CELLULAR AND EPITHELIAL ADJUSTMENTS TO ALTERED SALINITY IN THE GILL AND OPERCULAR EPITHELIUM OF A CICHLID FISH (OREOCHROMIS-MOSSAMBICUS)

Morphological features of the gill and opercular epithelia of tilapia (Oreochromis mossambicus) have been compared in fish acclimated to eit her fresh water (FW) or hypersaline water (60 parts per thousand S) by scanning electron and fluorescence microscopy. In hyperosmoregulating , FW-acclimated, tilapia only those mitochondria-rich (MR) cells prese nt on the filament epithelium of the gill were exposed to the external medium. After acclimation of fish to hypersaline water these cells be come more numerous, hypertrophy extensively, and form apical crypts no t only in the gill filament but also in the opercular epithelium. Rega rdless of salinity, MR cells were never found to be exposed to the ext ernal medium on the secondary lamellae. In addition, two types of pave ment cells were identified having distinct morphologies, which were un affected by salinity. The gill filaments and the inner operculum were generally found to be covered by pavement cells with microridges, wher eas the secondary lamellae were covered exclusively by smooth pavement cells.