Taxonomy & naming
Rhadinoloricaria stewarti was described in 2020 by Provenzano and Barriga Salazar in Zootaxa (4779(4): 485–500), in a paper that redescribed the genus Rhadinoloricaria and named this new species from Ecuador. It belongs to the subfamily Loricariinae of the family Loricariidae.
A point of caution: the epithet stewarti is also borne by Rineloricaria stewarti (Eigenmann, 1909). Despite the shared species name, the two are entirely distinct — different genera, different authorities, and separate entries in the Catalog of Fishes (Eschmeyer, CAS lists Rhadinoloricaria stewarti under species id 76164, genus id 8388). The authority and valid name used here are those of the Rhadinoloricaria species. The Catalog of Fishes is the authority for the valid name.
Morphology
Rhadinoloricaria stewarti reaches about 5 in standard length (SL), making it larger than the diminutive R. papillosa. The body is flattened in cross-section, as in other whiptail loricariines.
It is distinguished from its closest congener, R. macromystax, by a suite of proportional characters: the snout is more pointed and less projected, with a snout length of 11.3–13.3% SL (versus 14.6–15.1% in R. macromystax); the caudal peduncle is longer, with a postanal length of 48.9–50.8% SL (versus 45.2–46.1%); the postdorsal length is greater at 57.5–59.0% SL (versus 54.3%); and the abdominal plating is variable rather than completely covered. Sexual dimorphism has not been described in the published literature.
Habitat
Rhadinoloricaria stewarti is known from Ecuador, in the Napo River region of the western Amazon. It is a freshwater, demersal (bottom-associated), tropical fish. The Napo is a large Andean-fed tributary of the Amazon, carrying turbid, sediment-rich water from the eastern slopes of the Andes across the Ecuadorian lowlands. Beyond this general association, detailed microhabitat and water-chemistry data for the species have not been published.
Feeding
No feeding data have been published for Rhadinoloricaria stewarti. As a bottom-dwelling loricariine it most plausibly grazes biofilm, periphyton and fine detritus from the substrate, but the specific diet of this species has not been documented and any description would be speculation.
Mating
Courtship and mating behaviour have not been documented for this species. No information is published on pairing, territoriality, or reproductive cues.
Breeding
No breeding data have been published for Rhadinoloricaria stewarti. Reproductive mode, clutch size, and parental care are all unrecorded for this recently described species.
In the aquarium
Rhadinoloricaria stewarti has no established presence in the aquarium hobby, and no husbandry information has been published. Any care expectations would have to be extrapolated cautiously from related Amazonian loricariines that favour warm, well-oxygenated water with current and a fine substrate; these are general inferences only, and specific requirements for this species are unknown.
Conservation
Rhadinoloricaria stewarti has not been evaluated by the IUCN Red List (status: Not Evaluated). Described only in 2020 and known from the Napo region of Ecuador, its range, population size, and threats have not been formally assessed. The Napo basin faces regional pressures including oil extraction, deforestation, and sedimentation, but no species-specific conservation information is available.