EFFECT OF CYCLICAL SALINITY CHANGES ON CELL-VOLUME AND FUNCTION IN GEUKENSIA-DEMISSA GILLS

We acclimated the estuarine mussel Geukensia demissa to a regime of si nusoidal salinity cycling (12 h cycle between 100% and 60% seawater) a nd correlated changes in the volume of gill cells with changes in seve ral indicators of the functional status of gill cells (rate of O-2 con sumption, ATP content and amino acid transport). There was no indicati on of short-term volume regulation in the gill cells of mussels acclim ated to salinity cycling. When exposed to cycling salinity, cell water space consistently increased to approximately 3 ml g(-1) dry mass dur ing the cycle troughs (60% seawater) and returned to approximately 2 m l g(-1) dry mass at the cycle peaks (100% seawater). In mussels acclim ated for 2 weeks to cycling salinity, the gill contents of betaine, ta urine and K+ were unchanged (approximately 240, 230 and 160 mu mol g(- 1) dry mass, respectively) between the 60% and 100% seawater portions of the salinity cycle. The changes in cell volume did not appear to be associated with large perturbations in the functional status of cells , The rate of Oz consumption was approximately 100 mu l O-2 g(-1) dry mass min(-1), and ATP content was approximately 30 mu mol g(-1) protei n, in all salinities to which mussels were exposed. Rates of uptake of taurine, leucine and phenylalanine decreased by approximately 50% dur ing the first sinusoidal decrease to 60% seawater, but recovered follo wing re-exposure to 100% seawater. Uptake rates of all three amino aci ds were unaffected by any subsequent salinity cycles. These results su ggest (1) that the regulation of gill cell volume is normally absent f rom mussels exposed to repeated, gradual salinity changes, and (2) tha t any effects of changes in cell volume are not severe enough to justi fy the energetic expenditure that would be associated with repeated re gulation of cell volume. Unlike the response of gill cells to cycling salinity, there was a decrease in the solute contents of ventricles du ring the salinity troughs compared with the salinity peaks, suggesting that the presence of short-term volume regulation may be more critica l in the ventricle.