Osmotic and Ionic Regulation of North American Zebra Mussels (Dreissena polymorpha)

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Abstract
SYNOPSIS. Unlike other freshwater bivalves that survive for months in deionized water, Dreissena polymorpha requires minimal concentrations of Na, K, Mg, and Cl in the bathing medium for long-term survival. Although ion transport rates are higher in D. polymorpha compared to other freshwater bivalves, they tend to have lower blood solute concentrations. D. polymorpha has an unusually “leaky” epithelium with a high paracellular permeability to solutes. Thus, even with high transport rates, it may not be possible for zebra mussels to retain higher blood solutes because of the extensive passive loss of ions. Under a hyperosmotic stress, D. polymorpha will rapidly osmoconform (about 12 hr) due primarily to the diffusion of solutes and partially to the osmotic loss of water. D. polymorpha is not capable of surviving an imbalance of Na/K in the external medium. In the absence of K the cells will tend to lose volume to achieve isosmotic balance with the blood, but the animals usually die within a few days. If D. polymorpha is exposed to excess K in the environment (1 mM), they will accumulate K in the blood. If the K enters the cells, cellular volume would expand due to increase in osmolyte concentration, yet, if K remains in the blood, there will be an electrochemical imbalance. In either case, the animal cannot survive much longer than a day. When Na and K are present in the medium in a balanced combination approximated by artificial seawater (ASW), D. polymorpha will survive an acute transfer to 100 mosm ASW indefinitely (months). Our preliminary studies have shown that D. polymorpha will tolerate step-wise acclimation to solutions >250 mosm provided the changes in salinity do not exceed 50–100 mosm. Freshwater bivalves, unlike the marine bivalves, have limited free amino acids in their body fluids and must rely on inorganic ions for osmotic regulation. The free amino acids serve as an important osmolyte buffer for volume regulation when an animal experiences an environment of changing salinity. The inability of Dreissena, and perhaps other freshwater bivalves, to tolerate hyperosmotically induced dehydration may be due, in part, to the inability to accumulate or retain sufficient intracellular K to facilitate regulatory volume adjustments.

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