Taxonomy & naming
Periophthalmus pusing was formally described by the German ichthyologist B. Eggert in 1935. It is placed in the genus Periophthalmus — the largest and most widely distributed genus of mudskippers — within the family Oxudercidae (subfamily Oxudercinae), the family that the Catalog of Fishes now uses for the amphibious mudskippers and their close kin. Older literature may list mudskippers within a broadly defined Gobiidae; the Catalog of Fishes resolves that synonymy, recognising Oxudercidae as the valid family for Periophthalmus and its relatives.
The species epithet pusing is derived from a local Malay or Indonesian vernacular term; it does not have the wide literature trail of better-known congeners such as P. argentilineatus or P. modestus. Within its genus, P. pusing is notable for being among the very smallest species, with FishBase recording females at a maximum of 1.5 in standard length. It has been recorded as syntopic with P. argentilineatus and P. malaccensis in Indonesian intertidal mudflats, and separating small individuals of these species in the field requires close attention to morphological characters.
Morphology
Periophthalmus pusing is a compact, dorsoventrally flattened mudskipper reaching at most 1.5 in standard length in females — making it one of the smallest members of the genus. Like all Periophthalmus, it has the characteristic protruding, independently mobile eyes set high on the head, which can be retracted into moist sockets and which afford wide-angle vision across the mudflat surface. The pectoral fins are muscular and arm-like, used for crutching locomotion on land; the pelvic fins are fused into the sucker disc that is the defining morphological synapomorphy of the true gobies, distinguishing them from the eleotrids and gudgeons where the pelvic fins remain separate.
The body is generally brownish or olive-grey above with variable pale spotting or patterning that provides camouflage on muddy substrate. As with other small Periophthalmus, the first dorsal fin carries two or three spines with coloured markings used in intraspecific display, and the gill chamber is enlarged to retain a store of oxygenated water during terrestrial excursions. Detailed morphological data distinguishing P. pusing from syntopic congeners remains sparse in the available literature.
Habitat
Periophthalmus pusing is an estuarine species of tide-dominated coastal environments with relatively low wave action. FishBase characterises its habitat as mangrove forests and the muddy intertidal mudflats associated with them — systems dominated by brackish water with salinity cycling between near-freshwater at high rainfall and fully marine during dry season or high tides. These are warm, humid environments where physical disturbance is low and surface mud is nutrient-rich and heavily colonised by biofilm, diatoms, cyanobacteria and small invertebrates.
The species occurs in the Western Pacific, with Indonesia at the core of the documented range. It is found at the same sites as P. argentilineatus and P. malaccensis, which tend to be larger-bodied species; P. pusing likely partitions resources by microhabitat or prey size. As an amphibious air-breather, it moves freely between the water surface and exposed mudflat, and requires both dimensions to function. It cannot survive extended periods in fully fresh water.
Feeding
Mudskippers in the genus Periophthalmus are micro-predators of the intertidal mudflat surface. They feed visually on small invertebrates — tiny crustaceans, amphipods, worms, small insects and insect larvae encountered on the mud surface or in shallow water — and on occasion take small fish. In the aquarium, this translates to a requirement for small live and frozen invertebrate foods: bloodworm, brine shrimp, daphnia, small crickets and other soft-bodied terrestrial insects offered onto the land section.
Dry flake food is strongly contra-indicated for mudskippers; care guides note that it causes bloating and is poorly accepted by fish that hunt by sight for moving prey. A diet of live and frozen invertebrate food is essential for long-term condition. Because P. pusing is among the smallest Periophthalmus, prey items should be sized small accordingly — foods suitable for larger mudskippers such as P. barbarus may exceed what this species can comfortably take.
Mating
In the genus Periophthalmus, courtship takes place on exposed mudflat during low tide and is an elaborate visual and postural affair. Males hold territories centred on burrow entrances and display to rivals and to females by erecting the brightly coloured first dorsal fin, performing rapid fin-fanning movements, and adopting raised-body postures that advertise body size and vigour. Intraspecific aggression between males is high and territories are vigorously defended.
A receptive female is courted at the burrow entrance; the male performs repeated display bouts and leads her into the burrow once her readiness to spawn is established. Because burrow construction and tidal-cycle timing are so central to the mating system, replicating the conditions needed for courtship in a captive setting is very demanding and requires detailed tidal cycling, appropriate mud substrate for burrowing, and adequate territory size.
Breeding
Periophthalmus pusing breeds in the manner documented for Periophthalmus mudskippers generally: the male excavates a J- or Y-shaped burrow into the mud, with a near-horizontal shaft terminating in an upward-curving egg chamber near the top of the burrow. The female deposits her adhesive eggs on the domed ceiling of this air-filled chamber. The male then takes sole charge of the eggs, periodically delivering fresh air to the chamber by swallowing a mouthful at the surface and releasing it inside, thereby maintaining oxygen levels around the clutch during the low-tide period when the burrow entrance is above the waterline.
Research on mudskipper egg care has shown that the male senses hypoxia in the chamber and increases air delivery in response; hatching is triggered when the male deliberately floods the chamber on a nocturnal rising tide, synchronising the release of pelagic larvae into the water column. Those larvae develop in brackish or marine water before settling. Because successful breeding requires replication of tidal rhythm, deep mud burrows, appropriate brackish salinity, and the full behavioural repertoire of the male, P. pusing is effectively impossible to breed in the home aquarium — no reliable captive-breeding protocols exist for this species, and virtually all specimens in the hobby are wild-caught.
In the aquarium
Keeping P. pusing requires a PALUDARIUM rather than a conventional aquarium: roughly 50–70% of the enclosure surface should be land — fine mud or sand with mangrove roots and driftwood — and the remaining water section should be shallow (4–10 in). Salinity must be brackish, at specific gravity 1.005–1.015 (approximately 5–20 g of marine salt per litre), and water temperature should be maintained in the 68–82 °F range; pH should not fall below 7.0. High ambient humidity above the land section is essential because mudskippers respire partly through their moist skin. A tight-fitting lid is critical — these are strong jumpers.
Because P. pusing is one of the smallest members of its genus, it must not be housed with larger mudskipper species, which will outcompete or injure it. Males are territorial and require line-of-sight breaks and multiple burrow opportunities; in a small enclosure a single pair or one male with one female is more practical than a group. Diet must consist of live and frozen invertebrate food offered on the land section; dry flake food is not suitable. The realistic aquarist should approach P. pusing as a demanding, display-oriented species — one that rewards patient observation of its surface behaviour but that cannot be bred and will require replacement from wild-caught stock.
Conservation
Periophthalmus pusing was assessed as Data Deficient by the IUCN Red List in March 2024, reflecting the fundamental lack of population data, detailed range maps, and ecological studies for this small and easily overlooked mudskipper. No population trend is available and no conservation action is currently in place for the species.
The broader pressures on mangrove-dependent estuarine fish are well-documented: coastal development, shrimp-aquaculture pond construction, mangrove clearance, and pollution from agriculture and industry all reduce and fragment the intertidal mudflat habitat that species like P. pusing depend on. Indonesia — the core of its documented range — has experienced significant mangrove loss over recent decades. Because the species is both poorly known and restricted to a habitat under substantial conversion pressure, the Data Deficient assessment should not be read as reassurance; it reflects ignorance rather than security. Targeted surveys of Indonesian intertidal mudflat communities that include small Periophthalmus species would be a meaningful step toward a more informed assessment.