Abstract
The green mussel, Perna viridis, is a recent invasive species to the coastal waters of southwest Florida. Native to the Indo-Pacific it is a pesky biofouling organism ranging from coastal marine waters to high salinity estuaries. While it is currently only found in high salinity, sub-tidal areas in southwest Florida region, it is of concern that they may threaten native estuarine oyster (Crassostrea virginica) reefs by competing for food and substrate. The objective of this study was to assess the ability of the invasive green mussel to adapt to the salinity conditions prevailing in the estuaries of southwest Florida as compared to native oysters and to determine if green mussels potentially compete for resources with oysters by examining osmolality, clearance rates and survival in response to decreased salinities. Osmolality was assessed by exposing organisms to an acute salinity change from 30ppt to treatment salinities of 35, 30 (control), 25, 20, 15, 10, or 5ppt with three replicate tanks per salinity. Hemolymph and water was sampled regularly over a one week period and analyzed in a vapor pressure osmometer to assess changes in internal osmolality over time. The results showed an inability of the mussels to adapt at salinities below 15ppt while oysters were at equilibrium at all salinities with one week. Clearance rates of bivalves held at 30 ppt under laboratory conditions and exposed to an acute salinity change 35, 25, 15, and 10 ppt were examined after a 2 day acclimation period to treatment salinity with three replicates per treatment (N = 5 for each oysters and mussels). Bivalves were exposed to an initial concentration of the algae Isochrysis sp. At 20,000 cells mL-1 in static system. Water samples were taken every 15 minutes for one hour and analyzed using a flow cytometer to obtain cell counts for the calculation of clearance rates over time, Clearance rates of P.viridis (range 0.41 to 4.38 L h-1g-1 dry tissue weight) were two to three times that of C. virginica (range 0.09 to 2.56 L h-1g-1 dry tissue weight), however green mussels showed a signitificant decline in clearance rate at the lower salinities (10 and 15 ppt) while no statistical difference was observed between salinities for oysters clearance rates. To determine salinity tolerances bivalves were exposed to acute salinity changes between 5 – 35 ppt. Acute salinity changes resulted in poor survival of P. viridis at salinities below 10ppt (0%). However, gradual salinity changes (3 ppt / 2 days) resulted in excellent survival of green mussels at salinities 9 ppt (97%). Overall, P. viridis showed poor survival at salinities below 10 ppt compared to oysters. Results suggest that while green mussels are less tolerant of lower salinities ( 10 ppt), with gradual salinity changes they may invade estuarine portions, and given their higher clearance rates, may compete with oysters for resources such as food and substrate. However, results also suggest that high amounts of freshwater releases into the estuaries that will result in salinities 15 ppt for one week can be used as an adaptive strategy to control green mussel spread into the estuaries where native oysters thrive.