Salinity tolerance and structure of external and internal gills in tadpoles of the crab-eating frog, Rana cancrivora.

Salinity tolerance and histology of gills were studied in Rana cancrivora larvae. The tadpoles at the external gill stages (W stages 21-22) were able to survive in media containing up to 40% seawater, but died in water of higher salinity. Their external gills appear to have no critical role in adaptation to seawater. However, advanced tadpoles with internal gills (T-K stages I-XVIII) were able to tolerate 50% or higher seawater. In the internal gills, there are numerous mitochondria-rich cells (MR cells) scattered on the ventral and lateral epithelia of the gill arches and the gill tufts in both freshwater- and seawater-acclimated tadpoles. In freshwater-acclimated tadpoles there are three types of MR cell: (1) microplicated, (2) microvillous, and (3) apically vacuolated. In tadpoles acclimated to dilute seawater, the ratio of type-1 to type-2 cells is lower, although all three types of MR cell are present. In 60%-seawater-acclimated tadpoles, a few MR cells with a lumen and concave cytoplasm at the apical membrane (type 4) are present. The changes in MR cell morphology under ambient conditions of low or high salinity may reflect alterations in the physiological roles of the gills with regard to transport of ions.