Taxonomy & naming
Pseudomugil gertrudae was described by the Dutch zoologist Max Weber in 1911, from material collected in the Aru Islands of what is now Maluku, Indonesia. The type locality is a rivulet in sago forest at Ngaiguli on Terangan (Trangan) Island in the Aru Archipelago. Eschmeyer's Catalog of Fishes records the valid name as Pseudomugil gertrudae Weber, 1911; because the species has remained in the genus in which it was described, no parenthetical authority is used.
The genus Pseudomugil belongs to the family Pseudomugilidae, the blue-eyes, a small Australasian group that also includes the genera Kiunga and Scaturiginichthys and is closely allied to the true rainbowfishes (Melanotaeniidae) within the order Atheriniformes. The genus name combines the Greek pseudes, 'false', with the Latin mugil, 'grey mullet', a reference to a superficial resemblance. The specific epithet gertrudae is a patronym honouring Mrs Gertrude Merton, wife of the German naturalist Dr Hugo Merton, who accompanied her husband on his 1907–1908 expedition to the Aru Archipelago during which the species was obtained.
Among its congeners, P. gertrudae is most readily told apart by the bold dark spotting on the dorsal, anal and caudal fins — a pattern it shares only with P. paskai. Across the species' wide range, wild populations vary in colour and fin development, and aquarists recognise several distinct local forms, though none is a formally named taxon.
Morphology
A small, slender, fusiform fish: published maximum standard length runs to about 1.5 in, though most individuals fall in the 1–1.5 in range. The body is semi-transparent with a silvery to yellowish base, scattered with fine dark flecks, and the eye carries the blue iris ring that gives the blue-eyes their common name.
The fins are the species' showpiece. Pseudomugil gertrudae has the divided dorsal fin typical of the family, with 4–6 spines in the first dorsal and 6–8 soft rays in the second; the anal fin carries a single spine and 9–12 soft rays. In well-conditioned males the dorsal, anal and caudal fins are enlarged and elongated, peppered with the characteristic dark spots and edged in pale yellow or white, and are held spread and flickering during display. Females and unsexed juveniles are plainer, with shorter, less heavily patterned fins.
Sexual dimorphism is clear once fish mature: males are more colourful and more strongly marked, with the extended unpaired fins, while females are smaller-finned and plainer. As with many blue-eyes, colour intensity tracks mood, social context and condition, so a displaying male in good water looks markedly brighter than a stressed or subordinate one.
Habitat
Pseudomugil gertrudae is a fish of small, shaded, slow or standing waters. FishBase records it from small creeks, swampy marshes, heavily shaded rainforest streams, lily lagoons and backwaters adjacent to major water courses, frequently in Pandanus-lined creeks with muddy bottoms, where it aggregates in areas of abundant vegetation and submerged log debris. It occupies the shallow marginal zones of these habitats, among macrophytes, submerged wood and leaf litter.
The range spans northern Australia, southern New Guinea and the Aru Islands, between roughly 5°S and 19°S. In Australia it occurs across the Top End and Cape York — Darwin, Kakadu and Arnhem Land east to the Cape York Peninsula and the rivers draining to the Timor Sea and the Gulf of Carpentaria — as well as on Bathurst, Melville and Groot Eylandt and in the Torres Strait islands. In New Guinea it is known from the Bensbach, Pahoturi and Fly systems of Papua New Guinea, and it occurs on the Aru Islands of Indonesia.
Water conditions across this range vary enormously. Many populations live in blackwater stained brown with tannins — very low pH, low nutrients, soft and mineral-poor. FishBase gives a typical envelope of roughly 73–86 °F and pH 5.2–6.7, while Tappin's compilation of field measurements across many localities records extremes spanning temperatures of about 54–93 °F, pH from below 4 to over 9, conductivity from very low to several hundred microsiemens, and hardness from effectively zero to high values. The species as a whole is adaptable, then, even where a given population sits at one end of that range. The species is recorded as freshwater and occasionally brackish, and demersal in habit.
Feeding
The spotted blue-eye is a micropredator and opportunistic omnivore, feeding chiefly in the upper and middle water column. Its natural diet is made up of small invertebrates, zooplankton and phytoplankton — tiny crustaceans, insect larvae and other small prey taken from open water and from among vegetation. FishBase places it at a trophic level of about 3.3, consistent with a largely carnivorous micro-feeding habit.
In the aquarium it does best on small live and frozen foods that match its size and feeding style. Live Daphnia and Moina, freshly hatched brine shrimp (Artemia nauplii), microworm and similar small fare are taken eagerly. It will accept good-quality crushed dry foods too, but a diet leaning on small live and frozen items keeps colour and condition at their best. The mouth is tiny, so everything should be sized down, and frequent small feedings suit its constant grazing better than occasional large meals.
Mating
Pseudomugil gertrudae is an egg-scattering plant-spawner with no parental care. Mature males display almost constantly, spreading and flickering their elongated, spotted fins to court females and to spar with rivals; the fish looks its best when several males are kept together and compete. Spawning peaks in the late morning at temperatures of around 75–82 °F.
In the wild it spawns seasonally, with breeding concentrated in the wet-season months — roughly October to December in the Australian range. A female does not lay a single clutch. She deposits a few adhesive eggs each day over several days, attaching them to fine vegetation by sticky filaments. This drawn-out, trickle-spawning pattern is typical of the blue-eyes and of many small rainbowfishes.
Breeding
Breeding the spotted blue-eye in captivity follows directly from its natural habit. Conditioned adults spawn over fine-leaved plants such as Java moss or over synthetic spawning mops, the female hanging a few eggs at a time by adhesive filaments from the spawning medium. Because the fish trickle-spawn over days, breeders typically condition a group, leave a mop or mat of fine plant material in place, and lift it periodically to a separate rearing container, since the adults offer no care and will eat eggs and fry given the chance.
Incubation is comparatively long for so small a fish, taking roughly ten days at typical breeding temperatures. The fry are minute on hatching and cannot take brine shrimp at once: they require microscopic first foods such as infusoria or Paramecium for around the first five days before they are large enough for newly hatched Artemia nauplii and, later, finely powdered dry foods.
A short life shapes any breeding programme. Females are usually most productive in a single season and fall off in fecundity after about twelve to eighteen months, so breeders raise successive generations rather than lean on long-lived adults. Steady warmth, clean soft-to-neutral water and a continuous supply of suitably small foods grow on a strong brood.
In the aquarium
Pseudomugil gertrudae is a peaceful but timid shoaling fish best kept in groups of eight to ten or more, where males display against one another and the group settles and colours up. A modest aquarium suits it — a base footprint of around 45 by 12 in is workable for a group — but it is emphatically not a fish for the general community tank: it is small, slow-feeding and easily outcompeted or intimidated by larger, boisterous tankmates, and is best kept in a species setup or with similarly tiny, gentle companions.
Wild populations span an enormous chemistry range, but the ornamental strains usually offered do well in soft to moderately hard, slightly acidic to neutral water: aquarium guidance of roughly 70–82 °F, pH around 4.5–7.5 and moderate hardness reflects practical experience with tank-bred stock. A densely planted tank with fine-leaved plants, some surface cover and gentle filtration shows the fish at its best, and gives a timid micro-fish the spawning sites and refuge it needs. Small, short-lived and particular about company, it rewards the dedicated nano- or biotope aquarist rather than the beginner stocking a community tank.
Conservation
Pseudomugil gertrudae is assessed as Least Concern (LC) on the IUCN Red List, assessed on 12 February 2019. The rating reflects the species' very wide distribution across northern Australia, southern New Guinea and the Aru Islands, and its presence in a broad array of creek, swamp and lagoon habitats throughout that range, including within protected areas such as Kakadu.
The species is harvested for the international aquarium trade, but it breeds readily in captivity and tank-bred stock is widely available, which limits direct collecting pressure on wild populations. As with any small fish tied to shallow, vegetated wetlands, localised populations could be hit by habitat alteration, water extraction or introduced species. But there is no evidence of a broad decline, and the global assessment remains one of low conservation concern.