Taxonomy & naming
Lefua sayu was described by Albert William Herre and Shu-Yen Lin in 1936, hence the parentheses around the authority — a signal that the species has since been moved between genera at least once since its original description. It sits in the family Nemacheilidae, the stone and brook loaches, by far the largest of the loach families and a group whose genus-level classification has been repeatedly reshuffled as ichthyologists work through what was once a much broader, catch-all Nemacheilus/Noemacheilus.
A 2024 re-description of the holotype in Ichthyological Research revisited the species in detail, confirming diagnostic features such as dusky cross-bars on the dorsal area, a dusky bar beside the dorsal-fin base, and the arrangement of lateral-line and cephalic lateral-line pores. Notably, the authors treated its placement in Lefua as tentative, flagging that further study is needed to confirm the generic assignment. The Catalog of Fishes is the authority for the valid combination, and pending any future revision the species remains listed as Lefua sayu.
Morphology
The holotype — still the only known specimen — is a small, slender stone loach typical of the family: an elongate, only lightly compressed body, a relatively short nasal barbel with the anterior nostril set midway between the barbel's base and tip, and a completely inferior (underslung) mouth suited to picking food off the streambed. The body carries dusky cross-bars across the back and a dusky bar at the base of the dorsal fin, markings used to help distinguish it from other regional Lefua and Nemacheilidae.
Beyond these diagnostic points from the 2024 re-description, no maximum length or other size data have been published for this species. As with many Nemacheilidae, the skin is thin and the scales are small and deeply embedded, a trait shared broadly across the family that leaves these fish sensitive to harsh water conditions and medications even where it has not been documented species-by-species.
Habitat
The type locality is the upper Tsien Tang River (now more commonly romanised Qiantang River) in Zhejiang Province, eastern China — a subtropical Chinese river system, and FishBase records the species as a freshwater, demersal fish of subtropical Asia. Nemacheilidae as a family are largely bottom-dwelling stream fish of Eurasia, typically living over gravel, pebble or boulder substrate in flowing water, with the more current-loving species favouring cooler, well-oxygenated reaches.
No water-chemistry or temperature data specific to Lefua sayu have been published, and with only a single known specimen there is no information on how widely it ranges within the Tsien Tang system or whether it occurs elsewhere. This is a genuinely data-sparse species, and honesty about that gap is more useful than a guessed range.
Feeding
No dietary study of Lefua sayu itself exists, but FishBase lists a trophic level of 2.8, consistent with a small carnivore feeding on invertebrates rather than algae or detritus. This matches the general pattern for Nemacheilidae, which are bottom-hunting micro-predators taking small invertebrates, insect larvae and crustaceans off the streambed in fast, clear water.
In the absence of species-specific data, the safest inference is that Lefua sayu forages the same way as its relatives — picking small live prey from gravel and stones with its inferior mouth — but this is an inference from the family, not a documented observation of the species.
Mating
Nothing has been published on courtship or spawning behaviour in Lefua sayu; the species is known from a single preserved holotype, not from field or captive observation. Nemacheilidae as a family are not known for elaborate courtship or dimorphic breeding colours — most stone and brook loaches pair loosely and scatter eggs over gravel in flowing water rather than performing extended displays — but this cannot be confirmed for this particular species.
Breeding
There is no breeding record for Lefua sayu in the aquarium or otherwise; it has never been kept or bred in captivity as far as the literature shows. Based on the family pattern, the expected mode would be an egg-scatterer over gravel or among stones in fast water, with no parental care — but for this species that is an inference, not an observation, and it should be stated as such rather than presented as fact.
Across Nemacheilidae generally, aquarium breeding is rare and most species are simply never spawned in captivity; only isolated close relatives have documented captive spawnings. Lefua sayu, known from one wild-collected specimen, has no aquarium history at all.
In the aquarium
Lefua sayu is not and has never been part of the aquarium trade. There are no husbandry records, no verified photographs of live specimens, and no keeper reports to draw on — it exists in the hobby literature only as a taxonomic curiosity tied to its 2024 holotype re-description. Any care advice offered here would be invented rather than documented, so none is given.
For context, its relatives in Nemacheilidae that do reach the hobby are generally kept in cool-to-moderate, fast-flowing, well-oxygenated water over gravel, and — like most scaleless loaches — tend to be sensitive to copper and full-strength medication. Whether those generalities would hold for Lefua sayu specifically is unknown.
Conservation
Lefua sayu has not been assessed on the IUCN Red List and is recorded here as Not Evaluated rather than assigned a guessed status. With only a single known specimen, collected from one river system in Zhejiang, China, its population status, range and any threats it may face are effectively undocumented.
Stream-endemic Nemacheilidae in China face pressure from damming, pollution and habitat alteration across the region generally, but without further field surveys there is no basis for a species-specific conservation statement beyond noting that this is a poorly known, single-locality fish that would benefit from renewed field study.