Taxonomy & naming
Microgenys weyrauchi was described by the American ichthyologist Henry Weed Fowler in 1945, in Notulae Naturae (Philadelphia) No. 159. The holotype is ANSP 71627, held at the Academy of Natural Sciences of Drexel University in Philadelphia. The type locality is Ninabamba, near Ayacucho, in the Río Ucayali basin of Peru, at an elevation of approximately 1,2953 ft.
Eschmeyer's Catalog of Fishes (the authority for valid names) places the species in the family Stevardiidae, subfamily Creagrutinae — a group of predominantly Andean and sub-Andean small characids. The species was at one point considered a possible junior synonym of Bryconacidnus ellisi, but was subsequently upheld as a valid, distinct species. The top-level family field used on this site is Characidae in the broad sense, consistent with the site-wide treatment of this group.
No subspecies or synonyms are currently recognised. The genus Microgenys is a small Andean genus within Creagrutinae, and weyrauchi is the species epithet honouring the German entomologist Wilhelm Weyrauch, who collected natural history material in Peru.
Morphology
Microgenys weyrauchi is a very small fish, with FishBase recording a maximum standard length of 1 in. This places it among the smallest characids recorded from the upper Ucayali basin. Detailed published morphometric data are sparse in the accessible literature, and the original description in Notulae Naturae (1945) provides the primary morphological account.
As a member of subfamily Creagrutinae, the species is presumed to share the general body plan of that group: a moderately streamlined, laterally compressed body suited to rheophilic or hillstream habitats, with the typical characid fin arrangement including an adipose fin. Coloration in life is not well documented in the hobbyist or secondary scientific literature; the species is essentially unknown in the aquarium trade.
Given the paucity of specimens and the elevation of the type locality, data on sexual dimorphism, meristic counts, and live coloration remain sparse. Until additional material is examined, morphological description relies primarily on the type series.
Habitat
The type locality, Ninabamba near Ayacucho, sits in the Río Ucayali basin at approximately 1,2953 ft elevation, placing Microgenys weyrauchi firmly in the Andean montane zone. At this altitude, streams are typically cool, fast-flowing, well-oxygenated, and often low in dissolved solids — conditions very different from the warm lowland Amazon waters that most aquarists associate with South American tetras.
FishBase characterises the species as benthopelagic and freshwater, with a distribution across the Amazon River basin encompassing both Bolivia and Peru. Montane Andean streams at this elevation typically run over rocky or gravelly substrates, with riparian vegetation shading the channel and organic leaf or wood debris accumulating in pools and backwaters. Water temperature at nearly 2,0 ft would be notably cooler than lowland Amazonian sites, likely in the range of 57–68 °F, though precise temperature records from the type locality are not available in the cached sources.
The species is rarely encountered in collections, which may reflect genuine scarcity, collector effort concentrated at lower elevations, or both. Its benthopelagic habit suggests it spends time near the substrate, possibly in quiet stretches or behind boulders in otherwise fast montane streams.
Feeding
FishBase assigns Microgenys weyrauchi a trophic level of 2.8 ± 0.1, a value consistent with an omnivorous to insectivorous feeding strategy. At this trophic level the species is presumed to feed on small invertebrates — aquatic insect larvae, microcrustaceans, and similar prey — supplemented by algae, biofilm, and other plant-derived material available in montane stream habitats.
No detailed stomach-content studies are available in the accessible literature for this species. The trophic estimate follows the standard FishBase methodology of averaging across related taxa and close relatives when direct data are lacking. In the cold, fast waters of Andean foothill streams, prey availability is typically dominated by benthic invertebrates drifting in the current, and the feeding ecology of small Creagrutinae generally reflects this.
Nothing is known of its feeding behaviour in captivity, as the species is not established in the aquarium hobby.
Mating
No observational data on the mating behaviour of Microgenys weyrauchi have been published or recorded in the sources available for this account. As a small characid in subfamily Creagrutinae, it is expected to follow the general egg-scattering reproductive strategy typical of the broader characiform order — open spawning with no pair bond, adhesive or semi-adhesive eggs deposited among fine-leaved plants, submerged vegetation, or substrate, and no parental care.
The montane stream environment at the type locality, with its seasonal variation in water temperature and flow, may impose seasonal reproductive cycles tied to temperature or photoperiod cues, but this has not been documented. Males and females may show subtle external dimorphism at reproductive maturity, as is common in small Andean characids, but no specific accounts exist for this species.
Given its complete absence from the hobby and the rarity of type-locality collections, mating behaviour remains unobserved and undescribed.
Breeding
Microgenys weyrauchi has not been bred in captivity and no egg or larval descriptions are available. Based on the reproductive biology of related small Creagrutinae, breeding is inferred to involve egg-scattering with adhesive eggs deposited over substrate or among vegetation, followed by no parental care — a pattern universal in the characiform fishes.
At the elevations inhabited by this species, cooler water temperatures would likely be a prerequisite for successful conditioning and spawning should captive husbandry ever be attempted. Incubation times and fry requirements are entirely undocumented.
The species' very small adult size (under 1 in SL) suggests a short generation time and relatively early reproductive maturity, consistent with other miniature Andean characids. Beyond these inferences from related taxa, nothing specific is known about the breeding biology of M. weyrauchi.
In the aquarium
Microgenys weyrauchi is not established in the aquarium hobby and is essentially unavailable in the trade. No care accounts exist in the hobbyist literature. Given its type locality at roughly 1,2953 ft elevation in the Andes, it would require conditions markedly different from most Amazonian tetras: cooler water, good oxygenation, and moderate to strong flow would be the logical starting parameters for any experimental husbandry.
Its extremely small adult size — under 1 in — would make it suitable only for a quiet, species-specific or biotope setup if it were ever to become available. However, the complete absence of trade records and the scarcity of wild collections mean that practical care guidance cannot be responsibly offered at this time. Any aquarist encountering the species would be working from first principles.
Data on this species remain sparse, and this profile will be updated as information becomes available.
Conservation
Microgenys weyrauchi is assessed as Data Deficient (DD) on the IUCN Red List, a status reflecting the scarcity of information about its population size, distribution, and the threats it faces rather than a determination that it is not at risk. The assessment date is recorded as 2014 in FishBase.
The species is known from a restricted area of the upper Ucayali basin in the Andean highlands of Peru and Bolivia. Montane Andean stream habitats face ongoing pressures from agricultural encroachment, deforestation of riparian zones, and the downstream effects of Andean land-use change, though specific threat assessments for this locality are not documented in available sources. The Stevardiidae and related small Andean characids are generally poorly represented in the IUCN database, contributing to the Data Deficient status across many members of the group.
Given the narrow altitudinal and geographic range implied by the type locality, and the general vulnerability of Andean stream specialists to climate-driven shifts in hydrology and temperature, reassessment with modern survey data would be valuable. Until then, the conservation status of M. weyrauchi must be regarded as genuinely uncertain.