Pimelodus blochii (Siluriformes: Pimelodidae) is widely distributed in Amazon rivers. Several studies point out to large differences between specimens from different drainages, suggesting the existence of a complex of similar species. This work aimed to analyze specimens identified as P. blochii through morphological and molecular approaches. 157 specimens were used in the morphological analyses. These specimens were collected in the Purus, Madeira, Aripuanã, Negro, Branco, Solimões-Amazonas, Japurá, Araguaia, Apure, Urubamba and Marañon rivers, and specimens of Pimelodus blochii from the Corantijin River (Surinam) were treated as topotypes. The morphological analyses produced six new species of Pimelodus, and showed that the Brazilian specimens identified as P. blochii are morphologically different from the specimens from Surinam. Pimelodus sp. n. A is the most widely distributed, occurring in varzea areas in the Purus, Madeira, Japurá, Urubamba, Marañon and Solimões-Amazonas rivers. This species is described by a uniform coloration, caudal fin strongly forked, with pointed and thin lobes. Pimelodus sp. n. B from rio Branco was found in the middle course of the rio Negro and in the lower parts of rio Branco. This species is described by having large eyes, color pattern with a horizontal stripe above lateral line and snout short and turned ventrally. Pimelodus sp. n. C was recorded from the rio Apure, Venezuela, and can be characterized by having short eyes, and head and body short and broad. Pimelodus sp. n. D was recorded only from rio Aripuanã and can be characterized by the presence of small dots along the body. Pimelodus sp. n. E occurs in the lake Jenipapo, rio Aripuanã, and can be characterized by having numerous gill-rakers. Pimelodus sp. n. F occurs in rio Araguaia and can be characterized by having numerous gill-rakers and caudal fin with a dark band on the lobes. Based on the morphological results, Pimelodus blochii probably does not occur in the Brazilian Amazon and is restricted to Surinam. In the molecular analysis, a phylogenetic treatment was conducted to verify the monophyly of the blochii-complex when confronted with specimens of the Pimelodus albofasciatus-complex. The stricto consensus and the majority-rule cladograms obtained through parsimony produced six clades. According to the obtained phylogenies, neither the blochii nor the albofasciatus-
complex are monophyletic. The albofasciatus-complex was represented by three clades and the blochii-complex by three clades. The three blochii-complex clades are specimens analyzed in chapter 1 and classified as Pimelodus sp. n. A , Pimelodus sp. n. D and Pimelodus sp. n. E . Thus, the recognition of these clades under the molecular-cladistic treatment gave support to the morphological results. Pimelodus sp. n. E , more basal, occurs in the ancient Brazilian Shield. Pimelodus sp. n. A more derived, is found in more recent areas, as Madeira and Solimões drainage. As this hypothesis of relationship is the first to involve these Brazilian Amazon taxa, it is necessary to conduct further analyses to include more representatives from other drainages.
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