Resumen

Based on ten specimens, using morphometric, meristic and osteological characters, Astyanax leoni n. sp. is described from the upper Madre de Dios River drainage, Amazon River Basin of Peru. A. leoni n. sp. is a member of the Astyanax orthodus species group and is distinguished (except from A. multidens, A. villwocki, and A. gandhiae) in having an inconspicuous rod-shaped stripe on the caudal peduncle that extends anteriorly on top of a silver lateral stripe. It differs from the species mentioned above as follows: from A. multidens by having shorter chevron marks that cover up to only two series of scales above and below the apex of the chevron (vs. chevrons taller, with extensions that cover three series of scales); from A. villwocki by having chevron markings along the entire body (vs. no chevrons on the caudal peduncle); It differs from Astyanax gandhiae in having the caudal peduncle spot extending anteriorly to beyond the tip of the dorsal fin (vs. caudal peduncle spot not reaching anteriorly to tip of dorsal fin), and not of a uniform width as in A. villwocki. A dichotomous key to the species of Astyanax orthodus group is included.