Mercatoris

Mercatoris is a small, poorly known genus of freshwater fish that was first described in the late 19th century based on limited specimens collected in the basins of the Paraná River system in Brazil. The sole species that has been placed in this genus, Mercatoris sphaericus, is a slender, silvery fish that grows to a maximum length of approximately 12 cm. Morphologically, the fish is characterized by a long, streamlined body, a terminal mouth, and a distinctly compressed dorsal fin that runs almost to the tail. The caudal fin is forked, providing the fish with swift swimming capabilities adapted to moderately fast-moving streams.

Falco and colleagues originally proposed the genus to accommodate the species due to its unique combination of meristic counts, namely a dorsal fin with 13 spines and 7 rays, and a pectoral fin with 11 rays. Since its introduction, taxonomic work has remained sparse, and the status of Mercatoris in relation to other characiform families, such as Ctenoluciidae and Hepsetidae, has been debated. Molecular phylogenetic studies have yet to confirm the monophyly of the genus, and no recent revisions have incorporated genetic data.

Despite the limited information, Mercatoris sphaericus is considered of potential conservation concern due to its restricted geographic range and the ongoing degradation of its habitat caused by deforestation, dam construction, and pollution. Local fishing practices remain minimal, and the species is not known to have any significant commercial value. Continued field surveys and taxonomic scrutiny are necessary to clarify its ecological role, population status, and phylogenetic relationships within the Characiformes.