Osmoregulation in Dreissena polymorpha: the Importance of Na, Cl, K, and Particularly Mg

Zebra mussels (Dreissena polymorpha) are unusual in that they cannot survive in Mg-deficient water. Analysis of blood samples from mussels obtained in the field indicated a Mg concentration of 1.5-2.0 mM immediately after animal collection. However, Mg concentration in the blood decreased rapidly when the mussels were transferred to Mg-free artificial pondwater (PW); the t1/2 was 24 h. Blood Mg decreased to the limits of detection within 2 weeks, and the time to 50% mortality was about 17 days in Mg-free PW. When Mg-depleted specimens of D. polymorpha were returned to PW containing Mg, the net flux was 3 μmol Mg (g dry tissue.h)-1, and blood Mg concentration was restored within a day to 0.4-0.6 mM. Mussels depleted of Mg did not survive beyond 51 days. When mussels were acclimated to K-free pondwater (containing Mg), their osmoregulatory ability was impaired, and the total solute of the blood dropped from 30-36 to 21-24 mosm, with blood Na and Cl concentrations declining 30-50%. This ion-depleted condition was reversed within 45 h upon return of K to the pondwater bathing medium. D. polymorpha individuals were unable to survive beyond 5 days in deionized water and required minimal concentrations of Na, Cl, K, and Mg for prolonged storage (>51 days) under laboratory conditions. Mussels survived Ca-deficient solutions for more than 51 days, presumably because they were able to mobilize Ca from internal stores (shell) to maintain blood calcium at 1 mM.