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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Karyotypes and chromosomal characteristics with focus on B chromosomes of 2 species of the serrasalmid genus Metynnis, namely M. lippincottianus and M. maculatus, were examined using conventional (C-banding) and molecular (FISH mapping of minor and major rDNAs and Rex1, Rex3, and Rex6 retrotransposable elements) protocols. Both species possessed a diploid chromosome number of 2n = 62 and karyotypes composed of 32 metacentric + 28 submetacentric + 2 subtelocentric and 32 metacentric + 26 submetacentric + 4 subtelocentric, respectively; one small B element was found in the female genome of M. lippincottianus. C-banding revealed heterochromatin in the pericentromeric and terminal portions of all chromosomes of both species; the B chromosome was entirely heterochromatic. FISH showed 18S rDNA sites in 2 chromosome pairs in both species (pairs 19 and 22), and a large block in the B chromosome, while 5S rDNA signals were detected in the first pair of subtelocentric chromosomes in both species, moreover in M. maculatus an additional labeled pair 4 was observed. Mapping of the Rex1, Rex3, and Rex6 retrotransposable elements in the genomes of M. lippincottianus and M. maculatus indicated that they were dispersed throughout nearly all the chromosomes of the complement, except for the B chromosome of M. lippincottianus.

This study was developed as a research of first order streams fish assemblages located in two Conservation Units in Northwestern Paraná, Brazil, with the purpose of providing information about ichthyofauna composition of these water bodies while they are still under protection. There were captured 2,557 individuals belonging to six orders, nine families and 10 species in three streams. The most abundant species were Phalloceros harpagos (44%) from Conceição stream, Astyanax aff. paranae (14%) and P. harpagos (12%) from Jurema stream, and Melanorivulus apiamici (9%) from Conceição stream. Trichomycterus sp. could not be identified into a specific level, meaning it can represent an undescribed species to science, registered for the first time at the upper Paraná River basin.

The monotypic species Ossubtus xinguense was originally described based on scarce material putatively divided into juveniles and adults. Ossubtus xinguense has a restricted distribution and was previously known only from a few rapids downstream of the city of Altamira, in the Volta Grande stretch of the Middle Xingu River. Until recently, the species was rare in museums because its habitat (large rapids) is difficult to sample. Large-scale collecting efforts targeting rapids throughout the Xingu River basin have yielded an abundance of new material. Based on an analysis of the type series and freshly preserved specimens, we redescribe O. xinguense and provide detailed osteological descriptions along with comments about its relationships within Serrasalmidae. Furthermore, we expand the geographical distribution of the species and discuss its conservation status.

A new Hemigrammus is described from the rio Paraguai and rio Madeira basins, Mato Grosso and Rondônia States, Brazil. The new species is characterized by possessing a wide dark horizontal stripe across the eye, a vertically elongated humeral blotch, and 4-5 gill-rakers on upper branch and 9-10 on lower. The new species can be easily diagnosed from H. lunatus, the sympatric and morphologically most similar congener, by the shape of humeral blotch and the number of gill rakers. Data of the type material of both Hemigrammus lunatus and H. maxillaris, as well as extensive examination of specimens, allowed us to conclude that H. maxillaris is a junior subjective synonym of H. lunatus. A redescription of H. lunatus, as well as a formal restriction of its type locality, is provided. A putative monophyletic group within Hemigrammus, composed by H. barrigonae, Hemigrammus lunatus, H. machadoi new species, and H. ulreyi, named Hemigrammus lunatus group, is proposed based on overall body morphology and color pattern. Additionally, a discussion on the biogeographical relationships between the rio Paraguai and rio Guaporé basins is provided.Um Hemigrammus novo é descrito das bacias dos rios Paraguai e Madeira, estados de Mato Grosso e Rondônia, Brasil. A espécie nova é caracterizada por possuir uma ampla faixa escura horizontal atravessando o olho, mancha umeral verticalmente alongada, 4-5 rastros branquiais no ramo superior e 9-10 no inferior. A espécie nova pode ser facilmente diferenciada de H. lunatus, a congênere simpátrica e mais semelhante morfologicamente, pela forma da mancha umeral, e pelo número de rastros branquiais. Dados do material-tipo de Hemigrammus lunatus e H. maxillaris, bem como o exame de extenso material, levou à conclusão de que H. maxillaris é um sinônimo júnior de H. lunatus. É apresentada a redescrição de H. lunatus com a restrição formal de sua localidade-tipo. Um presumível grupo monofilético dentro de Hemigrammus, composto por H. barrigonae, H. machadoi espécie nova, Hemigrammus lunatus, and H. ulreyi, chamado grupo Hemigrammus lunatus, é proposto baseado na morfologia geral do corpo e padrão de colorido. Adicionalmente, uma discussão das relações biogeográficas entre as bacias dos rios Paraguai e Guaporé é apresentada.

Piaractus orinoquensis, a new species of serrasalmid fish, is described from the Orinoco River basin. The new species differs from congeners by having a slenderer body, relatively smaller head and snout, more compressed mid‐body, fewer scales above and below the lateral line and diagnostic molecular characters in the coI mitochondrial gene region. We also provide a re‐description of Piaractus brachypomus, restricting its geographic distribution to the Amazon River basin. Both species are economically important in their respective basins and need to be independently managed as distinct species.