, 1998; because it was described in Pseudomugil, where it remains, no parenthetical authority is used.
The genus Pseudomugil comprises the blue-eyes, small atheriniform fishes of the family Pseudomugilidae distributed across New Guinea and northern Australia. They sit within the order Atheriniformes, the silversides, and are close relatives of the true rainbowfishes (Melanotaeniidae). Within the genus, P. pellucidus is set apart by its transparent body — the trait behind its name — rather than by the bold finnage and colour of better-known congeners like P. gertrudae or P. signifer.
Morphology
Pseudomugil pellucidus is a very small fish, reaching a maximum recorded length of about 1 in SL (the figure given in the original description and carried by FishBase). This places it among the smaller blue-eyes and well within the size range typical of the genus, which contains some of the daintiest of all atheriniform fishes.
The defining feature, and the source of the epithet, is the body's translucence: the fish is largely clear, an adaptation common among small fishes of shaded, clear-water habitats. As in other Pseudomugil, the body is slender and laterally compressed, with two separate dorsal fins and the large eye that earns the group its 'blue-eye' name. Colour and finnage descriptions are sparse in the accessible literature. Beyond the diagnostic transparency, the fine points of its appearance are best left to the original description rather than asserted here.
Habitat
The species is restricted to the Timika–Tembagapura region of southern Indonesian Papua, where it is known from tributaries of the Aikwa (Iwaka) and Kopi Rivers — a narrow band of rainforest country running up from the lowlands toward the central highlands. FishBase characterises it as a freshwater, pelagic, tropical fish occurring in shallow streams that are usually slow-flowing and run through dense rainforest.
One feature is constant: water clarity. The streams are always clear, though often deeply stained with tannin — the dark 'blackwater' tea-colour familiar from leaf-litter-laden forest streams. Substrate varies from sand to gravel, pebble and rock. Reported temperatures run roughly 75–82 °F, in keeping with a warm lowland-to-foothill setting. That this clear-water stream habitat is naturally small and fragmented, confined to a single region of Papua, is the central fact of the species' biology and the reason its conservation status is of concern.
Feeding
No species-specific dietary study for Pseudomugil pellucidus is available in the accessible literature, and its feeding ecology is therefore inferred from the genus. FishBase places it at a trophic level of about 3.2, consistent with a small, invertebrate-feeding fish.
Blue-eyes are micro-predators that pick small invertebrates from the water column and surface — tiny crustaceans, insect larvae, other zooplankton — plus whatever small prey drifts through their slow forest streams. Given its small mouth and surface-and-midwater habits, P. pellucidus is almost certainly a similar planktonic and small-invertebrate feeder. Without a direct dietary record, no more specific claim is made here.
Mating
Reproductive behaviour has not been documented for Pseudomugil pellucidus specifically; the species is rarely kept and was described only in 1998, and no captive or field spawning account surfaced in research for this profile. What follows is therefore drawn from the well-established pattern of the genus rather than from observations of this fish.
Blue-eyes are continuous egg-scatterers. Males display to females among fine-leaved vegetation, courting with raised fins and heightened colour; spawning takes place among submerged plants or fine roots, where a receptive female deposits eggs a few at a time. Whether P. pellucidus follows this pattern in every detail is unconfirmed, and the specifics of its courtship remain data-sparse.
Breeding
There is no published captive-breeding account for Pseudomugil pellucidus, and the species is not established in the aquarium trade, so any description of its breeding must be treated as genus-level inference rather than fact for this fish.
Across the genus, blue-eyes are continuous spawners. They attach small adhesive eggs by fine filaments to fine-leaved plants or spawning mops, releasing a few eggs daily over weeks rather than one clutch. The adults give no parental care and will eat eggs and fry, so breeders of other Pseudomugil species move the spawning medium to a separate container for rearing and feed the tiny first-feeding fry on infusoria or very small live foods. Whether any of this translates to P. pellucidus is untested; the facts are simply absent for this species.
In the aquarium
Pseudomugil pellucidus is essentially absent from the hobby — a recently-described, narrowly-distributed blue-eye that doesn't appear in the trade, with no species-level care account. Anything written about keeping it is extrapolated from the genus, and offered here only with that caveat.
Blue-eyes as a group are small, peaceful shoaling fishes that do best in mature, well-planted tanks with soft to neutral, often tannin-stained water, gentle flow, and warmth. Given this species' clear, tannin-rich forest stream at roughly 75–82 °F, the same conditions — soft, slightly acidic to neutral blackwater, dense fine-leaved planting, a quiet group of conspecifics — would be the place to start, should wild or aquacultured stock ever turn up. In practice the keeping of this particular blue-eye is undocumented, and anyone drawn to the genus is far likelier to meet its established relatives.
Conservation
Pseudomugil pellucidus is assessed as Vulnerable (VU) on the IUCN Red List, under criterion B1ab(iii) (IUCN species ID 169523). The listing turns on the species' single-region range in the Timika–Tembagapura area of Indonesian Papua: a narrowly endemic fish confined to a small stretch of clear-water rainforest stream has little buffer against local degradation.
The assessment confines the species to tributaries of the Aikwa (Iwaka) and Kopi Rivers, basins covering an estimated extent of occurrence of about 14,48 mi², and names mining as the principal threat. The Grasberg copper-and-gold mine in the Sudirman Mountains discharges large volumes of tailings into the headwaters of the Aikwa, polluting it and neighbouring drainages and fouling the clear water the fish depends on. The species is of no fisheries interest and is rated low in vulnerability to fishing (about 10 out of 100), so the concern is habitat quality, not exploitation. With the fish all but absent from the trade, the hobby plays no part here — its long-term survival rests on protecting that restricted Papuan stream habitat.