The Pacu, Piaractus mesopotamicus, is a teleostei of the Characidae family, intensively cultivated in Brazil due to its rusticity, fast growth and easy adaptation. Morphological knowledge of the body systems including the lymphoid organs is necessary to improve fish production and supply subsidies for the maintenance of stocks. This study aimed to describe morphologically kidney and head kidney of Piaractus mesopotamicus, analyzing the cellular profile of each organ with the use of light microscopy and electronic transmission microscopy. The result of the macroscopic analysis showed that the localization of the kidney and head kidney is the same as found in the majority of the teleostei. The kidney presented a form in "H", where the medium region is expanded on the natatorium bladder. The head kidney presented a dilatation in the cranial region of the kidney. In the transmission electronic microscopy we found ultrastructural similarities with other teleostei fish. It can be concluded that the lymphoid organs (kidney and head kidney) of Piaractus mesopotamicus are histologically and ultra-structurally similar to the other teleostei.
Lymphoid organs; Piaractus mesopotamicus; kidney; cephalic kidney
