A new rheophilic South American darter (Crenuchidae:Characidium) from the rio Juruena basin, Brazil, with comments on morphological adaptations to life in fast-flowing waters

Characidium iaquira, a new species from the upper rio Juruena, rio Tapajos basin, Brazil, is described. The new species can be promptly distinguished from all congeners by having a unique v-shaped dark mark lying along the caudal-fin extension, in medium- and large-sized specimens, and a remarkable iridescent green colouration in life.Characidium iaquirais closely related toCharacidium crandelliiandCharacidium declivirostreby sharing unambiguous synapomorphies such as branchiostegal membranes united to each other across the isthmus, a scaleless area extending from the isthmus to the pectoral girdle, and dermal flaps surrounding anterior and posterior naris independent, but touching each other distally. Morphological specializations of the paired fins in the three riffle-dwellers species are discussed, including the wing-like shape, robustness, and inclination of the pectoral fin. (AU)