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Summary Length–weight (LWR) and length–length relationships (LLR) are presented for 135 freshwater fish species sampled by several types of fishing gear between April 2012 and July 2014 in the middle and lower Xingu River Basin, northern Brazil. The report represents the first references on LWRs and LLRs for 114 and 119 species, respectively, and also provides a new maximum size for 96 species.

Piranhas and pacus (Characiformes: Serrasalmidae) are a charismatic but understudied family of Neotropical fishes. Here, we analyse a DNA barcode dataset comprising 1,122 specimens, 69 species, 16 genera, 208 localities, and 34 major river drainages in order to make an inventory of diversity and to highlight taxa and biogeographic areas worthy of further sampling effort and conservation protection. Using four methods of species discovery—incorporating both tree and distance based techniques—we report between 76 and 99 species-like clusters, i.e. between 20% and 33% of a priori identified taxonomic species were represented by more than one mtDNA lineage. There was a high degree of congruence between clusters, with 60% supported by three or four methods. Pacus of the genus Myloplus exhibited the most intraspecific variation, with six of the 13 species sampled found to have multiple lineages. Conversely, piranhas of the Serrasalmus rhombeus group proved difficult to delimit with these methods due to genetic similarity and polyphyly. Overall, our results recognise substantially underestimated diversity in the serrasalmids, and emphasise the Guiana and Brazilian Shield rivers as biogeographically important areas with multiple cases of across-shield and within-shield diversifications. We additionally highlight the distinctiveness and complex phylogeographic history of rheophilic taxa in particular, and suggest multiple colonisations of these habitats by different serrasalmid lineages.

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Length-weight relationships (LWR) are presented for 27 freshwater fish species representing 11 families captured in the Trombetas River, a clearwater left margin tributary of the Amazon River. The allometry coefficient (b) of the LWR (W = aTL b ) ranged from 2.51 to 3.49, with a median value of 3.00. This study represents the first reference on LWR for 23 species and new records of maximum lengths for 15 species.

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A new species of Serrasalmidae, Tometes camunani, is described from the upper drainages of the rio Trombetas basin, Pará State, Brazil. The new species is distinguished from its congeners by having neurocranium with a slight concavity at the level of the frontal bone (vs. concavity absent, dorsal profile of neurocranium straight). It can be further distinguished from its congeners by having teeth with central cusp taller and acute (vs. central cusp shorter and with rounded edge in T. trilobatus), a terminal mouth (vs. upturned mouth in T. lebaili), and 12-26 prepelvic spines (vs. 0-9 in T. makue). The new species is strictly rheophilic like other species of Tometes, and occurs exclusively in the rapids of shield rivers, complex and fragile biotopes that are threatened by anthropogenic activities. An identification key to the species of the Myleus group is provided. Uma espécie nova de Serrasalmidae, Tometes camunani, é descrita para as drenagens superiores da bacia do rio Trombetas, estado do Pará, Brasil. A espécie nova distingue-se dos congêneres pela presença de uma ligeira concavidade no neurocrânio na altura do frontal (vs. concavidade ausente, perfil dorsal do neurocrânio reto). Também pode ser adicionalmente distinguido dos seus congêneres por possuir dentes com a cúspide central mais alta e cume agudo (vs. cúspide central mais baixa e com cume arredondado em T. trilobatus), a boca terminal (vs. boca orientada para cima em T. lebaili), e 12-26 espinhos pré-pélvicos (vs. 0-9 em T. makue). A espécie nova é estritamente reofílica, como as demais espécies de Tometes, e ocorre exclusivamente nas zonas encachoeiradas dos rios de escudo, biótopos complexos, frágeis e ameaçados por ações antropogênicas. Uma chave de identificação para as espécies do grupo Myleus é apresentada.