Taxonomy & naming
Stiphodon semoni was formally described by the Dutch-German naturalist Max Carl Wilhelm Weber in 1895, based on specimens from the region covered by his major faunistic surveys of the East Indies. The species epithet semoni honours the German zoologist Richard Semon, who collaborated with Weber on several collecting expeditions in the region. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) — the authority for valid names — places the species in genus Stiphodon, subfamily Sicydiinae, family Oxudercidae.
Oxudercidae, sometimes still called Gobiidae in older literature, are the true gobies. The defining character of the true gobies is the fusion of the pelvic fins into a single cup-shaped sucker disc; this structure allows Stiphodon and its relatives to cling to smooth, fast-current boulders in ways that sleeper gobies and gudgeons (family Eleotridae, whose pelvic fins remain separate) cannot. The genus Stiphodon presently contains more than a dozen recognised species distributed across South-East Asia and the western Pacific; all are freshwater rheophilic algae grazers and all are amphidromous. The species has also appeared in older regional lists under the broader genus Sicydium, now restricted to the New World; the valid name is Stiphodon semoni Weber, 1895.
Morphology
Stiphodon semoni is a small goby reaching roughly 1–2 in in standard length, typical for the genus. The body is slender and somewhat cylindrical, tapering to a rounded caudal fin; the head is broad and flattened ventrally, with a wide, underslung mouth bearing fine, comb-like teeth adapted for scraping biofilm and algae from rock. The paired pelvic fins are fused into a disc-like sucker that grips smooth substrate in fast current — the sucker disc is the single most diagnostic feature of true gobies.
In Stiphodon species generally, males are more brightly coloured than females and often display vivid iridescence, with blues, oranges and yellows that intensify during courtship; females tend to be plainer, brownish or grey. Specific colour details for S. semoni are incompletely documented in the literature, as is typical for the more poorly studied members of the genus. Both sexes share the characteristic blunt snout and compressed posterior body of the Sicydiinae.
Habitat
Stiphodon semoni inhabits swift, clear, well-oxygenated freshwater streams in South-East Asia, in the type-locality region surveyed by Weber in the late nineteenth century — likely the Indonesian archipelago (including Java or Sumatra) or nearby mainland areas. It is a rheophilic species: it lives on and among rocks in sections of stream where current is brisk and dissolved oxygen is high, clinging with its pelvic sucker disc and grazing the biofilm that accumulates on submerged stone surfaces.
Water conditions in these highland and foothill stream habitats are typically clean and relatively mineral-poor, with temperatures in the range of approximately 64–82 °F depending on altitude and season, and pH broadly neutral to mildly acidic (around 6.5 to 7.5). These are not still, blackwater or heavily vegetated environments; they are open, rock-bottomed runs with vigorous flow and high clarity. Stiphodon semoni is entirely freshwater and does not require or tolerate elevated salinity in the main part of its range.
Feeding
Stiphodon semoni is an aufwuchs grazer: it feeds by rasping biofilm, diatoms and fine filamentous algae from the surfaces of submerged rocks using its comb-toothed, underslung mouth. This dietary specialisation is shared across the entire Stiphodon genus and is the central challenge for keepers, because the fish depends on a living, mature film of microorganisms rather than on particulate or suspended food.
In the aquarium, the primary food source must be the natural biofilm that grows on rocks, glass and hardscape in a mature, well-lit tank — a tank that has been running for at least two to three months and has developed visible algal growth. Supplementary offerings of blanched spinach, spirulina-based wafers or disc foods, and small live or frozen invertebrates such as baby brine shrimp or micro-worm are accepted and useful for nutritional balance. A Stiphodon semoni placed in a bare or newly set-up aquarium will almost certainly starve: the absence of established biofilm is the single most common cause of failure with this species.
Mating
Courtship in Stiphodon semoni follows the pattern documented across the Sicydiinae cling-gobies. Males are territorial, defending patches of rock in the stream where spawning occurs, and they display intensified colouration and fin-spreading to attract females and repel rival males. Because current-swept substrate is the resource both diet and spawning depend on, male territories are centred on suitable flat or concave rock surfaces in well-oxygenated flow.
A receptive female approaches the male's territory; courtship involves the male leading or flanking the female toward the chosen spawning site. The pair is briefly in contact during egg deposition. Multiple females may spawn with a single territorial male during a season, which is characteristic of many Sicydiinae species.
Breeding
The breeding biology of Stiphodon semoni is amphidromous — one of the most ecologically distinctive reproductive strategies among freshwater fishes, and the reason the species cannot be bred in an ordinary aquarium. Adults spawn in fresh water: the female deposits adhesive eggs on a cleaned rock surface in current, and the male guards and fans them until they hatch. The newly hatched larvae are tiny and planktonic; they are carried downstream with the current, pass through the river mouth and enter the sea, where they develop as marine zooplankton for a period of weeks to months.
Post-larvae — sometimes called amphidromous post-larvae or 'gobies en montaison' — then return to fresh water, entering river mouths and migrating upstream, using their pelvic sucker disc to scale rapids and in some related species even waterfalls, until they settle in suitable stream habitat and grow to adulthood. This obligate marine larval phase makes the lifecycle impossible to complete in a home aquarium: the larvae cannot survive in fresh water and require open marine conditions that no aquarium can replicate. All Stiphodon semoni available in the hobby are wild-collected; there is no captive-bred supply.
In the aquarium
Stiphodon semoni is a specialist fish best suited to experienced keepers with an established stream-style aquarium. The tank should be mature — two months minimum, longer is better — with a generous coverage of biofilm and soft algae on rocks and glass. Strong filtration providing vigorous, well-oxygenated flow is essential; a powerhead or wave pump directed across a rock-scape replicates the rheophilic habitat and supports the biofilm that is the fish's primary food source. Temperature should be kept in the 68–79 °F range; the fish tolerates modest variation but does not suit warm, sluggish community tanks.
Coverage needs are moderate: the fish spends most of its time on open rock faces grazing, but it benefits from small overhangs or flat stones under which it can retreat. Males are territorial toward one another, so a single male with one or two females, or a larger tank with line-of-sight breaks between male territories, works best. The fish is not aggressive toward unrelated species but should not be mixed with anything fast enough to outcompete it for food at the bottom. Because specimens are wild-caught, an initial quarantine period is advisable to detect and treat any parasites or internal infections acquired before sale. This is not a fish for beginners, or for any tank less than several months old.
Conservation
Stiphodon semoni has not been formally evaluated on the IUCN Red List and carries no conservation category; it is recorded here as Not Evaluated (NE). Many members of the genus Stiphodon share this status: they are small-range endemics of island or upland stream systems in South-East Asia that have received little systematic assessment, partly because of taxonomic uncertainty within the genus and partly because of limited survey effort in their native habitats.
The species faces the pressures common to small South-East Asian stream gobies: watershed disturbance, deforestation, agricultural run-off and the associated siltation of the clear, fast streams it requires. Because Stiphodon semoni and all other members of the genus are amphidromous and cannot be bred in captivity, the hobby trade is entirely supplied by wild collection, which adds direct harvest pressure on top of habitat concerns. Buyers should be aware that purchasing this species always involves a wild-caught fish; responsible retailers typically source from operations with sustainable harvest limits, and aquarists should favour suppliers who can confirm the collection origin.