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Native Florida Crustacean Predators' Preferences Regarding the
Non-Indigenous Green Mussel, Perna viridis (Linnaeus 1758)
E.L. Mitchem, J.S. Fajans, and S.M. Baker
The invasive green mussel, Perna viridis, was first reported in Tampa Bay, on the west coast of Florida, in
1999 (Benson et al. 2001; Ingrao et al. 2001). To qualitatively assess whether native predators, the blue
crab Callinectes sapidus and the Caribbean spiny lobster Panulirus argus, could be a source of significant
green mussel mortality, we addressed the following questions using multiple-choice experiments: (1) Will
native crustacean predators prey on P. viridis? and, (2) Will these predators prefer native bivalves over P.
viridis? Predators were simultaneously offered equal numbers of three bivalve prey: P. viridis, eastern
oysters, Crassostrea virginica, and hard clams, Mercenaria mercenaria. Consumption was recorded for 13 days.
P. argus did not consume any P. viridis during the experiment, although green mussel shells showed evidence
of chipping along the posterior margin. C. sapidus specimens consumed fifteen P. viridis by the final day of
the experiment. Although the crabs consumed more than twice as many green mussels as hard clams or
oysters, prey species had no statistically significant effect on number consumed (p = 0.2067). We found that (i)
only blue crabs consumed non-indigenous P. viridis; spiny lobsters did not eat any, (ii) the number of green
mussels consumed by blue crabs increased with exposure time, and (iii) blue crabs and spiny lobsters differed in
their modes of prey handling.
The introduction of non-indigenous species through natural, intentional, or accidental transport by humans
threatens the integrity of natural communities (Carlton & Geller 1993) and may cause severe economic
impacts (Ingrao et al. 2001). The state of Florida receives many exotic species introductions and invasions owing
to the state's temperate to subtropical climate and heavy commercial and recreational use (Carlton &
Ruckelshaus 1997). One such new invader is the green mussel, Perna viridis (Linnaeus 1758), a native of the
coastal marine waters of the Indo-Pacific (Benson et al. 2001). Green mussels were first reported in Tampa Bay,
on the west coast of Florida, in 1999 (Benson et al. 2001; Ingrao et al. 2001). Dense assemblages of these
bivalves now clog seawater intakes, weigh down navigational buoys, and foul the hulls and engines of boats.
Green mussels have also been associated with oyster mortalities on reefs throughout Tampa Bay (Baker et al. 2003).
Since its first appearance in Tampa Bay, P. viridis has spread to Florida's coastal waters. Green mussels are
now distributed on the west coast from Anclote Key south to Marco Island. In October 2002, green mussels
were discovered on the east coast of Florida in the Ponce de Leon Inlet (Baker et al. 2003) and they are now
found from the Mosquito Lagoon north to the Georgia-South Carolina border (P. Baker & these authors, in
prep.). Based on physiological tolerances, this invasive species is expected to continue spreading throughout
much of Florida (Baker et al. 2004).
Predators, parasites, diseases, and viruses have been proposed as biological control agents for plant and
animal invaders (Thresher & Kuris 2004). Several predators of shellfish already inhabit the waters of Florida in
the current and potential range of P. viridis. These native predators include the blue crab, Callinectes sapidus,
and the Caribbean spiny lobster, Panulirus argus, both of which are commercially important crustaceans (Perera et
al. 2005, Muller et al. 1997). It is not known if these native predators will readily consume the non-indigenous
green mussel. To qualitatively assess whether these native predators could be a source of significant green
mussel mortality, we addressed the following questions using multiple-choice experiments: (1) Will native
crustacean predators prey on P. viridis? and, (2) Will these predators prefer native bivalves over P. viridis?
MATERIALS AND METHODS
Predator and prey specimens
Specimens of Perna viridis were collected by SCUBA from the 312 Bridge in St. Augustine, Florida on July 14,
2005. Caribbean spiny lobster, Panulirus argus, were collected by SCUBA from the Long Key Viaduct Bridge,
Long Key, Florida on June 24, 2005. Specimens of northern quahog (= hard clam), Mercenaria mercenaria
were purchased from a commercial supplier in Cedar Key, Florida (west coast) on July 12, 2005. Eastern
oysters, Crassostrea virginica, and blue crabs, Callinectes sapidus, were purchased from a retail market in
Ormond Beach, Florida (east coast) on July 12, 2005 and August 4, 2005, respectively. All specimens
were transported to the Department of Fisheries and Aquatic Sciences (FAS), University of Florida (UF),
in Gainesville. Crustacean predators and bivalve prey were placed in separate recirculating holding tanks (624
L) until use in experiments. Seawater (33.4 ppt ï¿½ 1.4 sd) was obtained from the UF Whitney Laboratory
in Marineland, Florida. Filtration systems included a protein skimmer, fluidized bed filter, wet-dry filter,
and ultraviolet sterilization. Temperature was held at 24.2 OC (ï¿½ 0.5 sd) by use of room air conditioners. P.
argus were fed every other day with frozen squid prior to the start of the experiment. Shell lengths (SL) of
bivalve specimens used in the experiments were as follows: green mussels ranged from approximately 8.5 to
11.5 cm; hard clams (littleneck size) ranged from approximately 5.0 to 6.5 cm; oysters ranged from
approximately 8.5 to 10.5 cm. Sizes of crustacean prey used in the experiments were as follows: spiny
lobsters (sub-legal) ranged from approximately 3.5 to 6.5 cm in carapace length; blue crabs ranged
from approximately 12.5 to 15.5 cm in carapace width.
Experiments were conducted in recirculating tanks (624 L) with filtration systems, as described above. P. argus
and C. sapidus feeding preferences were tested separately, with multiple prey species presented together. In
the spiny lobster experiment, begun on July 19, 2005, each of six tanks held seven lobsters each, for a total of
forty-two lobster specimens. Seven P. viridis, seven M. mercenaria, and seven C. virginica were placed in
each lobster tank. Experiments were conducted under a 13:11 h light:dark regime. Tanks were monitored
every twelve hours, at 8 am and 8 pm, for 13 days. At each tank check, water quality parameters
(temperature, salinity, pH, and dissolved oxygen) were measured, and any bivalve eaten was recorded and
replaced with the same species. The blue crab experiment, which began on August 5, 2005, was conducted in
the same manner as above, for 14 days. However, fewer crabs were available; five tanks held five crabs each, for
a total of twenty-five crab specimens. Bivalves were placed in tanks in the same ratio as in the lobster
experiment; five of each species in each tank. During both experiments, bivalve shells were qualitatively
examined for evidence of attempted predation.
For each predator species, a one-way analysis of variance was performed, with tanks as replicates, to test the
null hypothesis that there was no effect of prey species on cumulative number of prey consumed by the final day
of the experiment. If a null hypothesis was rejected, a Tukey test was performed to identify differences
in consumption between specific prey species. Statistical analyses were performed using Microsoft Excel
and StatsDirect version 2.4.3 statistical software (2005).
P. argus did not consume any P. viridis during the experiment (Figure 1A), although green mussel shells
showed evidence of chipping along the posterior margin (Figure 2A). On the morning of the first day, the lobsters
in one tank had consumed one C. virginica; no more oysters were consumed in any of the tanks during
the experiment. Spiny lobster specimens consumed seventeen M. mercenaria by the final day of the
experiment, apparently opening them by chipping large fragments off the posterior margin (Figure 2B).
The cumulative number of hard clams consumed in individual tanks (replicates) ranged from zero to eight.
Figure 1. Photo of shells removed from Panulirus argus tanks. A. Perna viridis shells with evidence
of chipping on posterior margin. Green mussels were not opened or consumed by spiny lobsters.
B. Mercenaria mercenaria shells with broken margins. Hard clams were consumed by spiny lobsters.
-*- P. vds --M ma cenar~f --C vrakca
A Pan uiirus argus
1 2 3 4 5 6 7 9 9 10 1 12 13
B. Callinectes sapidus
1 2 3 4 S 5 7 8 9 10 11 12 13 14
Figure 2. Cumulative consumption of bivalves by crustacean predators. A. Panulirus argus, forty-
two specimens. B. Callinectes sapidus, thirty-five specimens.
Prey species had a significant effect on number of prey consumed (p = 0.0219) by P. argus. Multiple
comparisons indicated that P. viridis were consumed in significantly lower numbers than M. mercenaria (p =
0.0331) and there was no significant difference in consumption between green mussels and C. virginica (p = 0.985).
C. sapidus specimens consumed fifteen P. viridis by the final day of the experiment (Figure 1B). The
cumulative number of green mussels consumed in individual tanks ranged from one to six. Typically, zero to
two green mussels were consumed per day. However, on the final day, six green mussels were consumed in
five tanks. Blue crabs consumed seven M. mercenaria and seven C. virginica over the course of the
experiment. Bivalves consumed by blue crabs had no evidence of shell damage; all empty shells were intact and
still hinged at the umbo. Although the crabs consumed more than twice as many green mussels as hard clams
or oysters, prey species had no statistically significant effect on number consumed (p = 0.2067).
In our study of the feeding preferences of two native crustacean predators, C. sapidus and P. argus, we found that
(i) only blue crabs consumed non-indigenous P. viridis; spiny lobsters did not eat any green mussels, (ii) the
number of green mussels consumed by blue crabs increased with exposure time, and (iii) blue crabs and
spiny lobsters differed in their modes of prey handling.
Non-chelate predators, such as P. argus, typically use the mandibles to chip or bite the edges of bivalve shells
and gain access to the soft parts through the resulting gap (Lau 1987). Although the P. argus in our study
attempted to open green mussels, attempts were unsuccessful and were apparently aborted. Optimal
Foraging Theory predicts that predators should select prey items that maximize net energy intake (Pyke
1984). Crustacean predators may evaluate prey value by assessing prey volume and shape (Mascar6 & Seed
2001). In this case, P. viridis may have been perceived to be of lesser value than M. mercenaria, simply because
of the thinner posterior margin.
Previous exposure to P. viridis may have had an impact on apparent food preference by the crustacean predators.
C. sapidus, collected on the east coast of Florida from the Halifax River (Hull's Seafood Market pers. comm.),
had most likely been previously exposed to green mussels (Baker et al. 2003). P. argus specimens, collected
from the Florida Keys, would have been naive (never exposed to green mussels) but had almost certainly
been exposed to other mussel species including Modiolus modiolus, M. americanus (squamosus according to
Leal 2005) and Brachidontes exustus (Morris 1975). In addition, lobsters in the genus Panulirus are known to prey
on P. perna (Berry 1971) and P. viridis (Radhakrishnan & Vivekanandan 2004). Previous exposure to
mussels, combined with the ability of crustaceans to transfer learned handling skills to novel prey items (Hughes
& O'Brien 2001), suggests that the lack of green mussel consumption by spiny lobsters was the result of chemical
or mechanical (ie: size, shape) inhibition, rather than novelty or insufficient mussel-handling skills.
Blue crabs, which are chelate predators, consumed all three species of bivalves, including P. viridis, without
causing any damage to the shells. This suggests that the blue crabs in our experiment levered the valves
apart. Prying behavior, using the dactyli of the first pereiopods, has previously been described for blue crabs
preying on eastern oysters (Menzel & Nichy, 1958). Other chelate decapods that have been observed employing
this technique include the crabs Liocardinus puber and Cancer pagurus, preying on the mussel Mytilus edulis; and
the crab, Ovalipes punctatus, opening Donax spp. (Cunningham & Hughes, 1984; Du Preez, 1984; ap Rheinallt
& Hughes, 1985; Lau, 1987). Prying is thought to be difficult, often resulting in broken shells and therefore
requiring time-consuming removal of shell fragments (Du Preez, 1984). In this study, blue crabs apparently used
the prying technique exclusively, and with no incidents of shell fragmentation.
During the early days of the experiment, C. sapidus consumed the three bivalve species in approximately
equal numbers, with the two native bivalves making up approximately 2/3 of their diet. On the final day of
the experiment, however, blue crabs consumed more P. viridis in one day (6) than both native species together
in any previous day (maximum total of 2 oysters and/or clams in a single day). Since blue crabs have the ability
to modify foraging behavior depending on experience (Micheli 1995, 1997), our data suggest that blue crabs
may increase their predation rates on green mussels with longer exposure duration.
Crustacean predators can have a significant impact on molluscan populations (Virnstein 1977; Sanchez-Salazar et
al. 1987; Richards et al. 1999) and augmentation of native predators has been suggested as an option for control
of introduced marine pests, such as invasive bivalves (Thresher & Kuris 2004). In this study, we found that
C. sapidus, will consume P. viridis along with native bivalves. This suggests that foraging by blue crabs may
provide a natural control mechanism for green mussels. However, green mussels have high fecundity, grow rapidly
-about 9 mm per month-and may attain sexual maturity within one year (Nair & Appukuttan 2003; Barber et
al. 2005). Therefore, unless blue crab numbers increase greatly, it is unlikely that they could significantly
impact green mussel populations, which can attain densities of 4000 per square meter (Baker & Benson 2002).
P. viridis is expected to continue spreading south along the Gulf Coast of Florida, possibly into the Florida
Keys (Benson et al. 2001), where P. argus is an important fishery species (Muller et al. 1997). While spiny
lobsters inhabit reefs, they are also frequently found around bridge pilings and other man-made structures.
Green mussels also prefer such habitats, entirely covering man-made structures and oyster reefs (these
authors, unpublished data). In this study we found that spiny lobsters were unable or unwilling to consume
green mussels. Therefore, spiny lobsters may be excluded from some habitats if green mussels out-
compete acceptable prey items. This could have negative impacts on the spiny lobster fishery.
Funding for this project was provided to ELM by the University Scholars Program, University of Florida. We would
like to thank the following volunteers who helped with animal collection, data collection, and tank maintenance:
Elise Hoover, Kenneth Black, Michelle Tishler, Shannon Osborn, Jennifer Helseth, Kelley Visentin, Lauren
Murphy, Robert Mitchem, Rebecca Mitchem, and Mary Mitchem.
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