Effect of acute salinity changes on hemolymph osmolality and clearance rate of the non-native mussel, Perna viridis, and the native oyster, Crassostrea virginica, in Southwest Florida

The green mussel Perna viridis is a recent invader to Southwest Florida and, though it is currently found only in high salinity areas, concerns abound that it may threaten native oysters. The objective of this study was to compare the responses of P. viridis and the native eastern oyster, Crassostrea virginica, to acute salinity changes by measuring hemolymph osmolality and clearance rate of algal cells over time. The osmolality of hemolymph in vivo and surrounding water were assessed regularly over a one-week period at seven test salinities ranging from 5 to 35. While oysters reached osmotic equilibrium at all salinities within 144 hours, hemolymph of green mussels remained hyperosmotic at salinities <= 10. Clearance rates of algae by P. viridis and C. virginica held in static tanks at four salinities (10, 15, 25, and 35) were measured, employing flow cytometry. At salinities of 25 and 35, green mussel clearance rates were approximately double those of oysters. Unlike native oysters, green mussel clearance rates decreased by an order of magnitude at salinities of 10 and 15. Further, at salinities of 10 and 15, P. viridis tended to close their valves. In a specific test of this behavior, 100% of mussels remained open at salinities of 25 and 35. At salinities of 10 and 15, mussels increasingly closed their valves over time, and within 120 hours of exposure all were either closed or dead. The chief concern about P. viridis is that it might compete with native bivalves for food and space. However, our results suggest that this recent invader may be salinity-limited, providing C. virginica with a refuge from competition in estuaries that experience acute periods of low salinity.