Taxonomy & naming
Pseudomugil tenellus was described by Taylor in 1964 from a large billabong at Oenpelli in Arnhem Land, Northern Territory, Australia, within the East Alligator River system. Eschmeyer's Catalog of Fishes records the valid name as Pseudomugil tenellus Taylor, 1964, with the species placed — as originally described — in the genus Pseudomugil, so no parenthetical authority is used.
The species belongs to the blue-eyes, family Pseudomugilidae, within the order Atheriniformes. The blue-eyes are a small Australasian family comprising the genera Pseudomugil, Kiunga, and Scaturiginichthys, and they sit close to the true rainbowfishes (Melanotaeniidae); the two families are often treated together in the aquarium hobby under the loose banner of 'rainbowfishes.' Pseudomugil is the most species-rich of the blue-eye genera, distributed across New Guinea and northern and eastern Australia.
The genus name Pseudomugil combines the Greek pseudes, 'false,' with the Latin mugil, 'grey mullet,' a reference to a superficial resemblance. The specific epithet tenellus is the Latin diminutive of tener, meaning 'soft,' 'tender,' or 'delicate,' the source of the usual common name, delicate blue-eye; the fish is also sometimes called the Arnhem Land blue-eye after its type region.
Morphology
Pseudomugil tenellus is a small, slender, fusiform fish. FishBase gives a maximum standard length of about 2 in, with a common length near 1.5 in; standard length in the species is generally reported in the range of roughly 1.5–2 in. The body is of normal proportions rather than deep, and like other blue-eyes the species carries two separated dorsal fins, the first short, the second longer. Meristics reported for the species include three to five dorsal spines, six to seven dorsal soft rays, no anal spines, and seven to ten anal soft rays.
The most useful field mark is a midlateral row of pale spots running along the flank, with darker spaces between them — enough to separate the delicate blue-eye from similar small blue-eyes. As in the family generally, the eye carries the bright iris ring that gives the group its 'blue-eye' name.
Sexual differences are clear in adults. Males are larger, more strongly patterned and more colourful than females, and they develop extended, filamentous tips to the unpaired fins; females and juveniles are plainer. With the male flaring those enlarged fins in display, the sexes are easy to tell apart once mature.
Habitat
The delicate blue-eye is a fish of the coastal lowlands of tropical northern Australia and southern New Guinea, recorded between roughly 8 and 16 degrees south. In Australia it ranges patchily but widely across the Top End and Cape York: in the Northern Territory it is known from the Alligator, Blyth, Daly, Finniss, Howard, Liverpool, and Mary drainages, with a western limit near Darwin (Leanyer Swamp, Rapid Creek, Benjamin Lagoon), and in Queensland it occurs east to the Cape York Peninsula in systems such as the Coleman, Edward, Jardine, Lockhart, and Watson rivers and the Jacky Jacky and Scrubby creeks. Beyond Australia it is reported from the Aru Islands of Maluku, Indonesia, and the Bensbach River of Papua New Guinea, and possibly more widely across southern New Guinea. Because of its small size it likely escapes detection and may be more widespread than records suggest.
It is a fish of still and slow-moving water: swamps, billabongs, marshy ponds, the quiet backwaters of streams, and the overflow pools of rivers and creeks, typically over mud or gravel bottoms with abundant vegetation including water lilies, and usually in places exposed to full sunlight. Many populations sit on coastal floodplains and are euryhaline — the species occurs in both fresh and brackish habitats, including tidal estuaries and salt marshes — while other populations live in purely freshwater upper streams.
Recorded wild water conditions are warm and span a wide chemistry. FishBase gives a temperature range of 25 to 35 degrees Celsius; field measurements reported by Tappin (2010) span 27 to 38 degrees Celsius, pH from about 5.0 to 7.1, and very low conductivity, on the order of 6 to 120 microsiemens per centimetre in the freshwater habitats. This breadth reflects the seasonal, flood-pulsed nature of the environment, which swings between wet-season inundation and dry-season shrinkage.
Feeding
In the wild Pseudomugil tenellus is a micro-predator and grazer of the upper water column, taking floating and suspended zooplankton, phytoplankton, algae, and small invertebrates. Its diet shifts seasonally with what the flooded lowland habitat offers, and FishBase places it at a trophic level of about 3.4 — squarely that of a small omnivore leaning carnivorous.
In the aquarium it is best fed small foods to match its small mouth. Live and frozen items such as Daphnia, Moina, Artemia nauplii, and micro worm are taken eagerly, alongside small floating dried foods that include plant or algal content. As an active, surface-oriented feeder it does best with frequent small feedings, and a varied diet with a live or frozen component keeps the fish in good colour and breeding condition.
Mating
The delicate blue-eye is a peaceful shoaling fish, and its breeding behaviour is best expressed in a group. Males court females with displays of their enlarged, filamentous unpaired fins, and because males display most strongly in the presence of rivals, the species shows its best colour and fin display when several males are kept together. Keeping groups of eight to ten or more is recommended both for the fishes' wellbeing and to bring out this display behaviour.
Spawning follows a daily rhythm tied to daylight. Activity peaks in the late morning to early afternoon, with spawning reported at water temperatures around 24 to 28 degrees Celsius. In nature the cycle is seasonal, concentrated in the wet season around October to December, when the floodplain habitats fill and conditions favour the rapid growth the species depends on.
Breeding
Pseudomugil tenellus is an egg-scatterer with no parental care; adults will eat their own eggs and fry, so any serious breeding effort separates eggs or fry from the parents. Rather than producing a single clutch, a female deposits a few eggs each day over several days, the eggs attaching to vegetation by fine adhesive filaments — the typical pattern for the family.
Incubation takes roughly ten days at spawning temperatures. The fry are very small and at first require microscopic food such as Paramecium (infusoria) for about the first five days, after which they can take Artemia nauplii and micro worm. Growth is fast, which suits a fish whose wild life is keyed to the brief productive window of the wet season.
The species is short-lived. Females are most fecund early in life and become noticeably less productive after about twelve to eighteen months; in the wild a female may complete only a single reproductive season. Rapid growth, early reproduction, and a short lifespan are the pattern in small annual-style floodplain fishes, and they dictate how the species is best kept — as continually renewed groups rather than long-lived individuals.
In the aquarium
The delicate blue-eye is a peaceful, active, shoaling fish best kept in groups of eight to ten or more, which both settles the fish and brings out the males' fin displays. A modest tank suits it — a base footprint of around 75 by 12 in is a sensible minimum for a group — but the water needs to be well-oxygenated with some gentle flow, as the species is sensitive to stagnant or poorly oxygenated conditions.
Reported aquarium parameters are warm and span a wide but stable middle ground: temperature around 25 to 32 degrees Celsius, pH from about 5.5 to 7.5, and hardness across a broad range. Although many wild populations are euryhaline, marine salt is unnecessary in the aquarium — fish from brackish or marine habitats breed in pure freshwater and vice versa. What matters is stability: the species reacts badly to sudden water-chemistry swings, so consistent conditions and good maintenance count for more than hitting any precise number.
Small, peaceful, and short-lived, it belongs in a quiet, well-planted tank with small, gentle tankmates or on its own — and it pays back best when bred in successive generations rather than kept as long-term individuals.
Conservation
Pseudomugil tenellus is assessed as Least Concern on the IUCN Red List, with the assessment dated 12 February 2019. The species is widespread across the coastal lowlands of northern Australia and southern New Guinea, occupies a range of habitats including resilient floodplain and brackish systems, and faces no identified range-wide threat sufficient to warrant a higher category.
Its small size and cryptic floodplain habits mean it is probably under-recorded, and it may be more widely distributed than current records show, which further supports the low-concern listing. As with other small lowland fishes, the chief watch-points for the future are local rather than global: degradation of floodplain wetlands, altered flooding regimes, and the spread of introduced fishes could affect particular populations even while the species as a whole remains secure.