Effects of hyperosmotic challenge on the freshwater bivalveDreissena polymorpha: importance of K+

The effects of elevated ion concentrations in the bathing medium on ionoregulation and mortality rate were examined in the freshwater bivalve Dreissena polymorpha. The survival of mussels in saline water depended on the balance of Na+and K+in the medium. Animals acclimated to artificial pond water tolerated acute transfer to 108 mosmol artificial seawater (3.5‰) for months with virtually no mortality. Dreissena polymorpha rapidly (~ 12 h) became isosmotic to the bathing medium when exposed to approximately 100 mosmol hyperosmotic solution composed of 45 mM NaCl or 3.5‰ artificial seawater. This was primarily due to a rapid accumulation of ions with some osmotic loss of water. During a hyperosmotic challenge the intracellular compartment gained K+but mussels required exogenous K+to maintain an intracellular to extracellular gradient similar to that of control animals. The epithelia of D. polymorpha were remarkably "leaky" to solutes. Under hyperosmotic conditions, sucrose, glucose, and inulin penetrated the epithelial tissue and appeared in the blood. The principal route of solute entry was likely paracellular pathways. However, the estimated net movement of sucrose or glucose was less than 2% of the net flux of NaCl. Dreissena polymorpha have the capacity to tolerate oligohaline water where sufficient K+is available to promote cellular homeostasis.