The tucuxi Sotalia fluviatilis was described by Gervais and Deville in 1853, and the costero Sotalia guianensis by Pierre-Joseph van Bénéden in 1864.
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The river is one of the best places to observe Amazonian wildlife, including the Amazon river dolphin and tucuxi.
The tucuxi (Sotalia fluviatilis) in the Amazon River is another species descended from oceanic dolphins; however, it does not perfectly fit the label of 'facultative' either, as it occurs only in freshwater.