A new species of Characidium (Characiformes: Crenuchidae) from the rio Madeira basin, Brazil.

This study describes Characidium nambiquara, a new species from the upper rio Guaporé, rio Madeira basin, Brazil. The new species differs from most congeners by the presence of isthmus and area between the contralateral pectoral-fin bases completely naked. From congeners with some degree of scaleless ventral surface of the body C. nambiquara differs by having 10 circumpeduncular scales. The new species is also distinguished from congeners by the spotted colour pattern on body of the large-sized specimens and by having black dashes on all fins and conspicuous midlateral longitudinal dark stripe or conspicuous vertical bars absent. Characidium nambiquara further differs from most congeners by the presence of 34-36 pored scales on the lateral line, 3 horizontal scale rows above the lateral line and 3 horizontal scale rows from the lateral line to the midventral scale series. Remarks on intraspecific colour variation within the genus, not related to sexual dimorphism, are also provided.