Experiments on Nile tilapia Oreochromis niloticus were conducted to assess serum osmolalities, ions and histopathological effects induced in gill tissues of 7 days exposure to different salinities (0, 10, 15 and 20 ppt). These tissues were investigated by light microscope. Blood serum osmolality (SO), sodium (Na+), chloride (Cl−) and potassium (K+) concentrations were assessed after 7 days of exposure. Serum osmolality and ionic content of exposed fish appeared differently affected by salinity throughout 7 days compared to the controls. Osmolality and Na+ were increased at the two tested salinities (15 and 20 ppt), Cl− increased at the three tested salinities (10, 15 and 20 ppt) but K+ contents remained unaffected due to salinity exposure. Therefore, tilapias exposed to high salinity present the increasing of osmotic and ionic content except for K+ contents. Na+/K+-ATPase (NKA) location was in the secondary lamellae in the gill of fi sh at 0 ppt salinity. However, NKA location was in the primary lamellae only in the gill of fi sh at 10 ppt salinity. The histological structure remain unaffected due to salinity changing. The osmoregulation and NKA location in gills of our fi ndings could be a protective response of animals to the environment changing. These data provide useful information for future reference and aquaculture practice as the information of salinity effect to osmoregulation of fish.
