Taxonomy & naming
Mystacoleucus greenwayi was described by the French ichthyologist Jacques Pellegrin and the Chinese ichthyologist Fang Ping-Wen in 1940. The Catalog of Fishes (Eschmeyer, CAS) records this as the valid combination and the authority pair is cited without parentheses, indicating the species has always been placed in Mystacoleucus since its original description.
The genus Mystacoleucus — which means roughly 'moustached white fish', from the Greek mystax (upper lip or moustache) and leukos (white, bright) — was established for a group of barbel-bearing cyprinids native to rivers of Myanmar, Thailand, Laos, and surrounding Indochinese drainages. Older literature may list some congeners under Barbus or Puntius, catch-all genera that once accommodated most of the world's barbs before systematic molecular and morphological revision. The specific epithet greenwayi honours a person named Greenway; the honoree is not identified in available sources.
Morphology
Mystacoleucus greenwayi is a moderately elongate, laterally compressed barb with the body plan typical of the genus: a streamlined profile adapted to flowing water, with two pairs of barbels (rostral and maxillary) that give the genus its name. The flanks are silvery and the dorsal surface is dusky, consistent with the countershaded pattern widespread among rheophilic cyprinids.
Detailed meristic data and colour descriptions from life are not well documented in accessible sources. Size at maturity and maximum total length have not been reliably recorded in the available hobby or scientific literature; the genus generally encompasses species in the 4–8 in size range, though M. greenwayi's precise maximum size is not confirmed here.
Habitat
Mystacoleucus greenwayi is native to mainland Southeast Asia, occurring in river drainages of the Indochinese region. The genus is centred on the major river systems draining from the mountains of Myanmar and southern China into Thailand, Laos, and adjacent countries — systems including the Mekong, Salween, and their tributaries.
Like other members of Mystacoleucus, the species is expected to inhabit clear to moderately turbid, well-oxygenated rivers and streams over sand, gravel, and cobble substrates. Water conditions in these Indochinese systems are typically warm (tropical lowlands) to seasonally cool (montane reaches), soft to moderately hard, and broadly neutral in pH, with notable seasonal fluctuation between dry and monsoon periods.
Feeding
No specific feeding studies have been published for Mystacoleucus greenwayi. By analogy with close relatives in the genus and the broader subfamily Cyprininae, it is presumed to be an opportunistic omnivore, taking small aquatic invertebrates — insect larvae, worms, crustaceans — along with algae, plant matter, and organic detritus from the river bottom and water column.
The two pairs of well-developed barbels suggest active substrate investigation, consistent with a benthic or benthopelagic foraging mode in which the fish roots through loose substrate and leaf litter for invertebrates while also taking drifting plant material. In the aquarium, members of this genus generally accept a broad diet of live, frozen, and dried foods.
Mating
Mystacoleucus greenwayi, in common with all members of family Cyprinidae, is an egg-scattering, non-guarding spawner. No published account of courtship behaviour in the wild or in captivity exists for this species, but the pattern characteristic of the family involves a driven male pursuing a gravid female through vegetation or over the substrate, with eggs and milt released in brief, close-contact bursts.
There is no nest construction, no pair bond beyond the spawning act, and no territory maintained. In large-river Indochinese cyprinids, seasonal flooding and water-temperature shifts associated with the monsoon cycle typically serve as spawning triggers, prompting movements into flooded margins or over gravel shallows where eggs are deposited and immediately abandoned.
Breeding
No aquarium breeding account exists for Mystacoleucus greenwayi in the hobby literature, and the species is essentially absent from the ornamental trade. Based on the reproductive pattern universal among barbs, spawning would involve adhesive eggs scattered over fine-leaved vegetation or onto a gravel substrate with no subsequent parental attention.
Were the species to be bred in captivity, a large dedicated tank with soft to moderately hard, slightly acidic to neutral water, a protective bottom layer to catch falling eggs away from the adults, and well-conditioned fish would represent the standard approach. Adults would need to be removed promptly after spawning to prevent egg predation. Larvae would require progressively larger live foods as they develop.
In the aquarium
Mystacoleucus greenwayi is not established in the aquarium hobby and is unlikely to be encountered in the mainstream ornamental trade. The genus as a whole receives minimal attention from hobbyists, with only a handful of species appearing sporadically through specialist importers of Southeast Asian riverine fishes.
Should specimens become available, the species would require a spacious, well-filtered aquarium with good water movement — reflecting its presumed origin in clear, flowing Indochinese rivers — kept in warm tropical conditions with soft to moderately hard, broadly neutral water. It should be maintained as a school to provide social cohesion. Robust, similarly sized companions from Indochinese river biotopes would be appropriate. Until more biological data emerge, specific husbandry recommendations remain speculative.
Conservation
Mystacoleucus greenwayi has not been assessed by the IUCN Red List, and its conservation status is Not Evaluated. This reflects the broader pattern for many small to medium Asian cyprinids: insufficient distributional and population data to support a formal assessment rather than confirmed abundance.
The major freshwater rivers of mainland Southeast Asia — the Mekong, Salween, and their tributaries — face escalating pressure from hydropower dam construction, sedimentation from upland agriculture and deforestation, overfishing, and water extraction. Species restricted to particular drainages or sub-basins within these systems can be vulnerable to local extinction before they are formally assessed. Baseline surveys of the ichthyofauna of these rivers remain an urgent conservation priority.