Taxonomy & naming
Diapoma pyrrhopteryx was described by Naércio A. Menezes and Stanley H. Weitzman in their 2011 systematic review of the genus Diapoma, published in Papéis Avulsos de Zoologia 51(5):59–82 alongside a second new species and an identification key to all known Diapoma. The valid name, as registered in Eschmeyer's Catalog of Fishes (Catalog of Fishes, California Academy of Sciences), is Diapoma pyrrhopteryx Menezes & Weitzman, 2011. The type locality is the Rio do Peixe, a tributary of the Rio Uruguay, in the municipality of Concórdia, Santa Catarina state, Brazil (27°28'S, 51°53'W).
Diapoma belongs to the subfamily Stevardiinae (formerly treated as the family Stevardiidae by some authors) within the speciose family Characidae. The tribe Diapomini is one of the smaller stevardiine lineages, distinguished by a suite of internal reproductive specialisations; iNaturalist records the species under the informal subfamily Diapominae, reflecting the alternative higher classification used by some taxonomists. Within Diapoma, pyrrhopteryx differs from its nearest congener D. speculiferum most obviously by the vivid red pigmentation of the unpaired fins in life and by a proportionally longer snout (21.3–24.5% of head length versus shorter values in D. speculiferum).
Morphology
Diapoma pyrrhopteryx is a small, moderately slender characid reaching a maximum recorded standard length of 2 in in males and unsexed specimens. The body is short and relatively deep for the genus. The dorsal fin carries 10 soft rays. Dentition includes pentacuspid teeth on the maxillary bone, a character noted in the original description as useful for distinguishing Diapoma from some allied genera.
The most striking feature of live males is coloration: the unpaired fins — dorsal, anal, and caudal — are red or orange-red, the character that inspired the species name. This fin pigmentation fades rapidly in preserved material, so much of what is recorded in museum specimens reflects body form and meristics rather than colour. The longer snout relative to congeners is a useful morphological marker in both fresh and preserved fish.
Habitat
Diapoma pyrrhopteryx is endemic to the upper Rio Uruguay basin in the states of Rio Grande do Sul and Santa Catarina, southern Brazil. The IUCN assessment records six sub-basins within this range: the Pelotas, Canoas, Passo Fundo, Rio do Peixe, Jacutinga, and Dourados drainages. The total extent of occurrence is estimated at only 1,520 mi², a small footprint for any freshwater fish.
The species occurs exclusively in river channels (calha dos rios), within inland freshwater systems classified by FishBase as benthopelagic and subtropical. The Rio Canoas basin is specifically noted as part of the distribution, lying within the broader Rio Uruguay headwater system. Water conditions in the upper Uruguay tributaries of this region are typically cool to moderate subtropical temperatures, reflecting the higher elevations of the Serra Geral highlands in southern Brazil.
Feeding
No detailed dietary studies of Diapoma pyrrhopteryx have been published in the available literature. Members of the genus Diapoma, like stevardiine characids generally, are presumed to be opportunistic micro-predators and omnivores, taking small invertebrates, microcrustaceans, and plant material from the water column and benthos of river channels.
The species is benthopelagic in habit according to FishBase, suggesting it forages across both mid-water and bottom substrate zones of its river-channel habitat. Given how little material exists for this fish — it is absent from iNaturalist and rarely collected — dietary data remain sparse, and no aquarium feeding observations have been recorded in the accessible literature.
Mating
Mating behaviour in Diapoma pyrrhopteryx has not been documented in the wild or in captivity. As a stevardiine characid the species belongs to a lineage defined in part by internal reproductive specialisations; stevardiines generally transfer sperm internally, and females may store sperm before fertilising eggs, a departure from the simple egg-scattering mode typical of many other characids.
Males of the species are distinguished by their red fin coloration, strongly suggesting that fin pigmentation functions in sexual display and mate choice, as it does in related red-finned characids. Beyond that inference, mating specifics — courtship sequences, pairing systems, timing — are unknown, and the rarity of the species means that aquarium observation data that might illuminate these questions do not exist in the available record.
Breeding
Reproductive biology details for Diapoma pyrrhopteryx are undescribed in the literature available from the cache. Stevardiine characids broadly show a range of reproductive modes from near-conventional egg scattering to more derived internal insemination; where internal fertilisation occurs, eggs may be adhesive and deposited among fine-leaved aquatic vegetation. No clutch size, fecundity, incubation period, or larval data have been recorded for this species in the accessible literature.
The species has never been established in the aquarium hobby, so captive-breeding observations are absent. Given its Endangered status, severely fragmented wild populations, and total absence from the ornamental trade, all information on its breeding mode remains inferred from generic stevardiine biology rather than direct study of pyrrhopteryx.
In the aquarium
Diapoma pyrrhopteryx is not kept in the aquarium hobby and has not been imported into the ornamental trade. The combination of its remote southern-Brazilian headwater range, Endangered status, extremely restricted distribution, and virtual absence from scientific collections means that living specimens have never been available to hobbyists. There are no aquarium husbandry records, no observations of behaviour in captivity, and no captive-bred stock in circulation.
Were the species ever to become available through responsible captive-breeding programmes, its small adult size (under 2.5 in SL), subtropical thermal preferences reflecting the cool highland tributaries of the upper Rio Uruguay, and presumed insectivore-omnivore diet would form the starting point for husbandry trials. For now it remains a species known almost entirely from museum specimens and field surveys, and its aquarium potential is entirely theoretical.
Conservation
Diapoma pyrrhopteryx is assessed as Endangered (EN) on the IUCN Red List under criterion B1ab(iii), reflecting an extent of occurrence of only 1,520 mi² with a severely fragmented population and continuing decline in habitat quality. The assessment, published in 2021 based on a 2018 evaluation by Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio), documents the species' restriction to six sub-basins in the upper Rio Uruguay — Pelotas, Canoas, Passo Fundo, Rio do Peixe, Jacutinga, and Dourados — with no records from any protected area.
Hydroelectric dams are the dominant threat. A series of large reservoirs — UHE Monjolinho, Passo Fundo, Itá, Machadinho, Barra Grande, Campos Novos, and the planned Pai Querê — have progressively inundated, fragmented, and degraded river-channel habitat across the basin. The species is no longer found in the Rio Passo Fundo after the filling of the UHE Monjolinho reservoir, demonstrating that reservoir creation causes local extirpation rather than mere displacement. No population size estimates exist, current trend is unknown, and no individuals are held ex situ. The species' entire known range falls outside protected areas, leaving hydroelectric development as an essentially unmitigated pressure on every remaining sub-population.