Cellular volume regulation in freshwater bivalves

The effect of ambient osmolality on the height of lateral ciliated cells from the gills of two freshwater bivalve species (Dreissena polymorpha, Toxolasma texasensis) was directly observed microscopically. The addition of 1 mmol·l^-1 KCl to an artificial pondwater (APW) superfusion medium resulted in an increase in cell height. When the superfusion solution was made hyperosmotic (~90 mmol·kg^-1 H₂O) by the addition of 45 mmol·l^-1 NaCl to APW, the cell height decreased by about 20-30% and there was no evidence of a regulatory volume increase over 20-30 min. In contrast, when 1 mmol·l^-1 KCl was added to the hyperosmotic medium the cell height always partially (40-50%) recovered. When the gill tissue was returned to APW following the hyperosmotic treatment the cells returned to the original cell height. Bivalve gills superfused with the hyperosmotic NaCl and KCl solution in the presence of 1 mmol·l^-1 ouabain experienced a similar 25% decrease in cell height. When the ouabain-treated tissues were returned to APW the cells swelled, overshooting the original cell height. These results indicate these fresh-water bivalves have a limited ability for cellular volume regulation using inorganic ions, but depend on a suitable balance of Na⁺ and K⁺ in the environment to effect regulatory volume changes.