Taxonomy & naming
Boleophthalmus pectinirostris was described by Carl Linnaeus in 1758, the founding year of modern zoological nomenclature, making it one of the oldest formally named mudskippers. The Catalog of Fishes — the authority for valid names — places it in the family Oxudercidae, subfamily Oxudercinae, the amphibious mudskippers. Older literature may place the genus under Gobiidae or list it as a subgenus of Gobius, reflecting the many reshufflings of the goby lineages over the past two centuries; the family Oxudercidae is the currently accepted circumscription.
The genus Boleophthalmus contains several species distributed across coastal Asia and is distinguished from the closely related Scartelaos by the presence of comb-like teeth. The epithet pectinirostris, meaning 'comb-snouted' in Latin, alludes to this dental arrangement. Boleophthalmus pectinirostris is the largest and most widely studied species in the genus, and it has been central to research on amphibious respiration, burrowing behaviour, and herbivory in the mudskipper radiation.
Morphology
The great blue-spotted mudskipper reaches a maximum of around 8 in total length — FishBase records 7 in for males and unsexed specimens, and up to 8.5 in for females — making it the largest species in the genus. The body is robust and slightly compressed, tapering from a large, broad head to a muscular tail. The overall ground colour is a drab greyish-olive, but close inspection reveals the brilliant blue-green iridescent spots scattered across the flanks and the large, highly mobile dorsal fin, which the male erects during territorial and courtship displays. The pectoral fins are leg-like and function as crutches for locomotion on the mud.
As a true goby (family Oxudercidae), Boleophthalmus pectinirostris carries the characteristic fused pelvic fins that form a disc-like sucker; this structure anchors the fish to hard surfaces and assists movement on slopes. The eyes sit high on the skull, can rotate independently, and retract into sockets to keep them moist during aerial exposure. The gill chambers are enlarged to hold a reservoir of atmospheric air, replenished regularly at the surface. Males are distinguished by the larger and more elaborately coloured first dorsal fin, which functions as a flag in territorial contests.
Habitat
Boleophthalmus pectinirostris is an estuarine species of the Indo-West Pacific, recorded from Japan (including the Ariake Sea), Taiwan, coastal China, and extending through Malaysia to Sumatra. It inhabits the soft intertidal mudflats, mangrove margins, and river mouths of warm sheltered estuaries — environments subject to intense and regular salinity fluctuation as tides push brackish water in and rainfall dilutes it. The species shows a strong preference for muddy substrates suitable for burrowing, and it is absent from sandy or rocky shores.
Activity in Japan peaks from March to November, with reduced surface exposure in colder months; in more tropical parts of its range it is active year-round. The species is amphibious in the fullest sense: individuals spend extended periods on the mud surface at low tide, returning to the water or to their burrows at high tide or when threatened. The water they occupy is brackish rather than fully marine, and like all mudskippers this species is physiologically adapted to wide swings in temperature, salinity, and dissolved oxygen — conditions that would rapidly kill most freshwater fish.
Feeding
Boleophthalmus pectinirostris is a specialist herbivorous deposit-feeder — an unusual feeding strategy among mudskippers, most of which are carnivores taking small crustaceans and invertebrates on the flat. Research published in Marine Biology (Springer) documents that this species targets diatoms and microalgae accumulating on the mudflat surface, including species such as Pleurosigma, Navicula, Nitzschia, and Gyrosigma. It is size-selective: the fish preferentially ingests diatoms larger than about 50 micrometres and rejects smaller forms, with the feeding apparatus shaped to minimise intake of mineral mud.
A notable aspect of its feeding ecology is an inverse seasonal pattern: diatom abundance on the mudflat is highest in cool months when mudskipper densities are lowest, and vice versa, suggesting that intense grazing in the warm months keeps algal standing crops depressed. In the aquarium, replicating this diet is genuinely difficult — live diatom and microalgal films must be cultivated on flat rock or glass surfaces, and prepared foods are a poor substitute. Mudskippers of other genera can be trained onto meaty foods, but the herbivorous Boleophthalmus is more demanding.
Mating
The great blue-spotted mudskipper is a territorial species in which males defend burrows and court females with conspicuous visual displays. Research on its reproductive ecology documents active breeding from March to November during the warm months, coinciding with peak surface activity. Rival males engage in direct combat and in elaborate fin-display contests: the large, brightly pigmented first dorsal fin is raised fully to signal dominance, and confrontations can escalate to chasing and biting along burrow boundaries.
Burrow ownership is the key resource — only males that hold a burrow can attract a mate and spawn. Males invest heavily in burrow maintenance, shaping the entrance and interior over days or weeks, and they actively defend the territory against competitors. Females visit multiple territories before choosing a spawning partner, and the male courts her at the burrow entrance with dorsal-fin displays and circling behaviour. Pair bonds are transient; after spawning the female leaves and the male takes over all parental duties.
Breeding
Boleophthalmus pectinirostris is a burrow-spawner. The male excavates a J- or Y-shaped tunnel in the soft mud, shaping an egg chamber at the lower end. Critically, he maintains an air pocket in this chamber: studies show that approximately 90% of active burrows in the breeding season contain stored air, which the male replenishes by carrying mouthfuls of atmospheric air down the tunnel. Eggs are attached to the burrow wall inside this air-filled chamber, where the higher oxygen content supports developing embryos in the otherwise anoxic mud.
The male guards the clutch and tends the burrow throughout incubation. After hatching, the larvae are pelagic and develop in brackish and marine water before returning to estuarine habitats. Because the larvae require conditions essentially impossible to replicate outside a specialised tidal facility, Boleophthalmus pectinirostris is not bred in the home aquarium. Specimens in the trade are wild-collected from their native estuaries.
In the aquarium
Keeping Boleophthalmus pectinirostris is a serious undertaking and not a project for beginners. This species requires a paludarium — a setup that provides both a substantial land area of soft mud or damp substrate and a shallow body of warm brackish water — rather than a conventional aquarium. The fish must be able to haul out onto land at will, the air space above the water must be maintained at high humidity to prevent the gill chambers from drying out, and the tank must be tightly covered, as mudskippers are determined jumpers.
Water conditions should be brackish, reflecting the estuarine habitat: a specific gravity of roughly 1.005–1.010 is often cited, with temperatures in the range of 75–86 °F. Substrate should be soft mud or fine sand deep enough to allow burrowing. Feeding presents the greatest challenge: the natural diet of diatoms and benthic microalgae is difficult to replicate, and most reports suggest this species is harder to acclimate to prepared foods than carnivorous mudskippers. Keepers have had some success cultivating diatom films on tiles and flat stones placed in or near the water, but long-term success remains the exception rather than the rule. A large, uncrowded paludarium with ample visual barriers between males is essential, as territory-holders will fight persistently when confined.
Conservation
Boleophthalmus pectinirostris has not yet been formally assessed on the IUCN Red List and carries no conservation category; it is recorded here as Not Evaluated rather than assigning a status that has not been established. The species is broadly distributed across coastal East and Southeast Asia and remains common in suitable habitat, but that habitat — soft intertidal mudflats and mangrove estuaries — is among the most heavily impacted coastal environments in the world.
Land reclamation for agriculture and coastal development has eliminated large areas of mudflat habitat across China, Taiwan, Japan, and Southeast Asia over recent decades. Mangrove clearance, pollution from river-borne agricultural runoff, and sedimentation from upland deforestation all degrade the intertidal zones this species depends on. The Ariake Sea population in Japan, which has been the subject of ecological research, has experienced habitat contraction. Pristine intertidal mudflat habitat is increasingly restricted to protected areas. Trade specimens are wild-caught, adding a further modest pressure, though the species is not commonly kept and demand is low. Its conservation trajectory is likely to track the broader fate of East Asian estuarine ecosystems.