Taxonomy & naming
Hypseleotris ejuncida was described by Douglas Hoese and Gerald Allen in 1982, and the Catalog of Fishes — the authority for valid names — continues to place it in the genus Hypseleotris and the family Eleotridae. The species name ejuncida is Latin for slender or lean, a reference to its elongated body plan compared with other carp gudgeons.
The first thing to understand about this fish is that it is emphatically not a true goby, despite the common trade habit of labelling any small bottom-dwelling fish a 'goby'. The true gobies of the families Gobiidae, Gobionellidae and Oxudercidae carry their paired pelvic fins fused together into a cup-shaped sucker disc; the sleepers and gudgeons of the Eleotridae keep their pelvic fins separate, and that single morphological character — visible with a hand lens or good photo — is the cleanest way to distinguish the two lineages. Hypseleotris ejuncida is an eleotrid through and through, more closely related to the purple-spotted gudgeons (Mogurnda) and the peacock gudgeon (Tateurndina) than to any true goby.
No genus transfer has been recorded since the original description, and no synonyms are known. The fish is sometimes placed informally in the 'carp gudgeon' group alongside the other eastern-Australian Hypseleotris species, a loose assemblage defined by their slim, minnow-like body plan and preference for slow lowland waters.
Morphology
The slender carp gudgeon is a small, streamlined fish, typically reaching 1.5–2 in standard length with a body noticeably more elongated than many of its congeners — the proportions that gave it its name. The body is pale, sandy to light brownish, with a faint lateral stripe and fine patterning that provides effective camouflage against a substrate of sand, silt or leaf litter. The fins are relatively small and rounded.
The two pelvic fins remain free from one another, never fusing into a sucker disc, confirming the fish's eleotrid identity. Sexual dimorphism is present but subtle: males in breeding condition develop more intense colouration and slightly larger and more pointed dorsal and anal fins, while females become noticeably rounder-bodied when ripe with eggs. Outside the breeding season the sexes can be difficult to distinguish in the field.
Size data for this species is sparse in the literature; the 1.5–2 in SL figure comes from the original type-series material from Queensland streams, and wild fish are unlikely to significantly exceed this in standard length.
Habitat
Hypseleotris ejuncida is endemic to eastern Queensland, Australia, where it occupies small freshwater creeks, backwaters and slow-flowing reaches of lowland stream systems. The type locality is in Queensland, and the species is considered a small-range endemic rather than a wide-ranging component of the Australian freshwater fish fauna. Habitat is typically still to gently flowing fresh water with fine substrates of sand or silt, leaf litter on the bottom, and riparian and submerged vegetation providing cover.
Water conditions in the subtropical Queensland streams it inhabits are fresh with neutral pH around 6.5–7.0 and temperatures roughly in the range of 72–79 °F through the warmer months, with cooler conditions possible in winter. This is not a warm tropical fish in the Indo-Pacific sense; Queensland's subtropical climate means the species experiences a genuine seasonal temperature range. It is strictly a freshwater species — there is no evidence of tolerance for brackish conditions — and it is not amphidromous.
Feeding
Hypseleotris ejuncida is a micro-predator, as are all the sleeper gudgeons. In its native Queensland streams it takes small aquatic invertebrates from among the substrate and leaf litter — insect larvae, tiny crustaceans, copepods and similar small prey that it hunts by sight from close range or ambushes from cover. It is not an algae grazer or a flake-food scavenger.
In the aquarium the same micro-predator approach applies: the species does best on a varied diet of small live and frozen foods — bloodworm, cyclops, daphnia and brine shrimp — sized to its small mouth. Dry flake and pellets are likely to be refused or accepted poorly; a diet reliant on dry food alone will lead to a fish that fades in colour and condition. Live and frozen foods are the key to long-term health and to bringing the fish into breeding condition.
Mating
Hypseleotris ejuncida is a resident freshwater cave or substrate spawner, the same breeding strategy used by most of the small resident eleotrids and gobies. Because the entire life cycle takes place in fresh water and does not require a marine larval phase, the species can in principle be bred in the home aquarium — a significant contrast to the many amphidromous stream gobies that are effectively impossible to captive-breed.
A ripe male establishes and defends a cave, crevice, overturned shell or firm concealed surface as a spawning site and courts females approaching his territory with intensified colouration and fin displays. Females gravid with eggs show a visibly swollen belly and will respond to a persistent, well-conditioned male's courtship by following him into the chosen spawning cavity.
Breeding
Spawning takes place inside the male's chosen cave or on a sheltered hard surface. The female deposits a clutch of adhesive eggs on the roof or wall of the site and then withdraws; from that point the male takes sole charge, guarding the eggs against disturbance and fanning them with his fins to maintain oxygenation and remove waste, until they hatch.
The fry are small on hatching and require appropriately tiny first foods such as infusoria or other micro-foods before graduating to newly hatched brine shrimp nauplii and microworm. Data on incubation periods and clutch sizes for this species specifically are sparse in the literature, but the general pattern for resident Hypseleotris is in keeping with other small eleotrids: hatching within roughly one to two weeks depending on temperature. The fact that the species is a substrate-bound cave-spawner with active male parental care and a fully freshwater life cycle means captive breeding is achievable for a patient keeper who can supply the fish, the conditions and the live foods it needs.
In the aquarium
The slender carp gudgeon is rarely encountered in the aquarium trade outside Australia, where it is occasionally available as a native fish. Its requirements are those of a small, cryptically coloured micro-predator from subtropical freshwater streams: a modest tank with gentle filtration, fine sandy substrate, hiding spots in the form of small caves, hollow wood and leaf litter, and stable, clean fresh water in the neutral pH range (6.5–7.0) at subtropical temperatures (72–79 °F).
Like most eleotrids it needs no salt and should not be kept brackish. It is a jumper when startled, so a well-fitted lid is necessary. Males are territorial towards one another, especially around caves and spawning sites, so a tank holding more than one male benefits from multiple hiding spots and visual breaks. It should not be mixed with fish large enough to swallow it; small, peaceful co-inhabitants are preferable if the fish is kept in a community context.
The key care demand is diet: this fish is a micro-predator that does not take well to dry foods, and small live and frozen invertebrates are the staple. Meet that requirement and the tank's other basic needs — cover, stability, a lid, neutral fresh water — and the slender carp gudgeon is manageable for an experienced keeper of small native fish, though it is a specialist's fish rather than a beginner's community species.
Conservation
Hypseleotris ejuncida has not been formally assessed by the IUCN Red List and carries no conservation category here; it is recorded as Not Evaluated rather than assigned a guessed status. As a small-range endemic restricted to eastern Queensland stream systems, it has the geographic profile of a species at potential risk from local habitat degradation — riparian clearing, sedimentation, altered hydrology from water extraction and drainage works, and the ongoing pressure of introduced predatory fish (such as tilapia and gambusia) that affect native Queensland freshwater fauna broadly.
There is no specific data in the available literature on population trends or any targeted conservation work for this species, and it is too rarely kept in the aquarium hobby to have a significant captive buffer. Its small range means that monitoring native fish surveys and protecting the stream catchments it inhabits are the most relevant safeguards. Keepers and observers in Queensland with access to its native waters are better placed than most to contribute occurrence records through citizen-science platforms, which can help fill the assessment gap.