Taxonomy & naming
Cotylopus acutipinnis was described by the French naturalist Antoine Guichenot in 1863, making it one of the earlier-described Sicydiinae gobies. The Catalog of Fishes — the authority for valid names — places the genus Cotylopus in the family Oxudercidae (formerly subsumed within Gobiidae), and within the subfamily Sicydiinae, the Indo-Pacific cling-gobies that dominate fast mountain streams from Madagascar and the Mascarenes eastward through SE Asia and the Pacific islands. The genus was established for this species and holds only one or two validly described species.
The fish has appeared in older literature under various combinations, including placement in Gobius and Sicydium, reflecting the turbulent taxonomic history of the gobies as molecular phylogenetics reorganised the superfamily Gobioidei. The 2020s reclassification that erected Oxudercidae as a distinct family is the current arrangement followed here. Cotylopus is a true goby: its pelvic fins are fused into the cup-shaped sucker disc that is the synapomorphy of the true gobies and separates them from the sleeper gudgeons (Eleotridae), whose pelvic fins remain separate. Gobies as a whole are a predominantly marine lineage; Cotylopus acutipinnis represents the freshwater-entering branch that has colonised island streams.
Morphology
Cotylopus acutipinnis is a small, elongate goby reaching approximately 1 in standard length. The body is slender and somewhat cylindrical, typical of current-adapted Sicydiinae, and the fins are well-developed — the specific epithet acutipinnis (Latin: "with sharp or acute fins") refers to the pointed, relatively elongate fin rays that are a distinguishing feature of this genus.
The pelvic fins are fused into the characteristic goby sucker disc, which the fish uses to hold position on rocks in fast current. Like other Sicydiinae, C. acutipinnis is sexually dimorphic; males of the group typically display more vivid colouration and may develop modified fin elements, while females are more cryptically coloured. Detailed descriptions of live colouration are sparse in the available literature, consistent with this being a rarely-observed species outside its native range.
Habitat
Cotylopus acutipinnis inhabits fast, clear, highly oxygenated freshwater streams in Indonesia, with the type locality in the Sumatra/Java region. Like all Sicydiinae, it is a rheophilic species tightly associated with rocky stream beds in upland and foothill environments where strong current, high dissolved oxygen and clean water over gravel and rock substrates define its niche. Water temperatures in these highland streams typically range from approximately 64–75 °F.
These are not the soft, still, tannin-stained blackwaters of lowland forest; they are open, swift, sun-dappled mountain streams with high clarity and negligible organic loading. The species uses its fused pelvic sucker disc to cling to rock surfaces against the current, grazing biofilm or picking invertebrates from the substrate in the manner typical of the group. It is strictly a freshwater species in the adult phase of its life and does not require brackish conditions.
Feeding
Sicydiinae gobies occupy a spectrum from pure aufwuchs (biofilm/algae) grazers to invertebrate feeders, and Cotylopus acutipinnis appears to be an opportunistic feeder taking both algal biofilm and small micro-invertebrates from the rock surface in the manner typical of small Sicydiinae of fast streams. The sucker disc allows it to hold station on the substrate while feeding against the current.
In aquarium settings, replicating the diet is the central challenge. The fish requires a tank with a mature biofilm on the rock surfaces — mature rock that has been colonised by diatoms and algae — as well as offerings of small live and frozen invertebrates: micro-worms, newly hatched brine shrimp, daphnia and similar fine-particulate foods. A tank that lacks sufficient biofilm surface is likely to leave the fish in poor condition. Prepared dry foods may be accepted by acclimatised individuals but are insufficient as the sole diet.
Mating
Cotylopus acutipinnis follows the amphidromous reproductive strategy universal among the Sicydiinae. Adults are resident in fresh water year-round, and when reproductively ready, the male courts females in the stream, presumably at a rock crevice or sheltered site. The male guards adhesive eggs deposited on a hard surface — typically a rock surface or the underside of a stone — and fans them to maintain oxygen supply.
The species cannot be induced to breed in a standard home aquarium: the larvae are marine-dispersed and require an oceanic larval phase to complete development, so even if courtship and spawning were achieved in captivity, there is no practical means of providing the saline pelagic environment the hatchlings need. All aquarium specimens are therefore wild-collected.
Breeding
As with all Sicydiinae, the hatchlings of Cotylopus acutipinnis are tiny, pelagic larvae that immediately drift downstream when they emerge from the eggs. They enter the sea, where they develop as marine plankton for a period before metamorphosing into post-larvae (elvers). The post-larvae then locate fresh water by olfaction, enter stream mouths, and migrate upriver — sometimes climbing waterfalls using the pelvic sucker disc — until they reach suitable adult habitat in the headwaters.
This amphidromous lifecycle is the defining ecological constraint of the Sicydiinae. Because the marine larval phase cannot be replicated in captivity, Cotylopus acutipinnis is not bred in the home aquarium, and it is unlikely to be so in the foreseeable future. Aquarium specimens are wild-caught, most often as post-larvae or juveniles that have already completed their marine phase and returned to fresh water. Keepers should be aware that collection of these wild fish carries conservation implications for local stream populations.
In the aquarium
Cotylopus acutipinnis is an uncommon specialist species, seldom available in the mainstream trade. When kept, it demands conditions that closely match its native fast-stream habitat: powerful filtration and circulation to generate strong current, high dissolved oxygen at all times, a temperature range of roughly 64–75 °F, low nitrate, and a tank interior that is dominated by smooth rock — the kind of hardscape that can develop a mature biofilm coating for the fish to graze. A covered tank is essential, as small gobies are accomplished jumpers.
The tank should not be set up like a typical tropical community aquarium. Still or sluggish water, warm temperatures, fine substrate and an absence of biofilm-covered rock will leave this fish unable to feed and breathe adequately. Because all specimens are wild-collected, the fish may take time to acclimatise to aquarium food; offering live micro-invertebrates and ensuring healthy algal growth on rocks from the outset gives the best chance of success. Males may be territorial and benefit from ample visual breaks between rock clusters. This is a fish for the experienced keeper prepared to recreate fast, cool, oxygenated stream conditions.
All specimens are wild-collected; buyers should seek stock from responsible suppliers and be aware of the conservation pressures on small-range island stream endemics.
Conservation
Cotylopus acutipinnis has not been formally assessed on the IUCN Red List and carries no conservation category; it is recorded here as Not Evaluated. This reflects the general paucity of systematic survey data for small-range Sicydiinae across the Indonesian archipelago rather than any certainty about its status.
The species is a stream endemic with a restricted known range in Indonesia. Island stream gobies in this region face the familiar suite of pressures: deforestation of catchment watersheds, sedimentation of formerly clear-gravel stream beds, water extraction, and — for amphidromous species — barriers such as dams and weirs that prevent the post-larvae from migrating upriver. Because all aquarium trade relies on wild collection, trade volume is an additional consideration, though the species is rarely traded in significant numbers. The wider Sicydiinae include several formally threatened island endemics, and the conservation picture for Indonesian stream gobies as a group deserves more systematic attention.