Paracellular solute uptake by the freshwater zebra mussel Dreissena polymorpha

A hyperosmotic solution of mannitol or glucose (100 mM) in pond water caused an increase in paracellular solute movement between the bathing medium and body fluids of Dreissena polymorpha. Small molecules (< 5,000 Da) in the bath entered the mussel, and 80-85% of the sodium and chloride in the blood was lost within 12 h. Blood total solute was elevated within 4 h of exposure to hyperosmotic conditions, but the rise was attributed to the gain of glucose or mannitol from the bath and not to an elevation of ion concentration as a result of the osmotic loss of water. Lanthanum in the bathing solution was able to penetrate the paracellular junctional complex between gill epithelial cells in mussels exposed to hyperosmotic conditions but was rarely observed in pond water-acclimated animals. Colloidal gold (6 nm diam) was unable to penetrate the paracellular space but was accumulated in endocytotic vesicles in many epithelial cells. The "leakiness" of the epithelial tissue may be a critical factor in the low blood solute concentrations in freshwater mussels despite high rates of ion transport in these animals.