Taxonomy & naming
Pseudomugil cyanodorsalis was described by Gerald R. Allen and N. A. Sarti in 1983. It belongs to the family Pseudomugilidae, the blue-eyes, a small group of Australasian fishes in the order Atheriniformes that sits close to the true rainbowfishes (Melanotaeniidae). Eschmeyer's Catalog of Fishes treats Pseudomugil cyanodorsalis Allen & Sarti, 1983 as the valid name; because the species was described in Pseudomugil, where it remains, no parenthetical authority is used.
The genus name Pseudomugil joins the Greek pseudes, false, with the Latin mugil, a grey mullet — 'false mullet,' for a superficial resemblance early naturalists drew to mullets. The epithet cyanodorsalis points to the blue colour over the dorsal region. The common name follows suit: the 'blue-eye' part is the family's shared hobby name, after the bright iris that marks the group.
The nomenclatural record is otherwise thin, and this remains one of the less-documented Pseudomugil. It is catalogued in the major databases — FishBase under SpecCode 15167, WoRMS under AphiaID 282446 — but has drawn far less literature than trade staples like the forktail or spotted blue-eyes.
Morphology
This is a small fish, reaching about 1.5 in total length at most. The body is elongate and laterally compressed — the slender, faintly translucent build characteristic of the genus. As the name suggests, the dorsal region carries blue tones, and the elongated rays and base of the second dorsal fin are marked with black.
The meristics show the divided dorsal arrangement of the blue-eyes. The first dorsal fin bears 3 to 5 spines; the second has no spines and 4 to 7 soft rays. The anal fin carries a single spine and 9 to 13 soft rays, with roughly 22 to 26 scales along the midlateral line. These counts place it comfortably within Pseudomugil, but, as with most of the genus, they overlap broadly with relatives and are not diagnostic on their own.
Males are the more ornamented sex — larger, with more produced fins and stronger colour than females, as is general for the genus. Species-specific descriptions of the sexual differences in P. cyanodorsalis are sparse, so the finer points of its dimorphism are best treated with caution.
Habitat
Pseudomugil cyanodorsalis is a brackish and marine fish of tropical northern Australia, recorded in the Western Pacific near Broome in Western Australia and between Darwin and Wyndham in the Northern Territory. Reports placing it in Papua New Guinea or Indonesia originate with unreliable hobby sources and are contradicted by FishBase, which restricts the verified range to northern Australia.
It is a genuine salinity specialist. It lives in mangrove swamps and highly saline creeks draining into estuaries, usually over muddy bottoms, and often gathers in large numbers. Recorded salinities run from roughly 28 to 40 parts per thousand — at the upper end, saltier than ordinary seawater. That tolerance of extreme, fluctuating salinity sets it apart from the many freshwater Pseudomugil aquarists know.
Recorded water temperatures sit in the tropical range, about 22 to 31 degrees Celsius. The species is pelagic-neritic, holding the water column of these shallow coastal and estuarine systems rather than tied to the substrate.
Feeding
Dietary studies of the species are sparse, but its trophic level is estimated at about 3.2 — a small micro-predatory omnivore. Like other blue-eyes, it presumably feeds in the water column on tiny invertebrates such as zooplankton, small crustaceans, and insect larvae, taken from open water and from among mangrove roots and submerged surfaces.
Where it is kept, generic Pseudomugil husbandry applies. It takes small live and frozen foods — baby brine shrimp, micro-worms, Cyclops, finely divided frozen bloodworm — along with suitably sized dry micro-foods, and a varied diet of small live and frozen items keeps up condition and colour. With a mouth this small, particle size is the main practical constraint.
Mating
Courtship in P. cyanodorsalis is not well documented, so what follows draws on the genus and should be read that way. Blue-eyes spawn more or less continuously under warm, stable conditions; males display to females by flaring their fins and deepening colour, with the produced dorsal and anal fins and the bright iris doing most of the work.
Displaying males stake out small patches among fine vegetation and court passing females, which follow a male into the plants to spawn. The display sharpens in the presence of rivals, so a group is at its most active and colourful when more than one male is present. Field and aquarium observations of this species are limited, and its courtship remains poorly described — data sparse.
Breeding
Breeding has been achieved in aquaria. The species is an egg-scattering plant-spawner: eggs are laid among fine-leaved plants or spawning mops, attaching by fine filaments, as is usual for the genus. There is no parental care — adults will eat eggs and fry given the chance — so breeders move the spawning medium or the eggs to a separate rearing container.
The developmental figures are unusually clear for an otherwise sparsely studied fish. Eggs take about two weeks to hatch, and the young reach adulthood in only about three months. Pair that quick maturation with a continuous-spawning habit and a conditioned group can be productive, provided the very small first foods the fry need are on hand — infusoria, rotifers, and similar micro-organisms, graduating to newly hatched brine shrimp.
Beyond this, species-specific detail is limited, and the brackish-to-hypersaline origin of the fish hints that salinity may matter for conditioning and rearing in ways no one has fully worked out for aquarium populations.
In the aquarium
The blueback blue-eye is rarely kept and is no fixture of the ornamental trade, so husbandry guidance is necessarily generic — drawn from the genus rather than from much species-specific experience. As a small, peaceful shoaler it belongs in a group of six or more, where males display against one another and settle into normal behaviour. Like other blue-eyes it jumps well, so a tight-fitting lid is not optional.
Water chemistry is the one consideration that sets this species apart. Unlike the freshwater Pseudomugil, it is a brackish-to-marine fish, occupying water in the wild from estuarine salinity up to around 28 to 40 parts per thousand. Expect to provide brackish — potentially strongly brackish — conditions rather than the soft fresh water its freshwater relatives want, alongside warm tropical temperatures in the low-to-high 20s Celsius. Precise optima are not established, so stable, well-filtered conditions and attention to salinity are what matter.
Given its size and specialised needs, this is a fish for the dedicated keeper, not a community tank. Tankmates, if any, should be similarly small, peaceful, and brackish-tolerant; keep it away from anything large or boisterous enough to outcompete or bully it.
Conservation
The IUCN Red List has not assessed Pseudomugil cyanodorsalis; FishBase records it as Not Evaluated under Version 2025-2, and it carries no CITES listing either. So there is no formal conservation status, and the claims on some hobby sites that it is 'Vulnerable' with a declining population are unsubstantiated and should be ignored.
It lives in the mangrove and estuarine systems of northern Australia — habitats that matter for coastal biodiversity and that face development and disturbance pressures elsewhere in the world, though no species-specific threat assessment exists for this fish. Its tolerance of a wide, fluctuating salinity range and its habit of gathering in large numbers hint at some resilience, but without a formal evaluation any statement about its population trend would be a guess.
For now the honest position is that the status is unknown and unassessed — data sparse — and that the fish is best documented from northern Australian estuaries, not the New Guinea or Indonesian localities wrongly pinned on it in some popular sources.