Taxonomy & naming
Pseudomugil furcatus was described by the American ichthyologist John Treadwell Nichols in 1955, originally — and currently — in the genus Pseudomugil. For a time it was placed in its own genus as Popondetta furcata, a name still occasionally encountered in older aquarium literature; because Popondetta was preoccupied (by an insect genus) the replacement name Popondichthys was also used. Current usage, following Eschmeyer's Catalog of Fishes, returns the species to Pseudomugil, so the valid name is Pseudomugil furcatus Nichols, 1955 — written without parentheses around the author because the species sits in its original genus rather than a later combination.
The genus Pseudomugil comprises the blue-eyes, small fishes of the family Pseudomugilidae — close relatives of the true rainbowfishes (Melanotaeniidae) within the order Atheriniformes, the silversides. The 'blue-eye' name refers to the iridescent blue ring around the eye shared across the genus. P. furcatus is distinguished within the genus by its forked caudal fin — the source of both the scientific epithet furcatus and the common name forktail — together with its yellow-and-black fin markings.
Morphology
Pseudomugil furcatus is a small, slender, slightly elongate fish reaching about 2 in in standard length, with males marginally larger and far more ornate than females. Like other blue-eyes it has the divided dorsal fin of the rainbowfish lineage and a bright, iridescent blue or blue-green ring around the eye.
The body colour is an understated translucent olive to yellow-green, sometimes with a faint silvery or greenish sheen along the flanks. The drama is in the unpaired fins. In males the first and second dorsal fins, the anal fin, and the upper and lower lobes of the conspicuously forked tail are washed and edged with lemon-yellow and marked with black, and the leading rays of the dorsal and anal fins can be drawn out into points. When a male displays — usually toward a rival or a female — he erects and spreads these yellow-and-black fins to their fullest and the colour deepens. Females are plainer, with shorter, less colourful fins, the usual sexual dimorphism of the group.
Habitat
Pseudomugil furcatus is native to the streams of eastern Papua New Guinea, where it is known from the Musa and Kwagila river basins — which drain into Dyke Ackland Bay and Collingwood Bay respectively — in the lowlands of the Northern (Oro) Province on the northeastern coast of the island. It favours clear, well-oxygenated, moderately flowing water with abundant aquatic and overhanging marginal vegetation, where the fish shoal in the upper water column.
Those waters run warm and on the soft-to-neutral side, clear and gently flowing — clean rainforest streams, not still swamps. Reported conditions centre on temperatures in the mid-20s Celsius and roughly neutral pH, though the species tolerates a moderate range of hardness. The flow, oxygen, and vegetation of its home streams carry straight over into its aquarium needs: this is a fish of clean, moving water, not a stagnant tank.
Feeding
The forktail blue-eye is a micro-predator and opportunistic omnivore that feeds in the upper part of the water column. In the wild it takes small invertebrates — tiny crustaceans, insect larvae, and aquatic and terrestrial insects drifting at or near the surface — along with some algal and plant matter.
In captivity it takes small prepared foods such as fine flake and micro-pellets, but it looks best, and breeds most reliably, on small live and frozen foods: Daphnia, newly hatched and adult brine shrimp, Cyclops, micro-worms. The mouth is tiny, so everything has to be sized to match. A varied diet with a strong live or frozen component brings up the yellow fin colour and conditions adults for spawning; on dry food alone the fish run paler and less vigorous. Being small and busy, it does best with several small feedings spread through the day.
Mating
Pseudomugil furcatus is an egg-scattering plant-spawner that breeds in small daily increments rather than in large clutches. Males are social and competitive, and much of the fish's appeal comes from the constant low-level sparring among them in a group. A male confronts a rival or courts a female by erecting his yellow-and-black fins to their full spread and quivering alongside, the fins flashing, the body colour deepening as he goes.
Display and spawning peak early in the day. A male that has won a female leads her into fine-leaved vegetation, where the pair press together among the fronds and release a small number of eggs. Because the males display hardest when they have rivals to display against, the fish are most colourful and most active in a group with several males, and breeders tend to condition and spawn them in groups rather than as isolated pairs.
Breeding
Breeding the forktail blue-eye is straightforward, which is why most aquarium stock is tank-bred. Conditioned adults deposit small, adhesive eggs that hang by fine filaments among fine-leaved plants such as Java moss or among synthetic spawning mops. Spawning runs continuous: a female releases a few eggs each day over an extended period rather than one batch, so a pair or group left in a mature, well-planted tank will quietly produce eggs day after day.
The relatively large eggs incubate for roughly one to two weeks, hatching faster in warmer water. Adults eat both eggs and fry, so anyone after good yields moves mops or eggs to a separate rearing container, or leans on a densely planted tank to give some fry cover. The fry are tiny and slow-growing and need very small first foods — infusoria, rotifers, or commercial liquid fry food — before they can manage newly hatched brine shrimp. Patience is most of the job: fry develop gradually, and males do not show their full fin colour until they near maturity. That steady trickle of fry is what lets a stable colony sustain itself in a quiet, planted aquarium.
In the aquarium
The forktail blue-eye is a peaceful, active, schooling nano fish that should always be kept in a group — ideally eight or more, with several males so the display behaviour stays on show. At that number the fish shoal confidently and colour up; kept singly or in pairs they turn shy and drab. Small as it is, it swims hard and wants open water above a planted layout; a modest planted tank suits a colony well, and a mature, stable one with some surface movement suits it best.
Water should be clean, warm, and well-oxygenated, reflecting the species' flowing-stream origins: temperatures in the mid-20s Celsius and roughly neutral, soft-to-moderately-hard water, with gentle current and good filtration. It reacts badly to poor water quality and to large swings in conditions, so stability and regular maintenance matter more than chasing exact parameters. These fish are small enough to be bullied or outcompeted, so tankmates should be other peaceful, small, non-nippy species — other blue-eyes and rainbowfishes, small rasboras, pygmy Corydoras, and small loaches all fit. They hang near the surface and jump well, so a tight-fitting cover is wise. Lifespan is short for the hobby, typically only a couple of years, but a self-sustaining colony renews itself, and the steady flicker of yellow fins makes a group one of the more rewarding nano subjects.
Conservation
Pseudomugil furcatus was assessed as Data Deficient (DD) on the IUCN Red List in 2020. The species is known only from a small number of localities — the Musa and Kwagila river basins of northeastern Papua New Guinea — and there is too little information on its population, range, and trends to judge its extinction risk, so it has not been placed in a threatened or non-threatened category.
As with many small Papua New Guinea fishes, the long-term concerns are local rather than global. Degradation of clear rainforest streams through deforestation, sedimentation, and changing land use can hit individual populations, and a fish that wants clean, flowing, well-vegetated water feels that kind of disturbance quickly. The ornamental trade is a comparatively minor pressure, mainly because the forktail blue-eye breeds so readily in captivity that the great majority of fish in the hobby are tank-bred rather than wild-caught. Buying captive-bred stock keeps collecting pressure low and helps maintain genetically diverse aquarium populations of a fish whose wild status is poorly known and which depends on its specific stream habitats staying intact.