Pacú (Piaractus mesopotamicus), pez chato ("flat fish"), mbiraí, or piraí, is a South American ray-finned fish that is endemic to the Paraguay-Paraná River basin, though it has been introduced by aquaculture activities in a wider area. P. mesopotamicus is a robust fish, with ovoid shape, flattened laterally. Its colour is dark grey to silver, with a white belly and a yellow breast, and the laterals have small darker spots.