Taxonomy & naming
Yunnanilus sichuanensis was described by the Chinese ichthyologist R.H. Ding in 1995, in a paper titled "A new species of the genus Yunnanilus from western Sichuan, China," published in Acta Zootaxonomica Sinica. The Catalog of Fishes, the standing authority for valid fish names, lists the species under its original combination, with no synonymy or genus transfer recorded.
It belongs to the genus Yunnanilus, a large assemblage of small stone loaches endemic to Yunnan and Sichuan in southwestern China, within the family Nemacheilidae — the stone or brook loaches, the largest family in the loach superfamily Cobitoidea with roughly 790 described species across Eurasia. The species epithet sichuanensis simply marks its home province, Sichuan.
Morphology
Yunnanilus sichuanensis is a small, elongate loach with a flattened, low-profile body plan typical of the genus: a long, laterally compressed trunk, a short but noticeably high caudal peduncle, a flattened head, and a blunt snout with large eyes set above the midline of the head. FishBase records a maximum length of about 2.5 in total length in females, making it one of the smaller members of a genus that runs uniformly small.
As in other Nemacheilidae, the skin carries reduced, deeply embedded scales rather than the coarse scaling of many other fish families, which leaves it more exposed than a heavily scaled species to skin damage and to medications absorbed through the body surface. Detailed morphological description beyond the original species account is sparse; no separate account of a suborbital spine is recorded for this species in the available sources.
Habitat
The species is known from the Shuyalong Jiang River, part of the Anning River drainage, in Mianning County, Sichuan, at roughly 28°41'N, 102°12'E and an elevation of about 2,66 ft — high-altitude mountain stream habitat. No broader range has been documented; it appears to be a narrow-range endemic restricted to this part of the upper Yangtze system in western Sichuan.
Direct habitat data for this species is limited, but closely related Yunnanilus species from the same region and elevation band — Y. chuanheensis and Y. niulanensis — are recorded from clear, slow- to moderate-flowing streams over sand and cobble bottoms at similarly high altitudes (around 1,2953 ft). Taken together, the genus and the type locality point to cool, clear, well-oxygenated mountain stream water rather than warm lowland rivers or still, tannin-stained blackwater.
Feeding
No species-specific feeding study has been published for Yunnanilus sichuanensis. Stone loaches of the family Nemacheilidae are, as a group, bottom-hugging micro-predators: they pick small invertebrates — aquatic insect larvae, worms, small crustaceans — off gravel, sand and cobble in fast, clear water, and this pattern almost certainly holds for this species too.
In the aquarium, related Yunnanilus and other small nemacheilids typically accept small live and frozen foods such as bloodworm, Tubifex and Artemia readily, along with sinking dried foods, but may ignore flake left to float. A keeper should plan around a bottom-feeding micro-predator diet rather than a general community flake regimen.
Mating
No description of courtship or spawning behaviour has been published for this species. Sexual dimorphism is not documented in the available sources, and given how little has been recorded even for the wider genus, nothing reliable can be said here beyond noting that data is sparse.
What is known from the family points toward the general Nemacheilidae pattern: pairs or small groups spawn without any elaborate courtship display, releasing eggs into moving water over gravel or cobble rather than building a nest or defending a territory.
Breeding
Stone and brook loaches of the genus Yunnanilus are egg-scatterers with no parental care, releasing eggs over gravel and among stones in flowing water; the fry and eggs are left to develop unguarded. This species has not been documented as bred in the aquarium hobby, and given how narrowly it is known even in the wild — a single high-altitude river system — captive spawning reports do not exist in the available literature.
The honest picture for Yunnanilus sichuanensis is that it sits among the many stone loaches that are simply not part of the aquarium breeding record: rarely if ever kept, let alone bred, outside specialist or scientific collections. Anyone attempting it would be working from the general Nemacheilidae template — a mature, well-oxygenated tank with current — rather than from any species-specific account.
In the aquarium
Yunnanilus sichuanensis is not an established aquarium species; there is no meaningful hobby literature or trade history to draw on, and it should be treated as effectively unavailable and largely undocumented in captivity. Any care approach has to be inferred from the genus and the family rather than from direct experience with this fish.
That inferred approach follows the standard stone-loach template: cool-to-moderate, clear, well-oxygenated water with steady flow, a sand-and-cobble substrate that mirrors its high-altitude stream habitat, and a mature aquarium, since nemacheilids as a group are sensitive to unstable water chemistry. Being effectively scaleless like other loaches, it should be assumed sensitive to copper and to full-strength medications, warranting the same cautious, half-dose approach used across the family. Given the small adult size recorded for the genus, a modest but current-driven tank rather than a large show aquarium would be the appropriate setup if the species were ever available.
Conservation
Yunnanilus sichuanensis has not been evaluated by the IUCN Red List; no assessment could be located, and the species is recorded here as Not Evaluated rather than assigned a guessed status. This is a common situation for narrow-range Chinese stream loaches described from a single river system and never revisited by a formal conservation review.
Its entire known range is a single high-altitude tributary system in Mianning County, Sichuan, which by itself makes it inherently vulnerable to any localized pressure — damming, water diversion, pollution or habitat alteration in the Anning River drainage — even though no specific threat has been documented. There is no evidence of an aquarium trade in this species, so collection pressure is not a known factor; the conservation picture here is one of data scarcity rather than of a documented decline.