Taxonomy & naming
Hypseleotris moolooboolaensis was described by D.F. Hoese and G.R. Allen in 1982 from material collected in the Mary River drainage of Queensland, Australia. The Catalog of Fishes places it within the family Eleotridae — the sleepers and gudgeons — not within the true gobies (Gobiidae and related families). This is the essential taxonomic point: although the hobby and common names often blur the line, the eleotrids differ from true gobies in that their pelvic fins remain distinct and separate. True gobies carry those fins fused into a characteristic cup-like sucker disc; in Hypseleotris and its relatives the fins are merely close-set, never joined, and the fish cannot grip substrate by suction.
The genus Hypseleotris has carried a range of older combinations in the literature — some species were formerly placed in Gobiomorphus, Eleotris or the broadly used wastebasket genus Eleotris — but current placement in Hypseleotris is stable. The species epithet moolooboolaensis is a Latinised toponym derived from 'Moolooboola', an Aboriginal name for the Mary River system from which the type specimens were taken, tying the fish's name directly to the Queensland waterway it calls home.
H. moolooboolaensis is further distinguished by its documented involvement in a hemiclonal hybrid lineage complex with related Hypseleotris species in the Murray–Darling and coastal drainages of eastern Australia. This sexually reproducing and unisexual hybridisation system has been the subject of ichthyological research, as it produces hybrid lineages that reproduce through a combination of sexual and clonal mechanisms.
Morphology
H. moolooboolaensis is a very small fish: males reach a maximum of approximately 1.5 in standard length (SL), females slightly smaller at around 1 in SL. Like other carp gudgeons it is a robust, slightly compressed fish with a relatively large head and upturned mouth well suited to picking invertebrates from the water column and bottom substrate.
The meristic counts — 8–9 dorsal spines, 10–12 dorsal soft rays, 1 anal spine and 12 anal soft rays, with 29–30 vertebrae — fall within the typical Hypseleotris range. Males in breeding condition develop elongated rays in the posterior portions of both the dorsal and anal fins, and these extended rays are tipped with pale orange bands, providing a reliable, if subtle, field character for sexing mature fish. Females are typically plainer and retain a more rounded body profile when gravid. The separate (non-fused) pelvic fins are the character that immediately places the fish outside the true gobies and into the eleotrids.
Habitat
The Mary carp gudgeon is restricted to the Mary River drainage of south-east Queensland — a relatively short coastal river system that drains from the Great Dividing Range to Hervey Bay. Within this system it occupies freshwater habitats: pools, runs and slower reaches with submerged structure and vegetation, often over sandy or silty substrates in lowland and foothill sections of the river.
Like other small Hypseleotris species, it is a demersal fish — spending most of its time in or near the bottom and among structure — and is classed as a tropical freshwater species, consistent with Queensland's subtropical to tropical climate. Water temperatures in the Mary River vary seasonally, and the species tolerates a degree of variability rather than requiring the stable tropical heat of some aquarium fish. The Mary River drainage is also notable for supporting other endemic freshwater species, and H. moolooboolaensis is one of several small fish whose ranges are restricted to that single catchment.
Feeding
H. moolooboolaensis is classified as an omnivore with a trophic level of approximately 3.1 — in practice it is a micro-predator of small aquatic invertebrates with some opportunistic feeding on plant material. In the wild it takes zooplankton, small benthic crustaceans, insect larvae and other tiny invertebrates encountered on or near the substrate and in the water column, in the manner typical of the genus.
In the aquarium it requires small live and frozen foods — bloodworm, daphnia, brine shrimp, cyclops and similar items — sized to its very small mouth. As with most gudgeons and small gobies, it does not eagerly accept dry flake as a dietary staple and is best maintained on varied live and frozen invertebrate food, with any dry food offered only as a supplement. Related Hypseleotris species such as H. compressa confirm the genus-wide pattern of invertebrate micro-predation and the need for suitably small, meaty food items.
Mating
H. moolooboolaensis is a resident freshwater cave-spawner. Breeding occurs in spring in the southern hemisphere — roughly November to January — when water temperatures rise to the 68–75 °F range that triggers reproductive behaviour. Males become territorial at this time, establishing and defending small cave territories (cavities under rocks, submerged logs, leaf litter accumulations or similar sheltered spaces) and intensifying their colouration, particularly the pale orange banding on the extended fin rays.
Courting males display to females from the entrance to the chosen cave, and a receptive female enters to spawn. The mating strategy is broadly consistent with that documented for Hypseleotris species generally: cave or substrate spawning within the freshwater range, with the male taking the territorial and guarding role. Because this is a resident freshwater breeder — not an amphidromous species reliant on marine larval development — the entire reproductive cycle can, in principle, be completed within a freshwater aquarium.
Breeding
Spawning in Hypseleotris follows a well-documented pattern: the female enters the defended cave or crevice and deposits 250–500 adhesive eggs on a hard surface — typically the underside or interior wall of the shelter — over a period of less than an hour. Once spawning is complete she plays no further parental role. The male takes sole charge of the clutch, guarding the cave against intruders and fanning the eggs continuously with his fins to maintain oxygenation and remove debris until they hatch. Under the warm spring temperatures at which spawning occurs, incubation takes approximately 6–9 days.
Because H. moolooboolaensis is a resident freshwater breeder, the whole cycle — courtship, spawning, egg-guarding and hatching — can take place in a home aquarium set up with appropriate caves and maintained at 68–75 °F for the breeding season. Newly hatched fry are small and require micro-foods initially before graduating to newly hatched brine shrimp and other fine live foods as they grow. The male's attentive guarding and the adhesive nature of the eggs mean that captive breeding is achievable for a dedicated keeper, in contrast to the amphidromous species in this atlas whose larvae require a marine phase.
In the aquarium
H. moolooboolaensis is a true nano fish — at under 1.5 in SL it is among the smallest of the carp gudgeons, and tank requirements are correspondingly modest in volume. A small to medium aquarium with a sandy or fine-gravel substrate, good filtration, and an assortment of caves and sheltered hiding spots suits it well. Submerged wood, smooth stones with cavities, and areas of dense planting provide both visual breaks that limit male territorial disputes and the spawning sites the fish actively seeks.
Water should be clean, freshwater (no salt required or tolerated), and maintained in the range of approximately 68–79 °F. The fish is not a demanding species in terms of water chemistry as long as extremes are avoided, but good filtration and regular water changes are advisable for any small fish held at high stocking densities. Males will assert territorial behaviour especially during the breeding season, so in small tanks a single pair or a group with a clear numerical skew is safer than multiple males together.
Feeding demands attention: like other small Hypseleotris, this species needs live or frozen invertebrate food rather than dry flake as its primary diet. Bloodworm, daphnia and brine shrimp are appropriate staples. A lid or tight-fitting cover is advisable, as small gudgeons can jump. Its very small size also makes it vulnerable to larger tank mates; the best companions are similarly scaled, peaceful small fish from comparable temperature ranges.
Conservation
H. moolooboolaensis has not been formally assessed by the IUCN Red List and carries a status of Not Evaluated. The species is endemic to the Mary River catchment — a single river system in south-east Queensland — which in principle places it at heightened risk from localised threats. The Mary River drainage faces pressure from water extraction, flow regulation, and land-use change in its catchment, and it supports several other endemic freshwater species of conservation concern.
The hemiclonal hybrid complex to which H. moolooboolaensis belongs adds a further layer of scientific interest: the genetic dynamics between the sexual parental form and hybrid lineages can be sensitive to changes in population structure or connectivity. While there is no specific published evidence of decline for this species, its narrow endemic range and dependence on a single river system make continued catchment-level management and monitoring relevant to its long-term security. Formal IUCN assessment would clarify whether current populations warrant targeted concern.