Taxonomy & naming
Petruichthys brevis was described by the Belgian-British ichthyologist George Albert Boulenger in 1893. It belongs to Nemacheilidae, the stone and brook loaches — the largest family in the loach superfamily Cobitoidea, made up mostly of small, slender-bodied fishes of clear running water across Eurasia. The genus Yunnanilus, as its name suggests, is centred on Yunnan Province in China and neighbouring parts of mainland Southeast Asia, though this species sits at the western edge of that range in Myanmar.
As with much of the historic Nemacheilus/Noemacheilus complex, older literature and some hobby sources may still carry this fish under a broader or superseded generic placement; the Catalog of Fishes is the authority for the current combination, which keeps it in Yunnanilus. Data on this species remain comparatively sparse — much of what is documented traces back to lake-survey and aquarium-trade sources rather than a large taxonomic literature.
This fish was long listed as Yunnanilus brevis; Eschmeyer's Catalog of Fishes places the species in Petruichthys, and the current combination Petruichthys brevis is followed here.
Morphology
This is a small, slender nemacheilid, with females reaching about 2.5 in and males slightly smaller at around 2.1 in — modest even by stone-loach standards. Like other members of the family it is an elongate, sub-cylindrical fish built for a benthic and near-benthic life, with a ventrally flattened head and belly suited to working over cover rather than open sand.
Detailed morphological description in the available sources is limited; a suborbital spine and the near-scaleless skin typical of many nemacheilids should be assumed present as family-level traits, but species-specific detail on markings, barbel count and fin shape is not well documented in the sources at hand. Prospective keepers should treat close identification features as data-sparse rather than assume they match better-known Yunnanilus species.
Habitat
Petruichthys brevis is known from Inle Lake and the He-Ho plain in Myanmar's Shan Hills, with additional, less-confirmed reports from the broader Salween river basin. Unlike the fast, cool riffle habitat typical of many stone loaches, this species is demersal in still or slow-running, shallow lake water, favouring areas with dense submerged and floating vegetation rather than open riffle bottom.
Water at Inle Lake is medium-hard and alkaline, with reported values around pH 7.8–8.0 and hardness up to about 12 dH, and a preferred temperature in roughly the 72–79 °F range — mild, not cold, and well within the warm-temperate to subtropical band typical of Shan Plateau lake systems. This still-water, vegetated lake setting is an important departure from the fast, clear-stream stereotype for the family and should guide how the species is kept.
Feeding
As with most small nemacheilids, Petruichthys brevis is best treated as a micro-predator and opportunistic bottom-forager, working through vegetation and substrate for small invertebrates rather than sifting open sand for insect larvae the way many riffle-dwelling stone loaches do. Specific dietary studies for this species are not available in the sources reviewed.
In the aquarium it takes standard prepared foods along with frozen and live fare such as brine shrimp, daphnia and bloodworm. A varied diet that includes small live or frozen invertebrates is likely to suit it better than flake alone, consistent with its lake habitat of dense vegetation, where small crustaceans and insect larvae would be the expected natural prey.
Mating
No dimorphism or courtship behaviour is documented for this species in the available sources. Breeding has not been recorded in the aquarium, and even in the wild the reproductive biology is not well characterised beyond a suggestion that it may mass-spawn, a pattern reported for some lake-dwelling loach and cyprinid relatives that gather in numbers during a spawning period rather than pairing off.
Breeding
Breeding is not known to have been achieved in home aquaria for Petruichthys brevis. As a nemacheilid it should be expected to follow the family pattern of egg-scattering with no parental care, though the specific act has not been documented; in nature it is thought possible that this lake-dwelling species mass-spawns, based on general reports for the species rather than direct observation.
This is an honestly data-sparse picture: no aquarium spawning reports exist in the sources consulted, and the wild reproductive strategy is inferred rather than confirmed. Keepers should not expect to breed this species and should treat any claim of captive breeding with caution absent documentation.
In the aquarium
The Inle loach is unusual among stone loaches for being a free-swimming, shoaling fish rather than a solitary bottom-hugger, and it is best kept that way — in a group of five or six or more, where it will move through the water column and over vegetation rather than hiding under cover. This shoaling, plant-associated habit should shape the tank: dense planting, both submerged and floating, recreates the structure it uses at Inle Lake far better than an open-sand riffle setup.
Water should be kept mild and alkaline rather than soft and acidic — around pH 7.8–8.0, medium-hard water, and a moderate temperature of roughly 75–81 °F sits close to its native lake conditions. As with other nemacheilids, expect a scaleless or near-scaleless, medication-sensitive fish, so treat any disease outbreak cautiously and at reduced dosing. At barely 2.4 in this is a fish for a modestly sized, well-planted community rather than a large, high-flow biotope tank.
Conservation
The IUCN Red List assesses Petruichthys brevis as Vulnerable, a status corroborated across the sources reviewed for this profile. Its documented range is essentially a single lake system — Inle Lake and the adjoining He-Ho plain in Myanmar's Shan Hills — which is the kind of narrow, single-water endemism that makes small loaches vulnerable even without heavy direct exploitation.
Inle Lake has faced well-documented environmental pressures, including sedimentation, agricultural runoff, invasive species and shrinking open-water and floating-vegetation habitat, any of which could bear on a species tied so closely to its submerged and floating plant beds. The Salween basin records noted in some sources are less confirmed, so the lake population should be treated as the core of what is known about this species' status and threats.