Taxonomy & naming
Psalidodon gymnodontus was originally described by Carl H. Eigenmann in 1911 as part of his extensive survey of South American freshwater fishes. The species was later revised and redescribed by Pavanelli and Oliveira (2009) in a paper published in Neotropical Ichthyology (volume 7, number 4, pages 569–578), which treated it as a new combination and addressed its notable polymorphism in dental and body characters. The epithet gymnodontus derives from the Greek gymnos (naked, bare) and odous (tooth), likely alluding to a dental feature observed in the type material.
FishBase has listed an alternative combination, Psalidodon gymnodontus, reflecting ongoing debate about generic limits within what has historically been treated as Astyanax — a very large, paraphyletic aggregate genus. However, the Catalog of Fishes (Eschmeyer, CAS) remains the governing authority for valid nomenclature on this site, and the species is treated here under Psalidodon gymnodontus (Eigenmann, 1911). Researchers following different phylogenetic classifications may encounter both names in the literature. The species is endemic to the Iguaçu River basin in Brazil, with no populations known from other drainages.
This fish was long listed as Astyanax gymnodontus; Eschmeyer's Catalog of Fishes places the species in Psalidodon, and the current combination Psalidodon gymnodontus is followed here.
Morphology
Psalidodon gymnodontus reaches a maximum recorded standard length of 6.7 in, making it a notably large member of its genus. The body form follows the characteristic Astyanax plan: laterally compressed, moderately deep-bodied, with a blunt snout, large eye, and the adipose fin that marks all characids. The silvery flanks may show a dark lateral stripe or humeral spot in varying degrees of expression.
The species is described as polymorphic, meaning that individuals within and across populations differ in dentition and, to some extent, body proportions — variation that historically created confusion about its taxonomic identity. The 2009 redescription by Pavanelli and Oliveira documented this variability and used meristic and morphometric analysis to delimit the species more precisely. Specific details of tooth form and fin-ray counts from that work are the most reliable morphological guide to identification.
Habitat
This species is endemic to the Iguaçu (Iguazu) River basin, a major river system draining the states of Paraná and Santa Catarina in southern Brazil and flowing westward to join the Paraná River at the famous Iguaçu Falls. The Iguaçu basin is geographically isolated below those falls, with a high degree of endemism in its fish fauna — many species found here occur nowhere else.
Psalidodon gymnodontus occupies freshwater environments within this basin. FishBase categorizes it as a pelagic freshwater species, suggesting it inhabits the open water column of rivers rather than being restricted to benthic or heavily structured microhabitats. The Iguaçu basin lies in a subtropical climate zone with warm summers and cooler winters, and the river system ranges from fast, rocky upland reaches to calmer, deeper sections. Data on specific microhabitat preferences, water chemistry tolerances, and seasonal movements for this species are sparse in the available literature.
Feeding
FishBase records a trophic level of 2.0 for Psalidodon gymnodontus and characterizes its diet as plants, placing it among the more herbivorous members of the genus. A trophic level near 2.0 is consistent with primary consumption of plant material, algae, or detritus derived from plant matter, with little or no significant predation on animal prey.
This herbivorous tendency is somewhat unusual within Astyanax, most of which are generalist omnivores or insectivores. Whether the diet reflects specialization on aquatic macrophytes, periphyton, or allochthonous plant material entering from the riparian zone has not been detailed in the sources available. The species' large size relative to other Astyanax would support the processing of coarser plant matter. In an aquarium context, a diet incorporating plant-based foods — spirulina flake, blanched vegetables, algae wafers — alongside standard omnivore foods would be appropriate given this profile.
Mating
No detailed field observations of mating behavior in Psalidodon gymnodontus appear in the sources available. Like most characids, the species is presumed to be an egg-scatterer that does not form lasting pair bonds. Males and females likely aggregate seasonally, with spawning triggered by environmental cues such as rising water temperatures or changes in river flow associated with seasonal rainfall in the Iguaçu basin.
Astyanax species generally engage in broadcast spawning, with males pursuing gravid females through the water column and vegetation, releasing sperm as eggs are shed. Sexual dimorphism, when present in the genus, is typically subtle and restricted to differences in body depth and the texture of the anal fin in males. Whether P. gymnodontus shows any notable secondary sexual characters has not been documented in the available literature.
Breeding
Reproductive details for Psalidodon gymnodontus specifically are not documented in the available sources. Based on the biology of the genus, breeding is assumed to follow the egg-scattering pattern typical of characids: adhesive or semi-adhesive eggs scattered among fine-leaved plants or over substrate, with no parental care provided by either sex. The eggs and any resulting fry would be ignored or consumed by the adults.
Batch fecundity and incubation times have not been reported for this species. Breeding in captivity would likely require a species-appropriate setup with temperature conditioning and spawning vegetation or mops, following the general approach used for larger Astyanax. The species has not featured prominently in the aquarium trade, so hobbyist breeding accounts are essentially absent from the literature.
In the aquarium
Psalidodon gymnodontus is not an established aquarium fish; it does not appear in the ornamental trade and hobbyist records are absent. Its large size — up to 6.7 in SL — and origins in the cooler subtropical waters of the Iguaçu basin would make it a demanding species to house appropriately, requiring a spacious aquarium and water temperatures somewhat below the tropical norm, with good oxygenation and flow to replicate its riverine habitat.
Any keeper attempting to maintain this species should provide substantial space and a plant-rich diet given the species' documented herbivorous tendencies. As with all Iguaçu endemics, wild collection is inadvisable given the basin's conservation pressures, and captive-sourced individuals would be preferred if the species were ever made available through the trade.
Conservation
Psalidodon gymnodontus has not been formally assessed by the IUCN Red List and carries no current conservation status. As an endemic of the Iguaçu River basin, it shares the fate of a fish fauna under substantial pressure. The Iguaçu basin has been heavily developed for hydroelectric power — the river's course is interrupted by multiple large dams, which fragment fish populations, alter flow regimes, block migratory routes, and inundate riverine habitat. Endemics with restricted ranges are among the most vulnerable to such cumulative impacts.
The species' polymorphism and its role in the broader Iguaçu fish community have been examined scientifically, but no targeted conservation assessment has been published. The 2009 redescription represents the most recent systematic treatment. Given the ongoing and planned hydroelectric development within the basin, formal assessment of all Iguaçu endemics, including P. gymnodontus, would be a valuable step in understanding the region's conservation priorities.