Taxonomy & naming
Phenacogaster retropinnus was formally described by Carlos Alberto Santos de Lucena and Luiz Roberto Malabarba in 2010, as part of a major taxonomic revision of the genus Phenacogaster published in Zoologia 27(2):263–304. That paper described nine new species simultaneously and provided a comprehensive identification key for the genus, anchoring each species to a suite of meristic and morphometric characters.
The species belongs to the family Characidae, the vast assemblage of New World characiform fishes that encompasses tetras, piranhas, and their relatives. Within Characidae, Phenacogaster is a group of small, translucent tetras inhabiting rivers and flooded systems across tropical South America. The valid name follows the Catalog of Fishes (Eschmeyer), which places it in Phenacogaster as Lucena & Malabarba, 2010, with no subsequent reclassification on record.
Morphology
Phenacogaster retropinnus reaches a maximum standard length of approximately 2 in. The body is relatively slender and low in profile — body depth in specimens larger than 1 in ranges from about 26.5 to 32.8% of standard length — giving the fish a streamlined, almost fusiform outline typical of open-water tetras in this genus. The scales are thin enough to render the flanks semi-transparent in life, contributing to the glassy appearance common across Phenacogaster.
The dorsal fin carries ii,9 rays, a count within the range typical for the genus. The most diagnostic feature, and the trait that gives the species its name, is the notably posterior origin of the anal fin relative to the insertion of the dorsal fin; the anal fin bears 28–34 branched rays. The humeral spot — a dark blotch behind the opercle — is rounded to slightly vertically elongated. A triangular dark mark on the caudal peduncle is also noted in the original description. Males bear small hooks on the rays of the anal, pelvic, and pectoral fins, a reproductive adaptation widespread in the family Characidae.
Habitat
Phenacogaster retropinnus is documented from the Rio Negro, Rio Madeira, and Rio Xingu river systems within the greater Amazon and Tapajós basins of Brazil. These are large, ecologically diverse drainages that span blackwater, clearwater, and whitewater river types; the Rio Negro is the world's largest blackwater river, while the Madeira and Xingu carry clearwater and whitewater tributaries.
FishBase classifies the species as a freshwater pelagic fish, suggesting it occupies open water rather than strictly benthic or heavily vegetated microhabitats. Across this range it encounters a broad spectrum of ambient water conditions depending on the specific tributary, from the highly acidic, ion-poor waters characteristic of blackwater Rio Negro tributaries to the more neutral, sediment-rich reaches of Madeira and Xingu margins. Precise habitat data at the microhabitat level remain sparse in the literature.
Feeding
No specific dietary study has been published for Phenacogaster retropinnus. Members of the genus Phenacogaster are small characids that, based on body form and guild membership, are presumed to feed on small aquatic and terrestrial invertebrates, microcrustaceans, and organic particles in the water column — the typical insectivore-omnivore niche of diminutive open-water tetras in Amazonian systems.
In an aquarium, such species generally accept a range of prepared foods including fine-grade flake, micro-pellets, and small live or frozen invertebrates such as Artemia nauplii, Daphnia, and micro-worms. A varied diet that includes live or frozen components supports conditioning and long-term health in small characids of this type.
Mating
No detailed spawning observations have been published for Phenacogaster retropinnus specifically. The presence of fin hooks on males — documented in the original description on the anal, pelvic, and pectoral fins — is a widespread characid trait associated with spawning behaviour; in many species, these hooks are thought to facilitate brief physical contact or clasping during the spawning act, helping the male maintain proximity to the female at the moment of egg and sperm release.
Based on the biology of small, open-water characids as a group, spawning is expected to follow the typical egg-scattering pattern: non-adhesive or slightly adhesive eggs broadcast among fine-leaved aquatic plants or over the substrate, with no parental care from either sex. Whether P. retropinnus shows any seasonal reproductive cycle tied to the flood pulse of Amazonian rivers — as many Neotropical characids do — has not been studied.
Breeding
Phenacogaster retropinnus has not been reported as bred in captivity in the accessible hobbyist or scientific literature, and the species is not known in the aquarium trade. By analogy with other small Amazonian egg-scattering characids, spawning would likely require soft, slightly acidic water in the lower to mid-20s Celsius to replicate the blackwater or clearwater conditions of its native range, with fine-leaved plants or synthetic mops as spawning substrate.
The eggs of egg-scattering characids are typically small and semi-adhesive, hatching within one to three days at tropical temperatures. Adults of such species offer no parental care and will consume their own eggs, so any breeding attempt would require the adults to be separated from eggs promptly. Fry would need infusoria or commercial liquid fry foods as first foods before graduating to newly hatched brine shrimp nauplii. These details remain speculative in the absence of confirmed captive breeding records.
In the aquarium
Phenacogaster retropinnus is essentially unknown in the aquarium hobby. The species was described scientifically only in 2010 and is not established in trade, so husbandry guidance based on direct experience is unavailable. What follows is therefore inferential, based on the ecotype and biology of the genus.
Small Phenacogaster tetras are delicate, open-water schooling fish best suited to a peaceful, species-appropriate community. A group of at least six individuals in a well-filtered, well-planted aquarium with subdued lighting and dark substrate would provide appropriate conditions; blackwater parameters — soft, acidic water in the range of pH 5.5–7.0 and temperatures around 75–82 °F — would reflect the conditions of much of its natural range. Tankmates should be similarly small and undemanding. Because the species is collected from large, relatively pristine river systems and is not commercially cultured, any specimens that appear in trade would almost certainly be wild-caught; demand for such fish is low, and wild collection for ornamental purposes is not recorded.
Conservation
Phenacogaster retropinnus has not been assessed by the IUCN Red List and carries a status of Not Evaluated (NE). No targeted conservation concern has been raised in the literature for this species specifically. Its distribution across three major Amazonian river systems — the Negro, Madeira, and Xingu — implies a reasonably broad natural range, though the actual population size and density are unknown.
The Rio Xingu drainage, part of the species' known range, is home to the Belo Monte hydroelectric complex, one of the world's largest dams, which has significantly altered hydrology and habitats in that basin since construction began in the 2010s. Broader Amazonian deforestation and river degradation continue to affect freshwater biodiversity across the region. For a species known from a 2010 taxonomic description and otherwise unstudied, baseline population data do not exist, and a formal assessment would require targeted fieldwork.