To determine the variation pattern of apical openings in mitochondrial-rich cells, short term and long term exposure to different salinities was studied. The results obtained suggest that morphological adaptations of mitochondria-rich cells reflect the animal's ionic and osmotic requirements. Yellowfin seabream, Acanthopagrus latus, are capable of tolerating direct exposure of salinities from 5 to 60‰ without showing mortalities. This species was also able to tolerate gradual decrease in salinity in the surrounding medium from seawater to freshwater through a period of 10 days and successfully adapt to freshwater without showing mortality. This is the shortest period reported in a true marine fish for adaptation to freshwater through an acceptable experimental duration. Three subtypes of mitochondria-rich cells were detected in photomicrographs created from scanning electron microscope; shallow basin, deep hole and wavy convex mitochondria-rich cells. After decrease in salinity to 20‰ the apical membrane of mitochondria-rich cells showed changes in morphology and on day 7 most of the mitochondria-rich cells were of the wavy convex or shallow basin subtypes. However, on day 21 they returned to their original state prior to changes in salinity. When transferred to hypoosmotic medium (5‰ and FW), rapid responses were exhibited in the apical membrane of mitochondria-rich cell which stabilized after 21 days and all subtypes of mitochondria-rich cells were observed in photomicrographs of gill filaments. Seabream transferred to 20‰ and hypoosmotic environments showed rapid decrease in mitochondria-rich cells, which was restored to normal levels as those in seawater within 7 days.
