Taxonomy & naming
Scartelaos tenuis was described by the British ichthyologist Francis Day in 1876 from the estuaries of Karachi — then British India, now Pakistan. The Catalog of Fishes (Eschmeyer, CAS) — the authority for valid names — places the genus Scartelaos in the family Oxudercidae, subfamily Oxudercinae, the subfamily that contains all mudskippers. This is a true goby: the pelvic fins are fused into a cup-shaped sucker disc, the defining character of the gobies that separates them from the gudgeons (Eleotridae), where the pelvic fins remain separate.
The gobies as a whole are overwhelmingly marine — the vast majority of species live in saltwater — and the mudskippers represent the extreme of the gobies' amphibious tendency, having colonised the intertidal zone so thoroughly that they spend more time out of water than in it. Older literature may place this species under Gobius or synonymise it with other Indo-Pacific Scartelaos species; the Catalog of Fishes should be consulted for the current valid name. Scartelaos is closely related to Boleophthalmus and Periophthalmodon, the other mudskipper genera of Asian mudflats.
Morphology
Scartelaos tenuis is a slender, elongated mudskipper, more gracile in build than the broad-headed Periophthalmus species that dominate aquarium trade and popular imagery. It reaches roughly 6–7 in in total length as an adult, with a cylindrical, tapering body, the large dorsal eyes mounted high on the head and capable of independent movement to survey the mudflat in all directions. The pectoral fins are muscular and crutch-like, used to propel the fish across mud in the characteristic skip-and-plant gait that gives the group its name.
The fused pelvic sucker disc is present as in all true gobies; it gives the fish a grip on slick mud surfaces and on vegetation. The first dorsal fin in Scartelaos is notably tall and sail-like, used in signalling and territorial display on the mud. Colouration is subdued — grey to olive-brown dorsally, paler below, with muted markings suited to a life on exposed tidal mud. The eyes can be retracted into their sockets to keep them moist while the fish is on land.
Habitat
Scartelaos tenuis inhabits the warm intertidal mudflats and mangrove-fringe zones of the northwestern Indian Ocean, ranging from the Persian Gulf and Gulf of Oman east to Pakistan. It is a species of soft, fine-grained tidal mud — the kind deposited in sheltered bays, estuary mouths and mangrove lagoons — where it excavates and defends burrows.
The habitat is brackish to near-marine in salinity, warm (tropical, typically 75–86 °F), and subject to regular tidal flooding and exposure. During low tide the fish is fully exposed on the mud surface; when the tide rises it retreats into its burrow or ventures into the shallow water at the margin. This is an amphibious fish in the strict sense: it breathes atmospheric air through its moist skin and the lining of its sealed gill chambers, and cannot remain submerged for extended periods without drowning. It does not inhabit inland freshwater habitats.
Feeding
Scartelaos tenuis is a micro-predator of the mudflat surface. It forages on exposed mud during low tide, taking small invertebrates — microcrustaceans, copepods, small insects, worms and other benthic invertebrates — and likely also grazes on diatoms and microalgae that coat the mud surface, a foraging mode seen in its close relative Boleophthalmus. It locates prey visually using its highly mobile, protruding eyes.
In the aquarium, like all mudskippers, it needs small live and frozen foods: bloodworm, brine shrimp, copepods, daphnia and other small invertebrates are the staples. It typically refuses dried flake entirely or takes it only poorly. A fish kept on flake alone will lose condition rapidly; consistent supply of live or frozen invertebrate foods is essential to maintain health and colour.
Mating
Male Scartelaos tenuis are territorial on the mudflat, each defending a burrow and the patch of mud around it. Display involves raising the tall first dorsal fin — the sail-like fin becomes fully erect during territorial and courtship encounters — combined with lateral posturing and, occasionally, physical grappling. These displays are elaborate and highly visual, readable from a distance across the flat mud surface.
A male courts a receptive female by displaying at the entrance to his burrow with his dorsal fin raised, and leads her inside to spawn. The burrow serves simultaneously as territory, nesting site and breeding chamber; males invest heavily in maintaining and enlarging it, ensuring the burrow has a deep, sealed inner chamber that can hold an air pocket when the tide covers the entrance.
Breeding
Breeding follows the mudskipper pattern: the pair spawn inside the male's burrow, attaching adhesive eggs to the wall of the deep inner chamber. Crucially, the male maintains an air pocket in this chamber by ferrying mouthfuls of air down the burrow and releasing them — the eggs develop in air, not water, which dramatically raises their oxygen availability. The male guards the clutch, refreshing the air supply throughout incubation.
After hatching, the larvae are pelagic and spend an early stage developing in brackish or coastal marine waters before settling. This dispersal phase means mudskippers are effectively impossible to rear through to adulthood in a home aquarium: even if spawning can be induced in a very large, well-designed paludarium, raising the marine-phase larvae requires specialist facilities far beyond standard aquarium practice. Scartelaos tenuis has not been bred in the home aquarium. Virtually all specimens in the trade are wild-collected.
In the aquarium
Scartelaos tenuis demands a paludarium, not a conventional aquarium — a large, covered enclosure with a substantial area of exposed, damp, fine-grained mud or substrate (mudskippers need to haul out, walk, display and burrow), a shallow brackish water section, and high ambient humidity. The water must be warm (75–86 °F) and brackish, with a specific gravity in the range of estuarine water; using marine salt mix to achieve a target of roughly 1.005–1.010 SG is appropriate. The cover is mandatory: mudskippers are excellent escape artists and will find any gap in a lid.
The tank must be large enough for territorial behaviour — males will fight aggressively without sufficient space and visual barriers. Live invertebrate foods are essential; this species does not adapt well to dry foods. Scartelaos tenuis is rarely available in the hobby, and when it does appear it is wild-caught. It is a demanding, specialist fish best suited to an experienced keeper who can maintain stable brackish paludarium conditions, supply live food consistently, and appreciate the animal's extraordinary biology rather than expecting a conventional aquarium inhabitant.
Conservation
Scartelaos tenuis has not been assessed on the IUCN Red List and carries no formal conservation category; it is recorded here as Not Evaluated. Across its Persian Gulf–Pakistan range it appears to be locally common on suitable mudflats, but the habitat it depends on — fine-grained tidal mudflats and mangrove-fringed estuaries — is among the most threatened coastal ecosystem types globally, facing reclamation, aquaculture development, pollution runoff, and the erosive effects of rising sea levels and intensifying storm activity.
Mangrove loss across South and Southeast Asia has been severe over recent decades, and species that depend exclusively on intact tidal mudflats and mangrove structure are directly exposed to that loss. The aquarium trade in this species is minor and mostly incidental; the larger conservation concern is habitat, not collecting pressure. A formal IUCN assessment would help establish the species' actual range and population trajectory.