Cyphocharax, Fowler (Pisces, Ostariophysi, 1906

Dagosta, Fernando C. P. & Pinna, Mário De, 2019, The Fishes Of The Amazon: Distribution And Biogeographical Patterns, With A Comprehensive List Of Species, Bulletin of the American Museum of Natural History 2019 (431), pp. 1-167 : 10-12

Cyphocharax in Vari (1992a), and 72% of Ster-

narchorhynchus in de Santana and Vari (2010).

Thus, the position of those genera among the most diverse in the Amazon, especially in the case of Cyphocharax and Sternarchorhynchus , may be an artifact resulting from the lack of taxonomic revisions in other potentially more diverse yet poorly studied genera, for example, Chaetostoma , Knodus , and Rineloricaria .

Interesting facts appear when the diversity of the richest Amazonian lineages is compared with equivalent data from outside the basin. The Amazonian fish community is formed both by typically Amazonian lineages (i.e., most or all of their diversity is in the Amazon basin) and by lineages that have greater diversity in other South American basins or in the ocean (fig. 3A). At the family level, it is clear that the Amazon dominates, by a wide margin, the alpha-diversity in the vast majority of primary-division lineages sensu Myers (1938), with relatively little diversity in the secondary and peripheral divisions. The only exclusively Amazonian family is the recently described Tarumaniidae ( Scoloplacidae and Lepidosirenidae occur also in the Paraguay basin).

Other interesting patterns are revealed by an examination of intrageneric diversity. The first one is that the most diverse genera in the Ama-

% of species ocurring in Amazon % of species not ocurring in Amazon

zon have the majority of their species in the basin (fig. 3B). Also, some megadiverse Neotropical genera are relatively poorly represented in the Amazon, such as Astyanax , Trichomycterus , Bryconamericus , Characidium , and Hemibrycon . This is probably related to the fact that the species of those genera in the Amazon are concentrated along its outer rims, which indicate that their presence in the region is relatively recent, caused by secondary geological events involving adjacent basins. Two cases that stand out are Astyanax and Trichomycterus , which have their Amazonian diversity concentrated respectively in the upper Tocantins and upper Ucayali, right at the divide with other drainages.

The richest Amazonian tributary is the Rio Madeira (fig. 4), a fact that is attributed not only to its large size, but also to its hybrid nature (Dagosta and Pinna, 2017). The Rio Madeira drains one-third of the Amazonian lowlands and at the same time has tributaries associated with the Andean range and the Brazilian Shield, both areas particularly rich in fish endemics. The Rio Negro is also one of the most diverse Amazonian tributaries (fig. 4), despite its nutrient-poor and extremely acidic waters, which represent ecological barriers to numerous fish species (Goulding et al., 1988; Lima and Ribeiro, 2011). Despite that, the Rio Negro also harbors a large number of endemics (79 spp) .

High levels of endemism are also seen in the Marañon and Ucayali (25% and 16%, respectively; fig. 4), a likely result of the Andean range in the diversification of the Amazonian biota. The largest absolute number of endemic taxa (87 spp.) is found in the upper Rio Tocantins followed by the Marañon (81 spp.). Other basins draining Amazonian Shield regions, such as Juruena, Teles Pires, Jari, and Madeira Shield tributaries, also show high levels of endemism, although relatively low total numbers of species. Such figures corroborate the hypothesis that highlands have proportionally fewer taxa with broad distributions and fewer cases of sympatry when compared with lowland rivers (Dagosta and Pinna, 2017) .

The majority of Amazonian fish diversity is exclusive, with 63% (1696 spp.) of its species found nowhere else (fig. 4). That number alone would place the Amazon as the richest basin in the world in fish species.