Investigation of Environmental
Tolerances of the Invasive Green Mussel,
Perna viridis, to Predict the Potential
Spread in Southwest Florida
K AT I E M C FA R L A N D , M O L LY RY B O V I C H ,
A S WA N I K . VO L E T Y
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 R T
MYERS, 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
•
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)
Bay Aquatic Preserve Management Plan, March 2013)
– Winter: 28-38 ppt
– Summer: 0-10 ppt
•
Temperature remains fairly stable ranging with averages from 16 - 32˚C (Barnes, et al.,
2007)
• 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
 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
1000
800
800
600
600
400
400
200
200
10 ppt
5 ppt
0
0
0
Osmolality (mOsm / L)
reach osmotic equilibrium
with the external environment
at salinities of 5 and 10 ppt
after 1 week of exposure
1000
20
40
60
80
100 120 140 160
1000
1000
800
800
600
600
400
400
200
0
20
40
60
80
100 120 140 160
0
20
40
60
80
100 120 140 160
200
15 ppt
0
20 ppt
0
0
20
40
60
80
100 120 140 160
1000
1000
800
800
600
600
400
400
200
200
25 ppt
0
30 ppt
0
0
20
40
60
80
100 120 140 160
0
Time (Hours)
20
40
60
80
100 120 140 160
Green mussels
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
% Survival
80
5 ppt
10 ppt
15 ppt
20 ppt
25 ppt
30 ppt
35 ppt
60
40
E
E
D
A
A
B
C
20
0
0
10
20
30
Time (days)
40
50
60
Gradual Salinity Decrease
 ≥97% survival at salinities of 9 ppt and above
 After only 13 days at 3 ppt 100% mortality was observed
100
Survival (%)
80
30 ppt
27 ppt
24 ppt
21 ppt
18 ppt
15 ppt
12 ppt
9 ppt
6 ppt
3 ppt
60
40
20
0
0
10
20
Time (days)
30
40
50
Methods: Desiccation
 Green mussels and oysters exposed
to desiccation under direct sunlight
(intertidal exposure), under shade
(mangrove canopy) or underwater
(subtidal) conditions for 0, 2, 4, 6,
and 8 hours
A
B
 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
C
 97% mortality in green
mussels while oysters
showed only 4% mortality
Cumulative Mortality
(%)
Desiccation: High Temperatures
90
80
70
60
50
40
30
20
10
0
Green Mussels
Oysters
0
10
20
30
Internal Temperature (OC)
40
 Both showed similar
internal temperatures, but
mussels died with increasing
frequency as temperature
increased
Internal Temp (˚C)
50
40
30
20
10
0
0
100
200
300
400
Elapsed Time (Minutes)
Average GM Internal Temp
Average O Internal Temp
500
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
3e+5
Average Brevetoxin Concentration (ng/g)

3e+5
2e+5
2e+5
1e+5
5e+4
0
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
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