KATIE MCFARLAND, MOLLY RYBOVICH, ASWANI K. VOLETY

F L O R I D A G U L F C O A S T U N I V E R S I T Y, M A R I N E A N D E C O L O G I C A L S C I E N C E S , 1 0 5 0 1 F G C U B LV D , F O RT

M Y E R S , F L

Investigation of Environmental Tolerances of the Invasive Green Mussel,

Perna viridis, to Predict the Potential Spread in Southwest Florida

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 reach osmotic equilibrium with the external environment at salinities of 5 and 10 ppt after 1 week of exposure

Oysters reached equilibrium at 10 ppt and above within 24 hours and as low as 5 ppt within the week exposure Well adapted for low salinities

prevailing in SW Florida summers

0 20 40 60 80 100 120 140 160

0

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1000

0 20 40 60 80 100 120 140 160

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1000

0 20 40 60 80 100 120 140 160

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0 20 40 60 80 100 120 140 160

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0 20 40 60 80 100 120 140 160

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0 20 40 60 80 100 120 140 160

0

200

400

600

800

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Green mussels

Oysters

Water

Time (Hours)

Osm

ola

lity

(mO

sm /

L)

5 ppt 10 ppt

15 ppt 20 ppt

30 ppt25 ppt

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

Time (days)

0 10 20 30 40 50 60

% S

urvi

val

0

20

40

60

80

100

5 ppt E

10 ppt E

15 ppt D

20 ppt A

25 ppt A

30 ppt B

35 ppt C

Poor survival below 20 ppt and 100% mortality at 5 and 10 ppt

Gradual Salinity Decrease

≥97% survival at salinities of 9 ppt and above

After only 13 days at 3 ppt 100% mortality was observed

Time (days)

0 10 20 30 40 50

Sur

viva

l (%

)

0

20

40

60

80

100

30 ppt

27 ppt

24 ppt

21 ppt

18 ppt

15 ppt

12 ppt

9 ppt

6 ppt

3 ppt

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

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

B C A

Desiccation: High Temperatures

97% mortality in green mussels while oysters showed only 4% mortality

Both showed similar internal temperatures, but mussels died with increasing frequency as temperature increased

0

10

20

30

40

50

0 100 200 300 400 500

Inte

rn

al

Te

mp

(˚C

)

Elapsed Time (Minutes)

Average GM Internal Temp Average O Internal Temp

0102030405060708090

0 10 20 30 40

Cu

mu

lati

ve

Mo

rta

lity

(%

)

Internal Temperature (OC)

Green Mussels

Oysters

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 Slowed growth Decreased juvenile recruitment

Brevetoxin ELISA’s showed an accumulation of toxins in the tissues

Lack of sufficient co-evolution period between Perna viridis and

Karenia brevis

Trophic Transfer?

Month

Feb Mar April June Aug

Ave

rage

Bre

veto

xin

Con

cent

ratio

n (n

g/g)

0

5e+4

1e+5

2e+5

2e+5

3e+5

3e+5

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. Journal of Shellfish Research 20:21-30.

Estero Bay Aquatic Preserve and Florida Department of Environmental Protection, March 2013. Estero Bay Aquatic Preserve: Management Plan. (Award No. NO11NOS4190077-CM227)

Firth, L. B., A. M. Knights, and S. S. Bell. 2011. Air temperature and winter mortality: Implications for the persistence of the invasive mussel,< i> Perna viridis</i> in the intertidal zone of the south-eastern United States. Journal of Experimental Marine Biology and Ecology 400:250-256.

Ingrao, D. A., P. M. Mikkelsen, and D. W. Hicks. 2001. Another introduced marine mollusk in the Gulf of Mexico: the Indo-Pacific green mussel, Perna viridis, in Tampa Bay, Florida. Journal of Shellfish Research 20:13-19.

Urian, A. G., J. D. Hatle, and M. R. Gilg. 2011. Thermal constraints for range expansion of the invasive green mussel, Perna viridis, in the southeastern United States. Journal of Experimental Zoology Part A: Ecological Genetics and Physiology 315:12-21.

Vakily, J. 1989. The biology and culture of mussels of the genus Perna. The World Fish Center.