Taxonomy & naming
Yunnanilus altus was described by Maurice Kottelat and Xin-Luo Chu in 1988 as part of a broader revision of the genus Yunnanilus, in which the authors split out several new species from Yunnan's stream systems. It was placed in the Y. nigromaculatus species group, one of several informal groupings Kottelat and Chu used to organise the genus's many similar-looking small loaches. The genus Yunnanilus itself sits in the family Nemacheilidae, the stone or brook loaches — the largest family within the loach superfamily Cobitoidea, and one that has been repeatedly reshuffled as older catch-all genera such as Nemacheilus and Noemacheilus were broken apart using better anatomical and molecular characters.
The species epithet altus is Latin for "high" or "deep," a reference to the fish's comparatively deep body — a useful field character among a genus whose many members otherwise look very alike. No taxonomic changes to this combination are documented in the available sources; it remains Yunnanilus altus Kottelat & Chu, 1988.
Morphology
Yunnanilus altus is a small nemacheilid, reported to reach about 3 in in standard length, with the deep, laterally compressed body that gives it its name relative to its more elongate congeners. Like other stone and brook loaches it is a slender-headed, benthic fish with barbels around the mouth used to probe the substrate for food, and — as is typical of the family — a reduced, deeply embedded scale covering that leaves the skin close to naked. This near-scaleless condition is a recurring theme across Nemacheilidae and matters directly for how sensitive these fish are to medication and to stress from poor water quality.
As in many small stream-dwelling nemacheilids, Nemacheilidae as a family carries an erectile suborbital spine below the eye in numerous genera; the available sources do not specifically confirm or describe this structure in Y. altus, so it is noted here as a family trait rather than a verified feature of this species. Detailed morphometric or coloration data beyond body depth and maximum size were not found in the cache and are not asserted here.
Habitat
Yunnanilus altus is known from the stream Xiangshuiho at Haijiashao in Yunnan Province, southwestern China — its type locality and, on current published information, effectively its only documented site. This places it among the many Yunnanilus species that are narrow-range endemics of Yunnan's hill and plateau stream systems, part of subtropical Asia's freshwater fauna. As a demersal stream fish it lives on or near the bottom, moving slowly in shoals over the streambed.
Stone and brook loaches of this family typically occupy fast-flowing, cool to mild, clear, well-oxygenated mountain and hill streams with gravel, stone or sand substrates — very different water from the warm, still, tannin-stained conditions associated with many lowland tropical fish. Nothing in the available sources suggests Y. altus departs from this pattern, and its Yunnan plateau setting is consistent with the cooler end of the genus's range.
Feeding
Yunnanilus altus is reported to feed on filamentous algae and insects, foraging over the stream bottom in the manner typical of small benthic loaches. This omnivorous-leaning-invertivorous diet — grazing algae and biofilm while also taking small invertebrate prey — matches the general pattern for stone and brook loaches, which use their barbels to detect food items in gravel and among stones in fast water.
In a hypothetical aquarium setting this diet profile would translate to a fish that benefits from a mix of sinking foods and access to natural algae and biofilm growth on hard surfaces, rather than relying on flake alone, though no aquarium feeding records for this specific species were found in the cache.
Mating
No courtship, spawning trigger, or sexual dimorphism information for Yunnanilus altus was found in the available sources. Data on reproduction in this species appears to be genuinely sparse, consistent with its status as a narrow-range stream endemic that has drawn little research or hobby attention since its 1988 description.
Breeding
Specific breeding data for Yunnanilus altus is not available in the sources consulted. Based on the documented pattern for stone and brook loaches (Nemacheilidae) generally, these fish are egg-scatterers that spawn over gravel in fast-flowing water with no parental care, often timed to late spring or early summer; they are rarely bred in captivity and many species, including narrow-endemic Yunnanilus, are simply not represented in the aquarium breeding record at all. Nothing here should be read as a documented account of this species' own reproduction — it is an honest extrapolation from the family, offered because no site-specific breeding record exists.
In the aquarium
Yunnanilus altus has no established presence in the aquarium trade, and the cache contains no husbandry record for it specifically. Related Yunnanilus species kept in the hobby — such as Y. brevis and Y. cruciatus — are reported to do well in cool-to-mild flowing water in roughly the 72–82 °F range, and the genus overall favours cool tropical to subtropical stream conditions rather than warm, still water. If ever kept, a small stone loach of this type would be expected to want a well-oxygenated tank with moderate to strong flow, a gravel or fine-stone substrate, and the same medication caution that applies across the near-scaleless Nemacheilidae — treatments should be gentle and dosed carefully, since scaleless loaches absorb medicines readily and are often quick to show signs of stress or disease. This is inference from the family and genus, not a documented care record for Y. altus itself.
Conservation
Yunnanilus altus has not been evaluated on the IUCN Red List, so it carries no formal conservation category; the honest label is Not Evaluated rather than a guessed status. Known only from its type locality — the stream Xiangshuiho at Haijiashao in Yunnan — it fits the broader pattern among Yunnanilus and other Nemacheilidae of narrow-range stream endemics whose true conservation status is unknown for lack of assessment, not because they are necessarily secure. Any pressures on its habitat from land use, water management or pollution in the Yunnan stream systems it depends on are undocumented in the available sources and are not asserted here.