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Fauna

The Tocantins basin (which includes the Araguaia River) is the home of several large aquatic mammals such as Amazonian manatee, Araguaian river dolphin and tucuxi, and larger reptiles such as black caiman, spectacled caiman and yellow-spotted river turtle.

The Tocantins River Basin has a high richness of fish species, although it is relatively low by Amazon basin standards. More than 350 fish species have been registered, including more than 175 endemics. The most species rich families are Characidae (tetras and allies), Loricariidae (pleco catfish and allies) and Rivulidae (South American killifish). While most species essentially are of Amazonian origin, there are also some showing a connection with the Paraná and São Francisco rivers. The Tocantins and these two rivers flow in different directions, but all have their source in the Brazilian Plateau in a region where a low watershed allows some exchange between them. There are several fish species that migrate along the Tocantins to spawn, but this has been restricted by the dams. Following the construction of the massive Tucuruí Dam, the flow of the river changed. Some species have been adversely affected and there has been a substantial reduction in species richness in parts of the river.

The São Domingos karst in the upper Tocantins basin is home to an unusually high number of cavefish species (more than any other region in the Americas): Ancistrus cryptophthalmus, several Ituglanis species, Pimelodella spelaea, Aspidoras mephisto, an undescribed Cetopsorhamdia species and Eigenmannia vicentespelaea. The last is the only known cave-adapted knifefish and one of only two known non-catfish in caves of the South American mainland (the other is the characid Stygichthys typhlops).

In its lower reaches, the Tocantins separates the Tocantins–Araguaia–Maranhão moist forests ecoregion to the east from the Xingu–Tocantins–Araguaia moist forests ecoregion to the west. It acts as a barrier that prevents dispersal of flora and fauna between these ecoregions.