RESPONSE OF CELL-VOLUME IN MYTILUS GILL TO ACUTE SALINITY CHANGE

The response of gill cell volume in Mytilus californianus and Mytilus trossolus (=edulis) to acute changes in salinity was assessed using th ree independent indicators: optical measurement of lateral cell height , measurement of intracellular water content using radiolabeled tracer s and measurement of the contents of the major osmolytes of the gills. Optical measurements indicated significant variation in the response of individual lateral cells of M. californianus to acute law-salinity shock. Lateral cell height increased by approximately 20 % shortly aft er abrupt exposure to 60 % artificial sea water (ASW). Following this initial swelling, we estimate that a substantial regulatory volume dec rease (RVD) was present in 25 % of the trials. More commonly, however, an RVD was either absent or minimal: cell height remained elevated fo r at least 1 h, then returned to the control height when gills were re -exposed to 100 % ASW, Changes in the combined water space of all cell s in the gill, measured as the difference between total water space an d extracellular space ([C-14]polyethylene glycol space), indicated tha t cell volume regulation in the gill as an organ was also absent or mi nimal. Cell water space was 2.16 ml g(-1) dry mass in isolated gills o f M. californianus acclimated to 100% sea water in the laboratory and increased to 2.83 ml g(-1) dry mass after a 6 min exposure to 60 % ASW . Cell water space was still 2.81 ml g(-1) dry mass after 1 h in 60 % ASW and returned to 2.06 ml g(-1) dry mass upon re-exposure to 100 % A SW. Consistent with these observations, the gill contents of the princ ipal cytoplasmic osmolytes (taurine, betaine and K+) were unchanged (a pproximately 450, 250 and 230 mu mol g(-1) dry mass,, respectively) fo llowing exposure of gills from 100 % ASW-acclimated mussels to 60 % AS W, A decrease in cell water space to 2.66 ml g(-1) dry mass after 4 we eks of acclimation to 60 % ASW corresponded with a 37% decrease in bet aine content; taurine and K+ contents were unchanged. The changes in w ater space and solute content of gills from freshly collected M. calif ornianus and M. trossolus were also consistent with the absence of vol ume regulation; cell water space remained elevated for at least 1 h af ter low-salinity exposure, and solute contents were unchanged after th is period. We calculated the potential energetic cost of cell volume r egulation for mussels exposed to 12 h of sinusoidal fluctuations betwe en 100 % and 50 % sea water; solute uptake for full volume regulation in all tissues would cost a minimum of approximately 30 % of the stand ard metabolic rate during the period of salinity increase, The routine absence of substantial cell volume regulation in Mytilus gill may ref lect the potentially high energetic cost of volume regulation in the f ace of the large and frequent salinity fluctuations that are regularly encountered by estuarine bivalves.