1. INFLUENCES OF PREDATORS ON PARASITE PREVALENCE
Meghan K. Tait, Jennafer C. Malek, James E. Byers,
Odum School of Ecology, University of Georigia, Athens, GA
Species interacLons can be important drivers in host-parasite
relaLonships. One of the most influenLal species interacLon is
predaLon. Predators can directly and indirectly affect prey
populaLons (Connell 1970) and thus substanLally influence
host-parasite relaLonships. One example of this influence is
the ‘healthy herd’ hypothesis, which suggests that predators
preferenLally feed on infected hosts and remove them from
the populaLon, thus lowering infecLon prevalence and
providing posiLve feedbacks for the host populaLons by
reducing parasite transmission (Packer et al. 2003).
Conversely, predators can increase parasite prevalence within
a host populaLon either through preferenLally feeding on
uninfected prey or acLng as ‘predator spreaders’ that increase
parasite dispersal when they consume infected hosts (Caceres
et al. 2009). In this study we invesLgated if the blue crab,
Callinectes sapius, exhibits preferenLal selecLon of eastern
oysters, Crassostrea virginica, infected by one of its most
prevalent parasites, Perkinsus marinus.
Trials Presen+ng One Uninfected
and One Infected Oyster
Crab ID Number of
Uninfected
Oysters
Selected
Number of
Infected
Oysters
Chosen
BC2 4 6
BC7 3 3
BC8 1 2
BC9 1 1
BC11 1 0
BC13 0 2
0
1
2
3
4
5
6
7
BC2 BC7 BC8 BC9 BC11 BC13
Number of Oysters Chosen
Crab ID
Trials Presen+ng One Infected and
One Uninfected Oyster
Uninfected
Infected
Discussion
Predators can have mulLple effects on host-parasite
relaLonships through preferenLal feeding. Though predator
influences through the healthy herd hypothesis and the
predator spreader hypothesis (Packer et al. 2003, Caceres et
al. 2009) have been documented, we found that blue crabs
do not influence the host-parasite relaLonship between
oysters and one of its most lethal parasites in either one of
these ways. Rather, we found that the blue crab predator
shows no preferenLal selecLon of oysters based on infecLon
status and thus, has no effect on parasite prevalence of P.
marinus in oysters. Blue crabs may lack the ability to
disLnguish between infected and uninfected oysters or they
may simply be indifferent to infecLon status. By evaluaLng
predator effects on oyster-parasite relaLonships, we have
helped expand our understanding of the influence that the
bioLc environment has on host-parasite systems.
0
10
20
30
40
50
60
Time (min)
Average Selec+on Time Between
One Infected and One Uninfected
Oyster
Uninfected
Infected
We also calculated the average
selecLon Lme for oysters as a
funcLon of infecLon status. We ran
a linear model using selecLon Lme
as our response variable and oyster
infecLon status and crab ID as our
predictor variables. We found no
significant difference in selecLon
Lme between oysters of different
infecLon status.
To test for preferenLal feeding by C. sapius on infected oysters,
we set up two experimental mesocosms in the lab. A GoPro Hero
300 was secured at the top of one end of each mesocosm to
observe crab feeding. For each trial, two oysters of similar length
were marked and placed in the water at one end of mesocosm
and a crab was placed at the other end. The GoPro was then
turned on and the start Lme was recorded. The behavior of the
crab was observed remotely from another room on an iPad
connected to the GoPro’s video feed to limit outside disturbances.
Once a crab was observed breaking into the shell and consuming
Lssue, the oysters were removed, and immediately tested for the
presence and intensity of P. marinus infecLon. We used the Ray’s
fluid thioglycollate medium (RFTM) method to assess P. marinus
infecLon in the gill, mantle, and rectal Lssue of the oysters (Ray
1954). If no oysters were consumed, the trial was disregarded.
When consumpLon occurred, trials in which one oyster was
infected and one was uninfected were used to examine whether
infecLon status affected prey choice. These trials, along with
those in which both oysters had the same infecLon status, were
used in separate subsequent analyses of prey selecLon Lme.
Methods
References
Cáceres CE, Knight CJ, Hall SR (2009) Predator–spreaders:
PredaLon can enhance parasite success in a planktonic host–
parasite system. Ecology 90:2850–2858
Connell JH (1970) A predator-prey system in the marine
interLdal region. I. Balanus glandula and several predatory
species of Thais. Ecol Monogr 40:49–78
Packer C, Holt RD, Hudson PJ, Lafferty KD, Dobson AP (2003)
Keeping the herds healthy and alert: implicaLons of predator
control for infecLous disease. Ecol Leh 6:797–802
Ray SM (1954) Biological studies of Dermocys<dium marinum, a
fungus parasite of oysters. Rice InsLtute Pamphlet Special Issue,
November 1954
Pictures:
Schepker, Nathan (2015) Blue Crab. NaLonal Geographic
Werthschulte, Moritz (2015) Oyster. NaLonal Geographic
Results
There were 24 trials across six crabs which contained one infected oyster and one
uninfected oyster. We ran a Chi-Squared analysis (X2=1.5, df=1, p=0.22) that indicated
no significant difference in the crabs’ choice between infected and uninfected
oysters.
0
10
20
30
40
50
60
Time (min)
Average Selec+on Time Between
Two Infected or Two Uninfected
Oysters
Uninfected
Infected
Lastly, we compared the Lme it took for
crabs to select an oyster when presented
with two uninfected oysters or two
infected oysters. We ran another linear
model using selecLon Lme as our
response variable and oyster infecLon
status and crab ID as our predictor
variables. We found highly similar Lmes
for prey selecLon as in the previous
analysis and again saw no significant
difference in consumpLon Lme as a
funcLon of infecLon status.
IntroducLon