Taxonomy & naming
Pseudomugil majusculus was described by Walter Ivantsoff and Gerald R. Allen in 1984. It is placed in the order Atheriniformes, family Pseudomugilidae — the blue-eyes — a family of small fishes closely allied to the true rainbowfishes (Melanotaeniidae). Eschmeyer's Catalog of Fishes records the valid name as Pseudomugil majusculus Ivantsoff & Allen, 1984; because the species was described in Pseudomugil, where it remains, no parenthetical authority is used.
The genus name Pseudomugil combines the Greek pseudes, false, with the Latin mugil, the grey mullet — a 'false mullet,' for the resemblance early naturalists drew between these slender silvery fishes and small mullets. The common name is Cape blue-eye. The range deserves a word, because the literature is inconsistent: some sources gloss it loosely, but the IUCN range account — drawing on G. R. Allen, the species' co-describer (pers. comm. 2019) — restricts P. majusculus to two eastern Papua New Guinea sites, Cape Ward Hunt in Northern Province and a locality near Araeda Village on Tagula Island in the Louisiade Archipelago, Milne Bay Province. The atlas follows that catalogued range and authority rather than looser hobby application.
Morphologically the species sits very close to Pseudomugil signifer, the Pacific blue-eye, and is easily mistaken for it. It is known from very little material — essentially a museum specimen and a small collection of live fish — so the boundaries of the species and its full distribution stay unresolved. Much of the blue-eye fauna of New Guinea is still that poorly mapped.
Morphology
Pseudomugil majusculus is a small fish, reaching a maximum recorded length of about 2 in total length. It has the slender, laterally compressed, somewhat translucent body typical of the blue-eyes, with the divided dorsal fin and the prominent, brightly ringed eye that gives the family its common name.
Meristic counts from the original material place the dorsal fin spines at four to five and the soft dorsal rays at seven to nine; the anal fin lacks spines and carries twelve to thirteen soft rays, and there are fourteen to fifteen gill rakers on the lower limb of the first arch. These counts overlap heavily with P. signifer's, which is the main reason the two are so readily confused; body proportions and counts alone won't separate them.
Life-colour and finnage descriptions for the species are thin in the catalogued sources. Rather than borrow the appearance of better-documented congeners, the atlas notes only that fine colour and pattern data specific to P. majusculus are limited.
Habitat
Pseudomugil majusculus is recorded from two widely separated sites in eastern Papua New Guinea: Cape Ward Hunt, about 118 mi north-east of Port Moresby in Northern Province, and a locality near Araeda Village on Tagula Island in the Louisiade Archipelago, Milne Bay Province — roughly 435 mi apart, with the species presumed to occur at intervening localities. The catalogued latitude lies in the tropics, at roughly 7 to 8 degrees south.
The holotype came from mangroves in salt water, and the species is usually described as a brackish, pelagic fish of shallow water, about nought to two metres deep. The picture isn't purely estuarine, though. The live aquarium specimens were taken from a small freshwater creek on Tagula Island, some 656–1312 ft upstream from the sea — a roughly two-metre-wide, coconut-shaded stream over sand and gravel. That tolerance of both salt-influenced and fresh water is one of the few firm ecological facts on record. In mangroves it is reported to feed the young of commercially important fishes.
Otherwise the ecology is poorly known, again because the fish is known from very little material. Outside these few recorded occurrences, the habitat picture is genuinely thin.
Feeding
No diet study specific to Pseudomugil majusculus is available in the catalogued sources, and the species' feeding biology is described as poorly known. FishBase places it at an estimated trophic level of about 3.3, which is consistent with a small carnivore or omnivore taking animal prey rather than a herbivore.
Blue-eyes generally are micro-predators of the upper and middle water column, taking small invertebrates — tiny crustaceans, insect larvae, and whatever drifts at or near the surface. In a brackish mangrove setting, zooplankton and small estuarine invertebrates would be the expected prey. These are inferences from family and habitat, not measured facts for this fish; no gut-content data for P. majusculus is on record.
Mating
Reproductive behaviour has not been documented specifically for Pseudomugil majusculus, and the catalogued sources offer no courtship account for the species. What follows is genus-level context, presented cautiously and not as confirmed observation of this fish.
Across the blue-eyes and their rainbowfish relatives, males are usually the more colourful, demonstrative sex, displaying to females with flared fins and heightened colour, often most intensely in the morning. Spawning tends to be drawn out — small numbers of adhesive eggs released over days among fine vegetation rather than in one clutch.
Whether P. majusculus does any of this is unrecorded. It is known from very little material and its biology stays poorly known, so its mating system should be treated as undescribed.
Breeding
There is no published account of captive or wild breeding for Pseudomugil majusculus in the catalogued sources, so any description here is necessarily genus-level and provisional.
Pseudomugil species in general are egg-scattering plant-spawners. Females lay small adhesive eggs that attach by fine filaments to fine-leaved plants or to spawning mops, a few at a time over several days rather than in one batch. Warmer water tends to trigger and sustain spawning in congeners, and parents give no care to eggs or fry. Well established for the genus — but unconfirmed for P. majusculus.
The species' brackish, estuarine ecology may also pull its real requirements away from the freshwater congeners that most hobby breeding knowledge rests on. The honest position is that the breeding biology of this blue-eye is unknown, and the atlas won't assert clutch sizes, incubation times, or fry-rearing details that have never been observed for it.
In the aquarium
Pseudomugil majusculus is not a commonly kept aquarium fish, and species-specific husbandry data could not be located; the guidance here is drawn from genus-level care for blue-eyes and should be read as cautious context rather than a tested protocol for this fish.
Pseudomugil blue-eyes are small, peaceful shoaling micro-rainbowfishes, best kept in groups. Reported genus-level parameters run roughly 75–83 °F and pH 6.0 to 8.0, in soft to moderately hard water. One caveat: the holotype came from brackish mangroves, yet the live specimens that reached aquariums came from a freshwater creek. The fish seems to take both salt-influenced and fresh water — so don't assume it needs brackish conditions, and don't assume it wants the soft-water defaults typical of the genus.
And because it is so easily confused with Pseudomugil signifer, fish sold under one name may be the other. Pin down the identification before you lean on any care advice. This is a poorly known species, and firm aquarium guidance for it does not yet exist.
Conservation
Pseudomugil majusculus is assessed as Data Deficient (DD) on the IUCN Red List, in a global assessment dated 4 June 2020. A Data Deficient listing means there is not enough information on the species' distribution, population, or threats to assign it to a category of extinction risk — it is neither a clean bill of health nor an alarm, but an acknowledgement of how little is known.
That status fits the rest of the record. The fish is known from very little material — essentially a single museum specimen and a 1993 collection of live aquarium fish — its biology is repeatedly called poorly known, and its catalogued range is just two localities in eastern Papua New Guinea, at Cape Ward Hunt and on Tagula Island in the Louisiade Archipelago, some 435 mi apart. A narrowly recorded, sparsely studied species is hard to monitor, and the Data Deficient label reflects that gap, not a documented decline.
The species is not listed under CITES or CMS. What would help most here is plain field data — confirmed distribution, population figures, basic life-history — enough to swap a real risk assessment in for the present holding category.