Morphological and histometric features of the caudal kidney in piranha Pygocentrus nattereri (Characiformes: Serrasalmidae).

Pygocentrus nattereri is a widely distributed species in the Neotropical region and a potential bio-indicator. Kidneys have functions in fish physiology, allowing them to live in different environments. We aimed to compare the histological characteristics of caudal kidneys between males and females, associating them with the renosomatic index (RSI). For this purpose, 15 males and 14 females were used for biometric and histological analyses. Structural volumetric density (SVD), renal corpuscle histometric measures, and hemosiderin and lipofuscin deposit frequency in macrophages melanogenic (MMs) were assessed. No biometric differences were observed between the sexes, but body weight and standard length were correlated with RSI. The SVD showed difference in hematopoietic tissue between female and males, whereas the density of the other structures was not different. The RSI was positively associated with hematopoietic tissue and proximal tubule density in contrast to distal tubules, blood vessels, collecting ducts and MMs. Females exhibited a higher renal corpuscle area, glomerulus area, distal tubule diameter, collecting tubule area, and collecting tubule lumen area. These differences may be due to metabolic differences between males and females. Sex effect in P. nattereri may define punctual differences in future studies on the metabolism and immunity of this species. [ABSTRACT FROM AUTHOR]