Abstract
The eVect of salinity was studied in natural populationsof the black-chinned tilapia (Sarotherodon melanotheron)from West Africa. This euryhaline species colonizesnearly all coastal environments from bays to lagoons characterizedby salinities ranging from fresh water to hypersalinewater over 100‰. Individuals were sampled during the dryseason at several locations characterized by diVerent levels ofsalinity (3–102‰). Their osmotic status and their gills wereanalyzed. The branchial mitochondria-rich cells (MRC),localized at the basis of the Wlaments and along the lamellaein Wsh taken from the saline stations, showed a wide plasticitywith signiWcant diVerences in their number and size. Themost striking results were a signiWcant larger area (t3x) anda higher number (t55x) of MRC at high salinity (102‰)compared to low salinity (3‰). The major ion transportersand channels were localized by immunocytochemistry anddiVerent expression patterns have been recorded between stations.Despite an increased Na+/K+-ATPase (NKA) -subunitexpression and NKA activity, pointing to an increased monovalention excretion, a severe osmotic imbalance wasrecorded in animals living in hypersaline environments.