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The freshwater and marine fish faunas of South America are the most diverse on Earth, with current species richness estimates standing above 9100 species. In addition, over the last decade at least 100 species were described every year. There are currently about 5160 freshwater fish species, and the estimate for the freshwater fish fauna alone points to a final diversity between 8000 and 9000 species. South America also has c. 4000 species of marine fishes. The mega-diverse fish faunas of South America evolved over a period of >100 million years, with most lineages tracing origins to Gondwana and the adjacent Tethys Sea. This high diversity was in part maintained by escaping the mass extinctions and biotic turnovers associated with Cenozoic climate cooling, the formation of boreal and temperate zones at high latitudes and aridification in many places at equatorial latitudes. The fresh waters of the continent are divided into 13 basin complexes, large basins consolidated as a single unit plus historically connected adjacent coastal drainages, and smaller coastal basins grouped together on the basis of biogeographic criteria. Species diversity, endemism, noteworthy groups and state of knowledge of each basin complex are described. Marine habitats around South America, both coastal and oceanic, are also described in terms of fish diversity, endemism and state of knowledge. Because of extensive land use changes, hydroelectric damming, water divergence for irrigation, urbanization, sedimentation and overfishing 4-10% of all fish species in South America face some degree of extinction risk, mainly due to habitat loss and degradation. These figures suggest that the conservation status of South American freshwater fish faunas is better than in most other regions of the world, but the marine fishes are as threatened as elsewhere. Conserving the remarkable aquatic habitats and fishes of South America is a growing challenge in face of the rapid anthropogenic changes of the 21st century, and deserves attention from conservationists and policy makers.

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South America harbours the most diverse freshwater fish fauna in the world, and recent estimates point to between 6000 and 8000 species in the Neotropical region. Such fauna has diverse historical origins, either having invaded and diversified in fresh water from marine ancestors during the Palaeogene or being isolated on the continent since the end of the break‐up of Gondwana in the Cretaceous. Taxonomic, morphological and ecological diversity of South American freshwater fishes is dramatic, as are the myriad freshwater habitats they inhabit. Unfortunately, many of these habitats are severely threatened by deforestation, water divergence for irrigation, industry and other uses by humans, hydroelectric damming, mining, pollution and invasive species. Despite these multiple threats, there are very few on‐the‐ground conservation initiatives in South America, although assessments of species‐extinction risks have been produced at regional and subregional levels in different countries.

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Osteological and soft anatomical features of representatives of poeciliine genera were studied to test the monophyly of the poeciliine tribes and to advance a hypothesis of relationships within the subfamily. The resultant hypothesis supports the proposal of a new classification for the subfamily Poeciliinae. Diagnoses are provided for suprageneric clades. The tribe Tomeurini is resurrected and the new tribes Brachyrhaphini and Priapichthyini as well as the supertribe Poeciliini are described. New usages of old tribe names are proposed based on the phylogenetic framework.Caracteres osteológicos e da anatomia mole de representantes dos gêneros de poeciliíneos foram estudados para se testar a monofilia das tribos de Poeciliinae e para propor uma hipótese de relações dentro da subfamília. A hipótese resultante suporta a proposição de uma nova classificação para a subfamília Poeciliinae. São fornecidas diagnoses para os clados supragenêricos. A tribo Tomeurini é ressuscitada e as novas tribos Brachyrhaphini e Priapichthyini bem como a supertribo Poeciliini são descritas. Novos usos para antigos nomes de tribos são propostos com base no arranjo filogenético.