Mitochondria-rich (MR) cells and the activities of the Na+/K+-ATPase and carbonic anhydrase in the gill and opercular epithelium of Oreochromis mossambicus adapted to various salinities

1. 1. The activities of the Na+K+-ATPase and carbonic anhydrase and the number and size of mitochondria rich cells were determined in the gills and opercular epithelia of tilapias adapted to freshwater, brackish water, seawater, 130% seawater and 170% seawater. 2. 2. In the gill all parameters examined increased significantly in correlation to an enhanced environmental salinity, whereas in the opercular epithelium the activities of the enzymes remained constant. 3. 3. On the basis of size-frequency analysis two classes of mitochondria rich cells were found.