Taxonomy & naming
Sicyopterus longifilis is a true goby of the family Oxudercidae, placed in the Sicydiinae, the amphidromous stream gobies of Indo-Pacific islands. The Catalog of Fishes lists it as valid under the authority Beaufort, 1912. The type locality is given there as: Upper course of Tubah River, western Ceram, Indonesia.
Morphology
Sicydiine gobies have the pelvic fins fused into a cup-shaped sucker disc, which lets them hold position on rock in fast current and climb wet rock faces on the way upstream. Males in this group are usually more colourful than females. FishBase gives a maximum of 4 in TL for this species, which is large for a sicydiine. A detailed colour description has not been compiled here.
Habitat
FishBase says the species lives in coastal streams with moderate to fast flow, usually in small, clear creeks, often in rainforest within about 31 mi of the sea. It is listed as freshwater, brackish and demersal, and amphidromous. Measured water chemistry has not been published.
Feeding
Sicyopterus species are strongly adapted for scraping algae and biofilm from rock, using a sucker-like mouth with a specialised lower lip. The diet of this species has not been described specifically.
Mating
Mating has not been described for this species. In better-studied sicydiines, males hold small territories on stream rock and court females at a nest site.
Breeding
FishBase lists the species as amphidromous. Sicydiine gobies are amphidromous: eggs are laid in fresh water, the larvae drift to sea, and the post-larvae return to rivers months later. Details specific to this species have not been published. It cannot be assumed to breed in a closed freshwater aquarium, and trade fish are wild-collected.
In the aquarium
Species in the trade under this name are demanding. They need cool to moderate, very clean, strongly oxygenated water, brisk flow, rock surfaces with mature biofilm, and a tightly closed lid. Husbandry for Sicyopterus longifilis itself has not been documented, so general sicydiine practice is the only guide. Identity of fish sold under this name should be treated with caution.
Conservation
The IUCN assessed Sicyopterus longifilis as Least Concern in 2019, meaning it is not currently considered at risk. Like other amphidromous gobies, it depends on unobstructed streams down to the sea, and dams, weirs and stream degradation are threats to the group.