SALINITY CHANGE AND CELL-VOLUME - THE RESPONSE OF TISSUES FROM THE ESTUARINE MUSSEL GEUKENSIA-DEMISSA

The response of cell volume to changes in external salinity was assess ed in four tissues (gill, mantle, hemolymph cells and ventricle) of th e estuarine mussel Geukensia demissa by using one or more of the follo wing three indicators of cell volume response: changes in cell dimensi ons, cell water space and cell solute content, All three techniques in dicated that short-term volume regulation was generally absent from gi ll tissue. Lateral cell height in gills, measured using differential i nterference contrast (DIG) microscopy, increased by approximately 20% after an abrupt exposure to reduced salinity (60% artificial sea water , ASW). There was significant variability in the observance of a regul atory volume decrease (RVD) subsequent to the initial swelling; cells remained swollen for Ih after low-salinity exposure in two-thirds of t he trials, while there was a return of cell volume towards control val ues in the remaining one-third of the trials. Lateral cell height incr eased linearly when salinity was gradually decreased from 100 to 60% A SW over 135 min. Cell height then returned to control values when the salinity was abruptly returned to 100% ASW, indicating that an RVD was not elicited by a slow change in salinity of the type normally encoun tered by estuarine mussels. Cumulative cell water space in gills incre ased by 47% after exposure to 60% ASW and the cells remained swollen f or at least 4 h, returning to control values when gills were returned to 100% ASW. Consistent with the overall lack of an RVD, there was onl y a small decrease (approximately 5%) in cumulative osmolyte content ( primarily taurine, betaine and K+) after 4 h in 60% ASW. Decreases in both cell water space and osmolyte content after 3 weeks of acclimatio n to 60% ASW indicated a long-term RVD of approximately 60%. Individua l cells in the mantle epithelium also generally lacked an RVD in respo nse to lowered salinity. Both abrupt and gradual decreases In salinity caused an increase in mantle cell height to a maximum of 25-30%, and cell height returned to the control height when salinity was abruptly returned to 100% ASW. Corresponding with the lack of an RVD in individ ual mantle cells, there was no change in solute content of the mantle tissue after 4 h of exposure to low salinity. The response of the volu me of spherical hemolymph cells to 1 h of abrupt exposure to low salin ity, calculated from measured cell diameters, likewise indicated that an RVD is generally lacking in these hemolymph cells. In the ventricle , however, there was a significant decrease in amino acid and betaine content after 4 h of exposure to low salinity, suggesting tissue-Speci fic variability in the cellular response to salinity change. The consi stent lack of a short-term RVD in many tissues may serve to avoid larg e energetic expenditures associated with repeated volume regulation in the face of the frequent, short-term changes in salinity encountered by estuarine mussels.