Taxonomy & naming
Yunnanilus qujinensis was described by Du, Lu & Chen in 2015 in Ichthyological Exploration of Freshwaters (volume 26, issue 3, pages 249–254), from a spring near the Hujiafen Reservoir at Qujin City, in the Nanpanjiang River basin of Yunnan, China — the fish's specific epithet simply names its type locality. It was placed in the nigromaculatus species group of Yunnanilus, one of several clusters recognised within the genus on the basis of diagnostic characters such as the processus dentiformis on the lower jaw and details of the gill rakers and pharyngeal teeth.
The family Nemacheilidae, and genus Yunnanilus within it, have been repeatedly reshuffled as molecular work outpaces morphology-only descriptions; the Catalog of Fishes notes that Yunnanilus qujinensis is now regarded by some authors as a junior synonym of Eonemachilus caohaiensis (Ding, 1992), following a taxonomic revision published in 2025. That question is not yet fully settled in the literature, and this entry keeps the original combination Yunnanilus qujinensis, the name under which the species was described and is still catalogued.
Morphology
Like other members of the nigromaculatus group, Yunnanilus qujinensis is a small, slender nemacheilid loach with a body fully covered in scales — a useful diagnostic feature, since many related stone loaches are naked or only partly scaled. The original description lists a processus dentiformis (a small tooth-like process on the lower jaw), the absence of an outer gill raker on the first gill arch, a distinct notch in the lower jaw, and eight pharyngeal teeth as characters separating it from its close relatives.
Detailed measurements of maximum length have not been reported in the sources available for this profile; as a member of a genus of diminutive plateau-and-spring loaches, it is reasonable to expect a total length in the small centimetre range typical of Yunnanilus, but no confirmed figure can be cited here without guessing. As in other nemacheilids, it almost certainly carries the erectile suborbital spine and thin, easily-damaged skin typical of the family, though this has not been explicitly documented for this species.
Habitat
The only confirmed locality for Yunnanilus qujinensis is a spring near the Hujiafen Reservoir, in the Nanpanjiang River basin near Qujin City, Yunnan Province, southwestern China. Springs of this kind on the Yunnan-Guizhou plateau are typically cool, clear and well-oxygenated, fed by groundwater rather than surface runoff, which is the setting in which most of the genus's tightly-endemic, single-locality species are found.
No published water-chemistry data — temperature, pH or hardness — exist for this exact spring in the sources reviewed here. A related species in the same genus, Yunnanilus chuanheensis, has been recorded in clear, slow-flowing water with a sand-and-cobble bottom, shallow depth and forested banks, which gives a plausible picture of the kind of cool, clean stream habitat this species likely shares — but that is an inference from a relative, not a direct record for Yunnanilus qujinensis itself.
Feeding
No feeding studies exist for Yunnanilus qujinensis specifically. As a small benthic nemacheilid from a clear spring habitat, it almost certainly forages as a micro-predator, picking small invertebrates, insect larvae and other invertebrate prey from the substrate and from among stones and sediment, in the manner typical of stone and brook loaches generally.
In the complete absence of aquarium-trade or captive-feeding records for this species, any statement more specific than this would be speculation. The safest and most honest summary is that it is presumed to be an invertebrate-feeding bottom forager, consistent with its genus and habitat, pending any direct observation.
Mating
Nothing has been published on courtship, spawning behaviour or sexual dimorphism in Yunnanilus qujinensis. No aquarium or field observations of reproductive behaviour appear in the sources available for this profile.
Given that stone and brook loaches (Nemacheilidae) as a family typically pair or aggregate briefly over gravel or fine substrate before scattering eggs into the current, it is reasonable to expect a broadly similar pattern in this species, but this is an inference from the family rather than a documented fact about Yunnanilus qujinensis itself, and should be read as such.
Breeding
There is no record of Yunnanilus qujinensis being bred in captivity, and no description of its reproduction in the wild beyond what can be inferred from its family. Nemacheilid stone and brook loaches as a group are typically egg-scatterers, releasing adhesive or semi-adhesive eggs over gravel or among stones in flowing water, with no parental care of eggs or fry afterward.
As a species that has never entered the aquarium trade and is known from a single spring locality, it is essentially certain that it has never been spawned by hobbyists. Data on this species' reproduction is sparse to the point of nonexistence, and this entry does not go beyond what the genus-level pattern reasonably suggests.
In the aquarium
Yunnanilus qujinensis is not known to be kept in the aquarium hobby. No trade name, import record or care account for this species could be found in the sources reviewed here, which is unsurprising for a fish described from a single spring locality in inland Yunnan with no apparent commercial collection.
Were it ever to appear in captivity, the responsible starting point — based on its genus, its spring habitat and the general rules for small nemacheilid loaches — would be cool, clean, well-oxygenated, gently flowing water, a fine sand or gravel substrate, and small live or frozen invertebrate foods; like other scaled or near-scaleless loaches, caution with copper-based and full-strength medications would be prudent. But none of this is drawn from an actual husbandry record, and this species should be treated as effectively unavailable and unknown in the hobby rather than as a viable aquarium fish.
Conservation
Yunnanilus qujinensis has not been formally assessed on the IUCN Red List and is recorded here as Not Evaluated. Known from a single spring near the Hujiafen Reservoir in the Nanpanjiang basin of Yunnan, it is exactly the kind of narrow-range, single-locality endemic that is vulnerable to any disturbance of its one known water source — reservoir management, groundwater extraction, pollution or habitat modification — even though no population trend data exist to document an actual decline.
Adding to the uncertainty, recent taxonomic work treats this name as a possible junior synonym of Eonemachilus caohaiensis, meaning the species' conservation status may in future be assessed, if at all, under that name rather than as Yunnanilus qujinensis. There is no evidence of any aquarium-trade collection pressure on this species; its principal risks, to the extent they can be inferred, are habitat-based and local to its single known spring.