Taxonomy & naming
Yunnanilus macrogaster was described by Maurice Kottelat and Xin-Luo Chu in 1988, placed from the outset in Yunnanilus, a genus of small nemacheilid loaches named for Yunnan province in southern China, where the great majority of its species are found. It belongs to the family Nemacheilidae, the stone and brook loaches — the largest family in the loach superfamily Cobitoidea, encompassing hundreds of small, mostly stream-dwelling species across Eurasia.
The genus Yunnanilus has been reshuffled repeatedly as revisions have split and re-split the old, broadly defined Nemacheilus and Noemacheilus into narrower genera; the Catalog of Fishes (Eschmeyer, CAS) is the authority for the currently valid placement, and it retains Y. macrogaster in Yunnanilus. The species is notable taxonomically for being one of three closely related Yunnanilus species confined to the same small marsh system — a pattern of local diversification that has drawn comparison to other isolated-basin species flocks.
Morphology
This is a small loach, reported to reach a maximum of about 3 in standard length, with the elongate, subcylindrical body typical of Nemacheilidae. Colouration is understated: FishBase describes a pattern of a few large, eye-sized dark spots along the flanks rather than the fine barring or mottling seen in many stream-dwelling relatives, a simplified pattern plausibly linked to life in a still, vegetated marsh rather than a fast, clear-bottomed stream. The lateral line is short and incomplete, carrying only 6 to 10 pores, and the fin ray counts are modest — 11 to 12 branched dorsal rays and 8 in the anal fin.
Like other nemacheilids, Y. macrogaster carries an erectile suborbital spine in a groove beneath the eye and has fine, deeply embedded scales that leave the skin close to naked — a combination that matters for handling and, especially, for medication sensitivity in captivity. FishBase describes the species as demersal and notes it as "hypogean," a term applied to species associated with subterranean or spring-fed waters, consistent with its marsh habitat's groundwater-linked hydrology.
Habitat
Yunnanilus macrogaster is endemic to the Datangzi Marsh system in Luoping County, Yunnan, China — an isolated, endorheic (closed) drainage basin with no connection to a larger river or the sea. The type locality is recorded as "Tatantze," an older transliteration of the same marsh. Within this small wetland the species lives among abundant aquatic vegetation, which both FishBase and the Encyclopedia of Life note as a defining feature of its habitat.
Because the marsh is spring-fed and closed, conditions there are likely stable and moderate rather than subject to the strong seasonal flood pulses that drive many other Southeast and East Asian loaches; specific temperature and chemistry data have not been published for the site. This is a warm-temperate to subtropical Yunnan wetland rather than either a cold montane stream or a fast hillstream torrent, and any aquarium approximation should aim for calm, well-planted, moderately warm water rather than strong current.
Feeding
In the wild, Y. macrogaster feeds on worms and insects picked from the marsh bottom and the tangle of aquatic vegetation it inhabits — the diet of a small, opportunistic benthic micro-predator rather than a grazer or scavenger. No data on feeding behaviour in captivity exists, since the species is not known to be kept.
A fish with this feeding ecology would be expected to take small live and frozen foods readily — bloodworm, small worms, daphnia and similar invertebrate fare — while likely ignoring dry flake in favour of anything that moves or sinks and can be rooted out of cover, in line with the general pattern for small insectivorous nemacheilids.
Mating
No sexual dimorphism has been documented for this species in the sources available, and nothing specific has been published on courtship or spawning behaviour. Data on Yunnanilus macrogaster overall is sparse — unsurprising for a fish confined to a single small, remote marsh system that has drawn little targeted study beyond its original description and basic taxonomic and distributional record-keeping.
What is known comes by inference from its genus and family: nemacheilid loaches as a group are egg-scatterers, releasing eggs into the water rather than depositing them in a nest or guarding a spawning site, and there is no indication that Y. macrogaster departs from this pattern.
Breeding
Sources describe Y. macrogaster as an egg-scatterer with no parental care, consistent with the wider pattern for stone and brook loaches: ripe fish release eggs, typically among vegetation or over the substrate, and the adults take no further part in rearing. The species has not been reported in the aquarium trade, and there is no record of it having been bred, or even kept, in captivity.
This is honestly about as much as can be said. As with many single-locality nemacheilid endemics, the breeding biology of Y. macrogaster has simply not been studied in detail; the egg-scattering, no-parental-care mode is the safe, genus-consistent description rather than a directly observed account for this species.
In the aquarium
Yunnanilus macrogaster is not a fish anyone should expect to find or keep — there is no record of it in the aquarium trade, and its extremely restricted range and marsh-specialist ecology make wild collection for the hobby both unlikely and undesirable. It is included here for completeness, as a documented member of the stone and brook loach family, not as a care recommendation.
If it were ever to be kept, its small adult size (under 3 in) and marsh origin suggest it would suit a modestly sized, densely planted, calm-water aquarium rather than the high-flow, high-oxygen hillstream setup appropriate for many of its faster-water relatives. As with all nemacheilids, its near-scaleless skin means any medication should be dosed cautiously — coppers and full-strength treatments are best avoided or used at reduced dose. These notes are inferential, drawn from its habitat and family, not from any documented aquarium history.
Conservation
Yunnanilus macrogaster is assessed by the IUCN Red List as Data Deficient, an assessment dating to 2007. This reflects the genuine state of knowledge: the species is known only from a single small, isolated marsh system, and there appears to be no recent population monitoring or targeted conservation assessment beyond that original listing.
A species restricted to one endorheic marsh is inherently vulnerable to habitat change — drainage, pollution, groundwater alteration or agricultural conversion of the wetland could affect the entire known population, and its two close relatives sharing the same marsh (Y. niger and Y. paludosus) would face the same exposure. There is no evidence of aquarium-trade collection pressure on this species; its main conservation relevance is as an example of the many narrow-range Nemacheilidae endemics across China and the wider Cobitoidea range whose status remains genuinely unresolved.