Investigation of Environmental Tolerances of the Invasive Green Mussel, Perna viridis, to Predict the Potential Spread in Southwest Florida KATIE MCFARLAND, MOLLY RYBOVICH, ASWANI K. VOLETY F L O R I D A GULF COAST UNIVERSITY, MARINE AND ECOLOGICAL SCIENCES, 10501 FGCU BLVD, FORT M Y E R S , FL Invasion of the Green Mussel Native to the Indo-Pacific (Vakily, 1989) Subtidal Tropical to subtropical First observed in Tampa Bay in 1999 (Benson et al., 2011; Ingrao et al., 2001) Ballast water and/or biofouling from ships coming to port from the Caribbean Free swimming larval stage has allowed for a rapid spread throughout Southwest Florida including Estero Bay Invasive species can pose a serious threat to ecosystems and infrastructure Biofouling organisms coat boat hulls, docks and pilings Compete with local bivalves for substrate and food Competition with Oysters Oysters form permanent 3-dimensional habitat essential to many economically and ecologically important species of fish and crab Oyster reefs form natural break walls that help prevent erosion and increase sedimentation Green mussels form more of a 2-dimensional mat over hard substrate and disarticulate upon death Tampa Bay observed a nearly 50% displacement of the oyster population upon the arrival of the green mussel (Baker et al., 2006) While locally green mussels are currently primarily found in the more marine portions of the estuary, some isolated individuals have been found on reefs within the bay Local Watershed and Environmental Characteristics • Shallow estuaries (average of 3 feet) allow for rapid environmental changes (Estero Bay Aquatic Preserve Management Plan, March 2013) • Temperature remains fairly stable ranging with averages from 16 - 32˚C (Barnes, et al., 2007) • Desiccation stress: With the already shallow water and most hard substrate in the intertidal region organism living on oyster reefs must be adapted to periods areal exposure • Extreme wet and dry seasons cause drastic variations in salinity (Barnes, et al., 2007) – Winter: 28-38 ppt – Summer: 0-10 ppt • Anthropogenic forces have drastically altered the watersheds of Southwest Florida estuaries - Estuaries have gained more tributaries many of which run through areas of increased urbanization Objectives Understand environmental boundaries of the invasive green mussel and predict the potential for spread Salinity Survival is a clear indication of environmental limits But sub-lethal effects can also limit the spread of a new species Desiccation Are green mussels capable of occupying hard substrate in the shallow waters of Southwest Florida estuaries? Do green mussels pose a threat to our native oyster reefs? How can we reduce this risk to oysters? Methods: Physiological Response to Decreased Salinities Osmolality An acute salinity change for both oysters and green mussels (5, 10, 15, 20, 25, 30, 35 ppt) Hemolymph was drawn at T = 0, 1, 4, 8, 12, 24, 48, 96, 120 hours Hemolymph osmolality of bivalves was compared with that of the exposure seawater using a vapor pressure osmometer Clearance Rate Clearance rates were measured for both oysters and green mussels following an acute salinity change (10, 15, 25, 35 ppt) Bivalves were fed the phytoplankton T. iso in a static system Algal cell concentration monitored over time using flow cytometery Internal Osmolality at Decreased Salinities Green mussels were unable to 1000 1000 800 800 reach osmotic equilibrium 600 600 with the external environment 400 400 200 200 5 ppt 10 ppt 0 0 at salinities of 5 and 10 ppt 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160 after 1 week of exposure 1000 1000 800 800 600 600 400 400 Oysters reached equilibrium 200 200 15 ppt 20 ppt 0 0 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160 at 10 ppt and above within 24 Osmolality (mOsm / L) 1000 1000 hours and as low as 5 ppt 800 800 600 600 within the week exposure 400 400 200 200 Well adapted for low salinities 25 ppt 30 ppt 0 0 prevailing in SW Florida summers 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160 Green mussels Time (Hours) Oysters Water Changes in Clearance Rates in response to an Acute Salinity Decrease Green mussels showed a significant decrease in clearance rates at salinities of 10 and 15 ppt compared to optimal salinities of 25 and 35 ppt Oysters did not show any significant differences in clearance rate at all salinities Methods: Survival in Response to Decreased Salinity Exposures Acute Salinity Changes Salinity adjusted from 30 ppt to 5, 10, 15, 20, 25, 30 (control), 35 ppt in triplicate tanks (N=20 / tank) Test conditions were maintained for 56 days with survival monitored daily Gradual Salinity Change Salinity was adjusted gradually from 30 ppt by 3 ppt every other day to final salinities of 30 (control), 27, 24, 21, 18, 15, 12, 9, 6, 3 ppt in triplicate tanks (N=20 / tank) Final salinities were maintained for an additional 28 days Acute Salinity Decrease Poor survival below 20 ppt and 100% mortality at 5 and 10 ppt 100 80 5 ppt E 10 ppt E 60 15 ppt D 20 ppt A 25 ppt A 30 ppt B 35 ppt C % Survival 40 20 0 0 10 20 30 40 50 60 Time (days) Gradual Salinity Decrease ≥97% survival at salinities of 9 ppt and above After only 13 days at 3 ppt 100% mortality was observed 100 80 30 ppt 27 ppt 60 24 ppt 21 ppt 18 ppt 15 ppt Survival (%) Survival 40 12 ppt 9 ppt 6 ppt 20 3 ppt 0 0 10 20 30 40 50 Time (days) Methods: Desiccation Green mussels and oysters exposed to desiccation under direct sunlight (intertidal exposure) or underwater (subtidal) conditions for 0, 2, 4, 6, and 8 hours A B C Internal temperature of organisms measured using a temperature probe inserted into the shell cavity through a narrow hole that was sealed after insertion External temperature measured using an aquarium thermometer Survival of oysters and green mussels were noted in both experimental and control treatments and expressed as cumulative mortality Desiccation: High Temperatures 90 80 Green Mussels 97% mortality in green 70 Oysters 60 50 mussels while oysters 40 30 (%) 20 showed only 4% mortality 10 0 0 10 20 30 40 Cumulative Mortality Internal Temperature (OC) 50 ) 40 Both showed similar 30 Temp (˚C Temp internal temperatures, but 20 10 Internal Internal mussels died with increasing 0 0 100 200 300 400 500 frequency as temperature Elapsed Time (Minutes) increased Average GM Internal Temp Average O Internal Temp Desiccation: Low temperatures Field observations in the winter of 2012 High numbers of juvenile recruitment was observed in the intertidal zone in December of 2011 A month later in January 2012 found dead Lab experiments documented in the literature confirm an intolerance of P. viridis to desiccation under cold air temperatures (Firth et al., 2011; Urian et al., 2011) Red Tide Blooms Previously documented die offs following Red Tide blooms in Tampa Bay (Baker et al., 2012) Field monitoring in Estero Bay (March 2011 – current): During periods of Red Tide observed: Increased mortalities 3e+5 Slowed growth 3e+5 Decreased juvenile recruitment 2e+5 2e+5 Brevetoxin ELISA’s showed 1e+5 5e+4 an accumulation of toxins in 0 Average Brevetoxin Concentration (ng/g) Brevetoxin Average the tissues Feb Mar April June Aug Month Lack of sufficient co-evolution period between Perna viridis and Karenia brevis Trophic Transfer? In Conclusion: What is the Potential Threat? Salinity is a limiting factor When the change is a acute P. viridis is unable to adapt If the change is gradual P. viridis may be capable of pushing into lower salinity regions of the estuary Desiccation With the shallow waters of Estero Bay, P. viridis is unlikely to be able to populate the intertidal zone Air temperatures can be significantly lower than winter water temperatures However, deep estuaries may be at risk even if salinities drop as low as 15ppt Oysters were able to adapt to all test salinities and showed high tolerance to desiccation stress even at high internal temperatures Well adapted to harsh conditions in SW Florida estuaries Will likely remain the dominant intertidal bivalve Acknowledgements Funding: U. S. Department of Education U. S. EPA Marco Island Shell Club South Florida Water Management District Technical and field support: Vester Marine Field and Research Station Coastal Watershed Institute Lesli Haynes, Robert Wasno, Jeffrey Devine, David Segal Rheannon Ketover, Julie Neurohr and Brooke Denkert. References Baker, P., J. S. Fajans, W. S. Arnold, D. A. Ingrao, D. C. Marelli, and S. M. Baker. 2007. Range and dispersal of a tropical marine invader, the Asian green mussel, Perna viridis, in subtropical waters of the southeastern United States. Journal of Shellfish Research 26:345-355. Baker, P., J. S. Fajans, and S. M. Baker. 2012. Habitat dominance of a nonindiginous tropical bivalve, Perna viridis (Linnaeus, 1758), in a subtropical estuary in the Gulf of Mexico Journal of Molluscan Studies 78:28-33. Baker, P., J. S. Fajans, S. M. Baker, and D. Bergquist. 2006. Green mussels in Florida, USA: Review of trends and research. World Aquaculture 37:43-67. Barnes, T., A. Volety, K. Chartier, F. Mazzotti, and L. Pearlstine. 2007. A habitat suitability index model for the eastern oyster (Crassostrea virginica), a tool for restoration of the Caloosahatchee Estuary, Florida. Journal of Shellfish Research 26:949-959. Benson, A. J., D. C. Marelli, M. E. Frischer, J. M. Danforth, and J. D. Williams. 2001. Establishment of the green mussel, Perna viridis (Linnaeus 1758)(Mollusca: Mytilidae) on the west coast of Florida.