Piranhas belong to the subfamily Serrasalminae, which includes closely related omnivorous fish such as pacus. Traditionally, only the four genera Pristobrycon, Pygocentrus, Pygopristis and Serrasalmus are considered to be true piranhas, due to their specialized teeth. The total number of piranha species is unknown and contested, and new species continue to be described. Estimates range from fewer than 30 to more than 60. Piranhas are indigenous to the Amazon basin, in the Orinoco, in rivers of the Guianas, in the Paraguay-Paraná, and the São Francisco River systems.