Taxonomy & naming
Yunnanilus macrositanus was described by the ichthyologist Li in 1999. It sits in the family Nemacheilidae — with more than 47 genera and over 750 species, the largest of the loach families and the one that holds the bulk of the small, unassuming stone and brook loaches of Eurasia's cool, clear streams. The genus Yunnanilus is itself a large radiation of slender micro-loaches concentrated in and named after Yunnan Province in southern China, a region of isolated karst basins and spring systems that has produced dozens of narrow-range endemics in this genus alone.
As with much of Cobitoidea, genus boundaries within Nemacheilidae have been repeatedly revised since the old catch-all genera Nemacheilus and Noemacheilus were broken apart; Yunnanilus itself is one of the products of that splitting. The Catalog of Fishes (Eschmeyer, CAS) is the standing authority for the valid combination, and it is retained here as Yunnanilus macrositanus.
The spelling Yunnanilus macroistainus has also been used for this fish — it is the form that appears on FishBase — but the two names are not a taxonomic disagreement, only a spelling one: Li's original 1999 publication used both spellings inconsistently, and under the ICZN's first-reviser provision, Maurice Kottelat's subsequent treatment of the genus fixed macrositanus as the correct spelling, which the Catalog of Fishes now follows and treats macroistainus as a synonym of.
Morphology
FishBase records an elongated body typical of the genus, with 10 dorsal soft rays and 7 anal soft rays. Like other nemacheilids it is a small, slender, benthic fish built for hugging the substrate in flowing water rather than for open swimming — a body plan shared across the stone and brook loaches, most of which never exceed a few centimetres in length.
Detailed morphometric data — precise adult size, fin-ray colouration, and barbel counts — are not available from the sources consulted here. As with many single-locality Yunnanilus species, much of the fine anatomical detail simply has not been published in an accessible form, and it would be dishonest to fill that gap with numbers borrowed from better-known congeners.
Habitat
The species is known from Lunan County, Yunnan Province, China, with its type locality at Heilongtan — a name that translates roughly to "black dragon pool," and describes the kind of spring-fed karst pool and stream system characteristic of Yunnan's limestone country. FishBase classifies it as a demersal, subtropical freshwater fish, consistent with life on or near the bottom of a clear, cool-to-mild spring-influenced stream rather than a warm, still lowland habitat.
Related Yunnanilus species are generally reported from shallow, slow-to-moderate flowing water with dense vegetation and soft sandy or silty substrate, and it is reasonable to expect a broadly similar setting here, though this has not been confirmed for macrositanus specifically. Precise temperature, pH and flow data for this species were not found in the sources consulted; given its karst-spring type locality, a cool-to-mild subtropical stream regime — not warm, static, blackwater conditions — is the safer inference.
Feeding
Specific dietary data for Yunnanilus macrositanus were not located in the sources consulted. Stone and brook loaches as a group, including other Yunnanilus species, are bottom-foraging micro-predators, picking small invertebrates, insect larvae and other tiny prey items from sediment, plant matter and stream substrate rather than grazing algae or taking large prey.
In the near-total absence of hobby husbandry for this species, feeding guidance can only be extrapolated from its relatives and genus-mates: a diet of small live or frozen foods and sinking preparations suited to a small benthic invertebrate-picker, rather than flake aimed at mid-water fish.
Mating
No mating behaviour, seasonality, or sexual dimorphism has been documented for this species in the sources consulted, and the sources found no published account of courtship or spawning triggers for it specifically. Related Yunnanilus species are reported as gregarious, benthic egg-scatterers, but nothing has been recorded for macrositanus itself.
Given the species' extremely restricted, single-locality range and its total absence from the aquarium trade, this is unsurprising — the field data needed to describe its reproductive biology in detail simply may not exist yet, or is not accessible in the literature surveyed here.
Breeding
There is no record of Yunnanilus macrositanus being bred, or even kept, in home aquaria. Consistent with stone and brook loaches generally, the expected mode — inferred from the genus and family rather than confirmed for this species — is an egg-scatterer that releases eggs over gravel or among stones in flowing water, with no parental care afterward.
Data on this species' reproductive biology is sparse to the point of being effectively absent from the literature surveyed. Nemacheilid stone loaches of this kind are rarely bred in captivity even when they are available to hobbyists, and this species is not known to be available at all, so there is no aquarium breeding record to report.
In the aquarium
Yunnanilus macrositanus does not appear to be established in the aquarium hobby; no care profile, husbandry record, or trade history was found in the sources consulted. It should not be assumed to be available, and no specific tank parameters can be responsibly given for a fish this poorly documented.
What can be said generally, drawn from the wider Nemacheilidae and from stone/brook loaches as a group, is that these are small, scaleless-skinned, bottom-dwelling fish of cool, clean, well-oxygenated flowing water — sensitive to poor water quality and, like other loaches, likely to be sensitive to copper and full-strength medications given their near-scaleless skin. Any future keeper would need to replicate a cool, clear, current-driven stream environment rather than a still, warm community tank, and should treat any claim of "macrositanus" in the trade with real skepticism given how narrow and undocumented its known range is.
Conservation
Yunnanilus macrositanus is assessed as Critically Endangered (CR) as of January 17, 2024, according to FishBase's attribution of the IUCN status. This is among the most severe threat categories the IUCN assigns, consistent with a species restricted to a single known locality — the Heilongtan spring system in Lunan County, Yunnan — where any pollution, groundwater extraction, habitat modification or water-diversion event could plausibly affect the entire known population.
A specific IUCN Red List assessment page with a numeric species ID was not located in the sources consulted, so no detailed assessment narrative (population trend, precise threats, area of occupancy) can be reported here beyond the CR status recorded by FishBase. This species is not part of the aquarium trade, so there is no ornamental collection pressure to report; its conservation risk, to the extent it is documented, appears to be a function of its extremely narrow karst-spring range in a region under broader development and water-use pressure.