Taxonomy & naming
Boleophthalmus caeruleomaculatus was described by the Australian ichthyologists Allan Riverstone McCulloch and Edgar Ravenswood Waite in 1918. The Catalog of Fishes places it in the family Oxudercidae, the amphibious gobies and mudskippers, within the subfamily Periophthalminae — the true mudskippers. It is the only described member of the genus Boleophthalmus native to Australia; its congeners (B. boddarti, B. pectinirostris, and others) range across the Indo-Pacific from the Persian Gulf to China and Southeast Asia.
The genus Boleophthalmus belongs squarely in the true gobies (order Gobiiformes), which is the first thing to establish about this fish. True gobies share the fused pelvic fins that form the cup-like sucker disc; mudskippers retain this character and use it to cling to sloped mud and mangrove pneumatophores when out of water. Gobies are overwhelmingly marine, and mudskippers like this species represent the amphibious, estuarine extension of that radiation into the terrestrial fringe — a quite different lineage from the freshwater sleepers (Eleotridae) or the cling-gobies (Sicydiinae) also covered in this atlas.
Older combinations sometimes placed Boleophthalmus species within broad, now-fragmented genera. Boleopthalmus caeruleomaculatus has no widely used synonyms in circulation, but the family Oxudercidae itself was only recently separated from the broader Gobiidae by modern molecular phylogenetics, so older references may list the species under Gobiidae.
Morphology
The blue-spotted mudskipper reaches a maximum of about 6.5 in in standard length (SL), making it a moderately large mudskipper. The body is elongate and laterally compressed, with the pectoral fins enlarged and arm-like — used to prop the fish upright on mud and to 'crutch-walk' across the flat in the characteristic mudskipper locomotion. The pelvic fins are fused into the sucker disc diagnostic of true gobies, here also aiding adhesion on wet, sloping surfaces.
Colouration is distinctive: the body is greenish-brown with small iridescent blue speckles scattered across the flanks. The first dorsal fin is greenish with bluish-white spots; the second dorsal fin is greenish with vertical rows of blue spots — the patterning that gives the species both its scientific name (caeruleomaculatus: blue-spotted) and its common name. Fin-ray counts run to 24–28 dorsal fin elements and 90–136 longitudinal scales.
Like all mudskippers, this species has mobile, periscope-like eyes set high on the head that can be retracted into moist sockets, enabling binocular vision in air. It is a facultative air-breather, respiring through the skin and the moist lining of the gill chamber when out of water.
Habitat
Boleophthalmus caeruleomaculatus is endemic to Australia, where it occupies the intertidal zone of tropical estuaries: tidal mudflats, mangrove margins, and the lower reaches of tidal creeks. These are warm, turbid, brackish to marine environments with a substrate of fine, oxygen-poor mud. The fish emerges at low tide to feed on the exposed mud surface and retreats into burrows or into the water at high tide or when threatened.
The burrow is central to its existence. Like other Boleophthalmus species, it excavates a J- or Y-shaped burrow into the mud — used for shelter, for retreating from rising tides and thermal extremes, and, critically, for spawning. Because it is tied to the intertidal mudflat ecosystem, it is sensitive to the health of mangrove and estuarine habitats. There is no freshwater phase to its life cycle: this is a brackish-estuarine fish through and through, and any captive setting must reflect that.
Feeding
Boleophthalmus caeruleomaculatus is a herbivorous deposit-feeder, browsing the thin layer of benthic microalgae — predominantly diatoms — from the surface of the mud. This makes it unusual among mudskippers: most members of related genera (Periophthalmus, Periophthalmodon) are carnivores that hunt small invertebrates, crustaceans, and other animals on the mudflat. The genus Boleophthalmus is the herbivorous outlier in the mudskipper radiation.
Research on the closely related B. pectinirostris has shown that the genus feeds selectively on centric and pennate diatoms larger than about 50 micrometres, with a specialised jaw and feeding apparatus that strips diatom cells from the mud surface. The same mode almost certainly applies to B. caeruleomaculatus. This diet has direct consequences for captive care: the fish needs a consistent source of benthic microalgae — a mature, illuminated, biofilm-colonised mud surface — and will not thrive on a standard carnivore menu of mysis or bloodworm.
Mating
Mudskipper mating behaviour is closely tied to the burrow. Male Boleophthalmus mudskippers are territorial, defending a patch of mudflat centred on their burrow entrance. During the breeding season males perform conspicuous courtship displays — raising and fanning the tall, spotted first dorsal fin as a visual signal — to attract females and warn rival males. These displays are highly visible on the open mudflat.
A gravid female is attracted to a male's burrow and assessed before entry. The courtship involves the male leading the female to and into the burrow, where spawning takes place in the dark, hidden from the tidal flat above. Because the burrow environment provides temperature and humidity stability, the male can maintain the physiological conditions needed for the eggs even when the tide covers the flat.
Breeding
Spawning occurs inside the mud burrow, where the female attaches adhesive eggs to the burrow wall. The male then guards the clutch in an air-filled chamber of the burrow — repeatedly visiting the surface to take mouthfuls of air, which he releases into the egg chamber to maintain the oxygen supply for the developing embryos. This is the defining mudskipper breeding strategy, and it is a remarkable adaptation to life on an anaerobic mudflat.
Once the eggs hatch, the larvae are pelagic and develop in the brackish or marine water column. This means the full breeding cycle spans two very different environments — terrestrial burrow and open water — and it is one reason mudskippers are effectively never bred in the home aquarium. The size and complexity of a functional burrow system, the tidal simulation required, and the pelagic larval phase make captive propagation extremely unlikely outside of specialist research facilities. Wild-collected specimens make up virtually all fish offered for sale.
In the aquarium
Boleophthalmus caeruleomaculatus is not a conventional aquarium fish. It requires a paludarium: a setup in which roughly two-thirds of the interior is land — fine damp sand or mud, deep enough to allow burrowing, with roots, cork or rockwork for cover — and one-third is warm, filtered brackish water. The land portion must remain consistently moist and the enclosure must be closed or humid; mudskippers breathe through their skin and will desiccate in dry air. A well-fitted lid is also essential, as they are capable climbers.
Water should be brackish, reflecting the estuarine habitat: a specific gravity of around 1.004–1.010 is typical for Boleophthalmus species in captivity. Temperature should be warm tropical, around 75–86 °F. Filtration is important but flow should be gentle; the fish need areas of still, shallow water they can rest at the surface of.
The herbivorous diet is the main challenge. Diatoms and benthic microalgae should be encouraged on flat rock or slate surfaces illuminated by full-spectrum light within or adjacent to the water zone. Many keepers supplement with small quantities of blanched vegetable matter, though biofilm is the preferred food. This species is best kept by experienced mudskipper specialists; it is not suited to a community tank or a standard aquarium setup. Because it cannot be captive-bred, any specimen acquired is wild-caught from an Australian coastal habitat.
Conservation
Boleophthalmus caeruleomaculatus is assessed as Least Concern (LC) on the IUCN Red List, most recently in February 2020. The species appears to be relatively widespread across the tropical coastal mudflats of northern Australia, and there is no evidence of population-level decline attributable to trade pressure. However, it is entirely dependent on healthy intertidal mudflat and mangrove systems — habitats under growing pressure from coastal development, port expansion, and hydrological modification in northern Australia.
Because all specimens in the aquarium trade are wild-caught (captive breeding does not occur), any significant trade pressure would fall directly on wild populations. In practice, demand for this specialist species is low compared with more easily kept mudskippers, limiting that risk. The broader conservation picture for its habitat — mangrove and estuarine systems along Australia's tropical north — is one of ongoing monitoring and, in parts of the range, targeted management under Australian environmental law.