“…Histological alterations constitute excellent biomarkers of exposure to toxicants because they represent the result of biochemical and physiological changes (Hinton et al, 1989). The most common injuries in gills are epithelial detachment (Machado and Fanta, 2003;Mallatt, 1985;Richmonds and Dutta, 1989;Schwaiger et al, 2004), hyperplasia (Annune and Ajike, 1999;Dutta et al, 1993;Fanta et al, 2003;Oliveira Ribeiro et al, 1996), fusion of secondary lamellae (Capinpin, 1994;Karan et al, 1998;Thophon et al, 2003) and aneurysm (Stentiford et al, 2003;Thophon et al, 2003;Van den Heuvel et al, 2000). Piaractus mesopotamicus is a freshwater native fish from tropical and subtropical rivers of the Paraná-Paraguay Basin and from flooded areas of the Pantanal region (State of Mato Grosso, Brazil) (Severi et al, 1997;Lima et al, 1998).…”