Taxonomy & naming
The species was described by Ding in 1992 from Lake Caohai ("Caohai" giving the species its epithet), a highland lake in Weining County, Guizhou Province, China, based on a holotype held at the Museum of Southwest University (MSINR 900077), an adult male of 3 in standard length. It was placed in Yunnanilus, a genus of small nemacheilid loaches named for Yunnan, the neighbouring province where many of its relatives were first found.
Taxonomy in this corner of Cobitoidea has moved since 1992: recent work (Du et al., 2021) reassigns the species to the genus Eonemachilus as Eonemachilus caohaiensis. Older and some current references, including regional survey literature, continue to use the original combination Yunnanilus caohaiensis, and this profile keeps that name pending a fuller resolution of the genus. Either way it remains within Nemacheilidae, the stone-and-brook-loach family, and the species has not been evaluated by the IUCN Red List.
Morphology
Like other small nemacheilids, Yunnanilus caohaiensis is a slender, low-slung loach built for holding position on the bottom of clear, flowing water rather than for open-water swimming. The holotype measured 3 in standard length, giving a working sense of adult size for the species, though published data on colour pattern, fin counts and other diagnostic detail is sparse and largely confined to the original 1992 description.
As in the wider family, an erectile suborbital spine below the eye and a near-scaleless or finely-scaled body are typical Nemacheilidae traits; no published account contradicts this for the species specifically, but detailed morphological description beyond the type material has not been located. This is one of the many stone loaches for which the honest answer is that far more is known about the family in general than about this species in particular.
Habitat
The species is endemic to Guizhou Province in southern China, first known from Lake Caohai and its associated waters, and documented from the wider Jinsha and Nanpanjiang river systems in the region. It was long considered locally extirpated from at least part of its range before being rediscovered in 2022–2023 in the Yangwanqiao River, a tributary of the Luoze River within the Yangtze drainage — a reminder of how easily a narrow-range stream loach can disappear from view for years without formal reassessment.
Guizhou's karst plateau streams and lakes typical of this region run cool, clear and well-oxygenated, and the family's broader habitat preference — clean, flowing, oxygen-rich water over stone, gravel or sand — almost certainly applies here, though site-specific data on temperature, flow and substrate for Yunnanilus caohaiensis itself has not been published in detail.
Feeding
No diet study specific to this species has been located. Nemacheilid stone and brook loaches as a family are bottom-foraging micro-predators, working over gravel and stones for small aquatic invertebrates — insect larvae, small crustaceans and similar prey — and it is reasonable to expect Yunnanilus caohaiensis follows the same pattern, but this should be read as family-level inference rather than a documented fact for the species.
In the unlikely event this fish were kept, the safe approach for any small nemacheilid is to offer small sinking live or frozen foods (bloodworm, small crustaceans) alongside sinking pellets suited to a small-mouthed bottom feeder, since flake aimed at mid-water fish is easily missed.
Mating
Nothing specific has been published on courtship or spawning behaviour in Yunnanilus caohaiensis. Sexual dimorphism has not been documented for the species, and with the fish essentially unknown outside a handful of survey and taxonomic records, there is no basis to describe pairing behaviour, spawning triggers or seasonal timing beyond what is typical of the wider stone-and-brook-loach family — egg-scattering with no courtship ritual reported in any detail.
Breeding
As with the great majority of Nemacheilidae, Yunnanilus caohaiensis is not known to have been bred in captivity. Stone and brook loaches typically scatter eggs over gravel or among stones in flowing water and provide no parental care, and there is no reason to expect this species departs from that pattern, but no direct spawning observation — wild or captive — has been located for it specifically. It is not present in the aquarium trade, so there is no hobbyist breeding record to draw on either.
In the aquarium
Yunnanilus caohaiensis is not an aquarium fish. It is a narrow-range Chinese endemic recently rediscovered after a period of presumed local loss, known chiefly from taxonomic and survey literature rather than from the trade, and there is no reliable record of it being kept or sold. This profile exists to document what is known of the species, not to recommend or describe husbandry that has never been demonstrated.
Were it ever to appear via specialist or scientific channels, the sensible default for a small nemacheilid from cool, clean, flowing highland water would be a well-oxygenated, unheated or lightly-heated tank with a hard or sand-and-gravel substrate and gentle-to-moderate current — but this is inference from the family, not documented care for the species.
Conservation
Yunnanilus caohaiensis has not been formally assessed by the IUCN Red List and is recorded here as Not Evaluated. What is documented is instructive: the species was considered locally extirpated from part of its historical range before being rediscovered in the Yangwanqiao River in 2022–2023, a pattern consistent with a narrow-range stream endemic under pressure from habitat change in a heavily developed karst landscape.
With its known range confined to a small part of Guizhou Province and its population status still poorly resolved even after the recent rediscovery, this is a species where the responsible position is to flag data scarcity rather than assign a conservation status the evidence does not support.