Taxonomy & naming
Oligolepis acutipennis was first described by the French zoologist Achille Valenciennes in 1837, one of a vast number of fish species he worked on as co-author of the monumental Histoire Naturelle des Poissons. The genus name Oligolepis means 'few scales', from the Greek oligos (few) and lepis (scale), referring to the relatively sparse scalation of these small gobies. The species epithet acutipennis comes from the Latin acutus (sharp) and pinna (fin), alluding to the pointed or acute form of the fins. The Catalog of Fishes places the genus in the family Oxudercidae — the true gobies, a lineage defined above all by the fusion of the paired pelvic fins into a cup-shaped sucker disc on the underside. This is the key morphological character that separates real gobies from the superficially similar sleepers (Eleotridae) and gudgeons, whose pelvic fins remain separate.
The species has passed through older combinations over its taxonomic history: early workers placed various small Indo-Pacific gobies in the catch-all genus Gobius or in Vaimosa, and Oligolepis itself has sometimes been lumped with Gobiopterus or treated as close to Stenogobius. The Catalog of Fishes stabilises the name as Oligolepis acutipennis (Valenciennes, 1837). The genus contains a small number of species — estimates vary from three to six valid names — of which acutipennis is the one most likely to appear in the brackish-water aquarium trade.
Morphology
The sharptail goby is a slender, cylindrical fish reaching approximately 1–2 in in total length in the wild; most aquarium specimens stay toward the lower end of this range. The body is pale and somewhat translucent, often marked with a series of small blotches or a lateral stripe, and the fins are lightly marked or clear. The first dorsal fin carries a small number of spines; the second dorsal and anal fins are soft-rayed. The caudal fin is pointed or slightly rounded — the feature the species name nods to. Like all true gobies it carries the fused pelvic fins that form the characteristic sucker disc on the belly, visible as a shallow cup when the fish is at rest on the substrate.
Sexual dimorphism is subtle but present in breeding condition. Males tend to be slimmer and may show brighter or more contrasting colouration when displaying, while ripe females become noticeably plumper in the abdomen. Neither sex develops the dramatic nuchal hump seen in some gudgeons or the vivid colour contrasts of the peacock gudgeon; this is an understated, subtly pretty fish rather than a showpiece.
Habitat
Oligolepis acutipennis is an Indo-West Pacific estuarine species, recorded from coastal and tidal habitats across South and Southeast Asia and neighbouring island groups. It inhabits tidal mudflats, mangrove channels, estuarine creeks and the brackish reaches of coastal rivers where fresh and salt water mix. The bottom is typically soft mud or fine sand, the water shallow and warm, and the habitat often fringed by mangrove or other coastal vegetation. This is not a fish of clear, fast-flowing hill streams or open ocean reefs — it occupies the messy, productive interface zone between river and sea.
Salinity in these habitats is variable and subject to tidal fluctuation, ranging from near-fresh at flood after heavy rain to moderate brackish (roughly 6 to 18 parts per thousand) on the ebb. Water temperature is warm throughout: typically 75–82 °F in the tropical belt of its range. The species does not penetrate far into pure fresh water as an adult; juveniles may briefly tolerate lower salinities, but both wild ecology and aquarium experience confirm that adults fail in plain tap water over time due to osmoregulatory stress. It is not an amphidromous species — unlike the cling-gobies (Stiphodon, Sicyopterus) it does not require a marine larval phase — and the whole life cycle can be completed in brackish water.
Feeding
Like all small gobies, Oligolepis acutipennis is a micro-predator. In its tidal mudflat and estuarine habitat it hunts small benthic invertebrates — copepods, amphipods, small worms, insect larvae and tiny crustaceans — picking them from the surface of the mud and among organic debris. It is a visual, active hunter rather than a passive filter-feeder or algae-grazer.
In the aquarium this translates directly: the sharptail goby needs small live or frozen meaty foods. Bloodworm (chironomid larvae), brine shrimp, daphnia, cyclops and copepods are all taken readily. Dry flake food is typically refused outright or accepted only reluctantly and should not be relied upon as a staple; a fish kept on flake alone will lose condition. Small sinking pellets may be investigated if introduced carefully, but live and frozen micro-prey remain the foundation of a healthy diet. This is a keeper-failure axis — many small brackish gobies decline slowly in captivity simply because their owners do not provide enough live or frozen food.
Mating
Oligolepis acutipennis is a resident cave-spawner, not an amphidromous species. Pairs or small groups form in a brackish aquarium when conditions are stable, and males establish loose territories centred on a chosen cave or crevice — a hollow, a section of PVC pipe, a coconut shell or a gap among rocks all serve. The male courts a ripe female by intensifying his posture and displaying at the cave entrance, drawing her in to inspect the spawning site.
Courtship is not aggressive by the standards of cichlids or larger gobies, but males will spar with rival males over sites, and a tank with multiple males benefits from caves spaced so fish cannot hold line-of-sight on one another continuously. The salinity must be appropriate — measurable brackish water — for breeding behaviour to be triggered reliably; a pair in pure fresh water is unlikely to condition and spawn.
Breeding
Spawning follows the pattern standard for the resident small gobies: the female enters the male's cave and lays a clutch of adhesive eggs on the roof or wall of the cavity, then withdraws. From that point the male takes over completely, guarding the clutch and fanning the eggs continuously with his pectoral fins to oxygenate them and keep fungus at bay. This is genuine parental care — not cursory — and a devoted male will aggressively defend his cave against tankmates during incubation. Eggs hatch in approximately five to seven days at tropical temperatures.
The hatchlings are tiny and need appropriately small first foods: rotifers, infusoria or commercial liquid fry food initially, progressing to newly hatched brine shrimp nauplii and micro-worm as they grow. Because the complete life cycle is in brackish water — there is no marine larval phase — this species can in principle be raised through to adulthood in a dedicated brackish breeding tank. This is a meaningful distinction: many of the most sought-after gobies in the hobby, including the cling-gobies (Stiphodon, Sicyopterus), are amphidromous and effectively cannot be captive-bred; Oligolepis acutipennis is not in that category. Success requires stable salinity, good water quality, and adequate live food for the fry.
In the aquarium
A species tank of 10–15 US gal suits a pair or a small group of sharptail gobies well. The non-negotiable is brackish water: a specific gravity of roughly 1.004 to 1.010 (approximately 6 to 15 parts per thousand salt) is a sensible target range, and plain fresh water will stress the fish over weeks and months even if they appear to survive initially. Aquarium salt or marine salt mix both work; marine salt provides a more complete ionic mix. Temperature should be maintained in the 75–82 °F range.
Decoration should provide plenty of small caves and hiding places — the fish is retiring by nature and genuinely needs retreats, not simply for aesthetics but to reduce inter-male stress and to provide spawning sites. Sand or fine gravel substrate suits the species' substrate-hugging lifestyle. Plants tolerant of brackish conditions (Java fern, Vallisneria, Anubias in low-salinity setups) can be used but are not essential. The tank lid must fit well: gobies of all kinds are accomplished jumpers and will find any gap. Do not mix this species with fish that require pure fresh water, and avoid large, predatory tankmates. On appropriate diet — live and frozen micro-prey foremost — and in correctly prepared brackish water, the sharptail goby is reasonably hardy and can be a rewarding breeding project for the patient keeper.
Conservation
Oligolepis acutipennis has not been formally assessed on the IUCN Red List and carries a status of Not Evaluated; no assessment was found in the available sources. For a widespread estuarine species distributed across the Indo-West Pacific coastal zone, the most likely formal status would be Least Concern were it to be assessed, given its broad range and tolerance of disturbed, tidal habitat — but this should not be assumed, and the species is recorded here as NE to reflect the actual state of knowledge.
The broader pressures on estuarine fish in the Indo-Pacific are substantial: mangrove clearance, coastal development, pollution from agricultural and industrial runoff, and estuarine water quality degradation all affect the brackish mudflat and tidal-creek habitats this species depends on. Collection for the aquarium trade appears to be the main route by which it enters hobbyist hands; because it is a resident breeder rather than an amphidromous species, captive breeding is possible and should be encouraged as a way to reduce reliance on wild-caught stock. Keepers who successfully breed this species in brackish aquaria make a small but genuine contribution to reducing pressure on wild populations.