Taxonomy & naming
Psalidodon rivularis was originally described by Christian Frederik Lütken in 1875 from specimens collected in the Rio das Velhas, a major tributary of the São Francisco River in Minas Gerais State, Brazil. The type locality is thus the highland reaches of that river system in south-central Brazil. Eschmeyer's Catalog of Fishes (CAS) records the current valid name as Psalidodon rivularis (Lütken, 1875) — the parenthetical authority indicating that the species has been transferred from the genus in which it was originally described.
The reassignment to Psalidodon reflects broader molecular and morphological revisions of the historically unwieldy Astyanax, one of the largest and most species-rich genera in Characidae. Rodrigues-Oliveira et al. (2025) conducted an integrative study drawing on meristic and morphometric data, karyotypic analysis (2n = 46 chromosomes in the true P. rivularis morphotype), and phylogenomics, concluding that what had been treated as a single species is in fact a complex of at least five distinct species. The name P. rivularis is now restricted to morphotype 1, which is endemic to the São Francisco River Basin. The lectotype was formally selected in that same 2025 study, providing a stable nomenclatural anchor for the restricted concept of the species.
This fish was long listed as Astyanax rivularis; Eschmeyer's Catalog of Fishes places the species in Psalidodon, and the current combination Psalidodon rivularis is followed here.
Morphology
Psalidodon rivularis is a small, laterally compressed characid reaching a maximum recorded standard length of approximately 3.9 in, though most individuals encountered in the wild are considerably smaller. Like most members of its generic alliance, the body is silvery, often with a darker lateral stripe running from behind the gill cover toward the caudal peduncle, and the fins are largely hyaline or lightly pigmented.
The species carries the typical characid complement of teeth and the adipose fin characteristic of the family. Detailed meristic and morphometric characters — counts of lateral-line scales, vertebrae, and fin rays — were used by Rodrigues-Oliveira et al. (2025) to delimit the species complex, and the true P. rivularis (morphotype 1) shows a combination of those counts that distinguishes it from the four other cryptic lineages now recognised. Precise published meristic data for aquarists are sparse, reflecting the species' limited presence in the hobby literature.
Habitat
The species is endemic to the São Francisco River basin, the great inland drainage of east-central Brazil that flows north through the cerrado and caatinga biomes before turning east to the Atlantic. The type locality in the Rio das Velhas places the species in the southern, highland portion of that catchment in Minas Gerais State, where streams run clearer and cooler than in the semi-arid middle and lower basin.
As the common name piaba do córrego (creek piaba) suggests, P. rivularis is associated with smaller watercourses — rivulets and stream reaches rather than large rivers — in the São Francisco drainage. Specific microhabitat data are limited in the available literature, but small Astyanax-alliance characids in similar cerrado and cerradão streams typically occupy marginal vegetation, riffles, and pool edges, exploiting both the water column and the surface film. Water conditions in the highland São Francisco tributaries are generally soft to moderately hard, with temperatures that can span roughly 68–82 °F across seasons.
Feeding
Psalidodon rivularis is an omnivore in the broad sense typical of the Astyanax alliance. Small characids in the São Francisco highlands exploit whatever invertebrate and plant material the stream offers: aquatic insect larvae, small crustaceans, terrestrial insects that fall onto the surface, algae, and fine plant matter. Opportunism rather than dietary specialisation characterises the group.
In captivity, such species accept a wide range of standard aquarium foods — flake, micro-pellets, frozen and live foods including Daphnia, brine shrimp, and bloodworm. Offering variety and some live or frozen invertebrate food is advisable for conditioning fish and observing natural behaviours.
Mating
Psalidodon rivularis, like the great majority of Astyanax-alliance characids, is presumed to be an open, egg-scattering spawner without parental care. The genus is well characterised by this reproductive mode across many studied species: males pursue females during spawning, eggs are scattered among fine-leaved vegetation or over the substrate, and adults offer no subsequent care and will consume the eggs if not removed.
Specific courtship observations for P. rivularis have not been published in the available literature. By analogy with closely related species, spawning likely involves brief chasing behaviour, a scatter of small adhesive eggs among aquatic plants or substrate, and immediate departure of both sexes. The restricted distribution of the species in highland São Francisco streams suggests possible seasonal cues linked to dry- and wet-season temperature and flow changes, but this has not been formally documented.
Breeding
No detailed captive-breeding accounts specific to Psalidodon rivularis appear in the available hobby or scientific literature, reflecting the species' limited representation in the aquarium trade. The reproductive biology can be inferred from the genus: egg-scattering over fine-leaved plants or spawning mops, small adhesive eggs, no parental care, and adults that will consume their own eggs if not separated. Incubation at tropical temperatures in related species is typically 24–48 hours, with free-swimming fry emerging shortly thereafter.
Fry of small Astyanax-alliance species are typically tiny and require infusoria or commercial liquid fry food as first foods, graduating to newly hatched brine shrimp and finely powdered flake as they grow. Data sparse for this species specifically.
In the aquarium
Psalidodon rivularis is not an established aquarium species in the international hobby and is unlikely to be encountered commercially under this name. Its closest traded relatives are the various 'piaba' and 'tetra' species from the São Francisco drainage that appear intermittently in specialist shipments from Brazil. Given its size — under 3.9 in SL — and the social, schooling habits typical of the genus, a group of six or more individuals in a well-filtered, planted aquarium of 31.5 in or more would suit it well.
Water conditions matching the highland São Francisco environment — soft to moderately hard, slightly acidic to neutral water in the range of 68–79 °F — would be appropriate, though Astyanax-alliance species are generally hardy and adaptable. The species would be compatible with other peaceful, similarly sized South American fish. Any specimens in the trade may have been collected from wild populations in a restricted range, and sourcing should be considered accordingly.
Conservation
Psalidodon rivularis has not been assessed by the IUCN Red List as of the date of this article, and neither has the current valid name Psalidodon rivularis. The species' restricted endemic range within the São Francisco River basin of Minas Gerais — a river system that has experienced significant deforestation, agricultural runoff, water abstraction, and impoundment in its catchment — warrants conservation attention even in the absence of a formal assessment.
The discovery by Rodrigues-Oliveira et al. (2025) that the taxon comprises at least five distinct species rather than one broadly distributed fish substantially reduces the effective range of each lineage and elevates the conservation concern for the true P. rivularis, which appears to be endemic to the São Francisco basin proper. The description and formal naming of the cryptic species in the complex is an important prerequisite for targeted conservation evaluation.