This document summarizes an experiment that examined the effects of predator interactions between green crabs and dogwhelks on the consumption of blue mussel prey. The experiment tested single-predator treatments of just dogwhelks or just crabs, as well as a multiple-predator treatment with both species present. Results showed that dogwhelk feeding rates decreased significantly in the presence of green crabs, while crab foraging increased when dogwhelks were present. Additionally, significantly more total mussels were consumed in the multiple-predator treatment than in the single-predator treatments. This indicates that the presence of another predator species impacts the feeding behavior and rates of these invertebrate predators.

1. Predator interactions among green crabs (Carcinus maenas) and dogwhelks (Buccinum
undatum) in the presence of blue mussel (Mytilus edulis) prey
Rachel Brodie
September 22, 2012

2. 1 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
Abstract
In many coastal marine environments, blue mussels (Mytilus edulis) play a critical role in
determining the biodiversity and providing a basal food source for many predators. The
present study examined the effects on mussel predation by single and multiple predator
systems of green crabs (Carcinus maenas) and dogwhelks (Nucella lapillus). Total number and
mean size of mussels consumed by each species was determined for comparison between
single and multiple predator treatments. Feeding rates of whelks decreased significantly in the
presence of green crabs. The presence of whelks had a positive effect on crab foraging, in
which higher mussel consumption was approaching significance for crabs experiencing
interspecific competition for food in comparison to crabs only competing with conspecifics.
There was no statistically significant difference between the sizes of mussels consumed in each
treatment. When comparing the mean number of mussels consumed for each treatment,
significantly more mussels were consumed in multiple-predator treatments. Results indicate
that the presence of another predator has a significant impact on the feeding pattern of other
invertebrate predators of blue mussels. Interspecific predator interactions are therefore an
important part of mussel bed community dynamics.
1. Introduction
Community dynamics are impacted by predation, competition, species diversity and
species density (McQueen et al., 1989). In natural systems, most prey face risk of mortality
from a variety of different predators. Predators may interact while foraging, causing a
deviation from the predicted consumption when the activity of isolated predators is summed
(Sih et al., 1998). When risk reduction is observed, the number of prey consumed is less than

3. 2 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
predicted from isolated predators due to the interactions between predators or prey behaviour
reducing foraging rates (Griffen and Byers, 2006). On the contrary, risk enhancement results
when prey behaviours or competitive predator facilitation increases foraging success (Mansour
and Lipcius, 1991). Predator facilitation is an example of a positive interaction within
community dynamics, in which the presence of one species assists the ability of another species
to forage (Soluk, 1993). Non-independent multiple predator effects on prey can include both
conspecific and interspecific pairs of predators (Wong et al., 2010). When the observed and
predicted consumption rate differs for conspecific and interspecific predator cases, the effects
of multiple predator species is evident (Vance-Chalcraft et al, 2004). This indicates that the
effects of interspecific predators on consumption rates are independent of predator density.
Filter-feeding invertebrates serve as the key basal food source for a variety of different
food web interactions within intertidal communities (Menge and Branch, 2001). Intertidal
communities are often used to investigate the effects of predation, competition, and various
other interactions among predators and prey. This environment provides many benefits to
ecologists, including small, sessile or slow moving organisms which can be easily manipulated, a
simple assemblage, and predators that often share a similar resource which commonly causes
high levels of competition (Bertness et al., 2002). The blue mussel Mytilus edulis is present in
dense beds in the intertidal zone and helps to support rich communities of species in
Passamaquoddy Bay, Bay of Fundy, Canada (Quinn et al., 2012). Blue mussels are abundant
filter feeders in this region and are predated upon by dogwhelks (Buccinum undatum), sea stars
(Asterias spp.), and green crabs (Carcinus maenas) (Hamilton, 2000).

4. 3 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
The green crab is an invasive decapods crustacean that originated from Europe and
Northern Africa (Jensen, et al 2007). Crabs are highly aggressive competitors, with interference
by conspecifics potentially inhibiting crab feeding (Griffen and Williamson, 2008). The green
crab is an omnivorous predator feeding primarily on bivalves, but also on plants, small
arthropods and gastropods (Ropes, 1968).
Dogwhelks are carnivorous gastropod molluscs that feed on mussels, periwinkles, and
barnacles (Crothers, 1985). In order to search and consume prey, whelks utilize both physical
and chemical techniques. Olfactory chemical senses lead whelks to food sources, where they
use a combination of chemical dissolution and radular scraping to bore through the shells of
prey (Carriker and Williams, 1984). Foraging by whelks has been found to be impacted by intra-
and interspecific interactions. Dogwhelks are occasionally eaten by green crabs; this is an
example of intraguild predation (Trussel, et al., 2003). Chemical risk cues in the seawater serve
as a signal to dogwhelks that predatory crabs are present in their environment. Dogwhelks
often respond to these risk cues by reducing their feeding rate. The presence of other
dogwhelks can also impact foraging activity. Chemical cues from feeding whelks have been
suggested to stimulate conspecifics to feed (Dunkin & Hughes, 1984). The same principle may
be at play for crabs as they use olfaction to detect prey (Crothers, 1985).
Along the mid-Atlantic coast of North America, two of the most common predators of
mussels are the green crab and the dogwhelk (Crothers, 1968). Two studies have been
conducted on the competition between green crabs and dogwhelks in an intertidal community
near St. Andrews, New Brunswick. d’Entremont (2005) performed a field and lab experiment to
investigate the effect of competition and density of predators on the level of consumption of

5. 4 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
blue mussel prey. d’Entremont found that whelk feeding was depressed by crab presence, but
crabs were unaffected by the presence or absence of whelks. A field study conducted by Quinn
(2012) looked at the interactions between two blue mussel predators, the green crab and
dogwhelks. In contrast to the findings in 2005 by d’Entremont, Quinn found that crabs in the
presence of whelks tended to consume more biomass than in the whelk-free treatments.
The aim of this project is to examine the interspecific interactions on feeding rates. It is
hypothesized that negative interactions between predators at high densities will lead to
depression of foraging rates in comparison to single-predator treatments. It is predicted that
interspecific competition will cause a decrease in whelk feeding rates because of the presence
of crabs and associated crab risk cues. Competitive intra- and interspecific interactions at high
densities is also hypothesized to cause predators to alter prey size selection in order to
minimize competitive interactions. It is predicted that predators will become less selective
under higher competitive stress of interspecific competition and feed on mussels of sub-
optimal sizes.
The conflicting results of these green crab-dogwhelk interactions have inspired a further
investigation into the effect of competition on their feeding rates. Understanding what affects
predation rates helps us gain a better understanding of entire systems. It is important to
recognize that predators not only have an obvious effect on their prey, but also on each other.
These predation rates dictate the density of the mussel bed, and therefore the structure of the
entire mussel bed community.

6. 5 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
2. Materials and Methods
2.1 Experimental Setup
Experiments were conducted at the Huntsman Marine Science Center, St. Andrews,
New Brunswick, Canada. Green crabs, dogwhelks and mussels were kept in separate 30cm x 60
cm aquaria in a flow-through set up for five days prior to experimental trials. Predators were
starved for this five day period to standardize hunger levels and to acclimate them to lab
conditions. During experimental trials, species were placed in a flow-through aquarium
measuring 30cm x 60cm. In the center of the each aquarium was a 10cm x 30cm tile where
mussels attached. This arrangement allowed for foraging to take place within an area of
0.03m2, consistently across all tanks. Black covers were placed over each tank during
experimental trials to exclude external variables, such as movement, view of other predators in
adjacent tanks and changes in light exposure. Water temperature of all trials and holding tanks
remained fairly consistent ranging from 14-14.5⁰C.
2.2 Treatments
Predator densities were determined based on the area of the tile (0.03m2), in which all
foraging predator interactions would take place. The density of all predator species was kept at
a consistent level in order to specifically examine the effect of the presence or absence of
another predator on feeding rates. High predator density was based on analysis of the littoral
zone of Passamaquoddy Bay in the Bay of Fundy conducted by Quinn (2012). To achieve a high
density of predators based on Quinn (2012), the number of predators used within the 0.03m2
foraging area was 2 green crabs and 7 whelks. Mussels per aquaria should be approximately 50

7. 6 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
individuals to reflect normal distribution in natural environments (Boudreau, 2011). However,
due to resource limitations, only 36 mussels were added to each tank.
Mussel-covered tiles and predators were randomly assigned to one of three
treatments. Experimentation was conducted from August 13-16, 2012. Eight replicates were
completed for each treatment. There were three different treatments examined: 1) only
dogwhelks observed at a high density (7) with blue mussels, 2) dogwhelks (7) and green crabs
(2) together at a high density with blue mussels, and 3) only green crabs at a high density (2)
with blue mussel prey. Treatments with single predator species (1 and 3) tested the effects of
intraspecific interactions on predator foraging, while treatment 2 with both predators present
tested the effects of interspecific interactions on mussel consumption.
2.3 Predator and Prey collection
All species were collected within the intertidal zone at Indian Point in St. Andrews, New
Brunswick, Canada. Collections took place at low tide from August 7-10, 2012. Crab sizes were
within a range of 48-68mm in carapace width. To avoid any behavioural or morphological
biases, only male crabs were used. Dogwhelks ranged from 25-40mm in height. Mussels were
collected and ranged in size from 30-50mm. These specific ranges in species size ensure that
prey species are edible by both predator species (Quinn, 2010).
2.4 Data Collection
The duration of each trial was 12 hours. After each test, all predators and mussels were
removed for analysis. Mussels with at least one bore hole in an otherwise intact shell were
classified as killed by a whelk. A mussel shell with chips was classified as being consumed by a
crab. This sorting method is consistent with the method outlined by Quinn (2012).

8. 7 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
The total number of mussels consumed by each species in single and multiple-predator
treatments was recorded. The size of mussels that were consumed was also measured.
2.5 Statistical methods
All statistical analysis was carried out using SPSS 20 IBM processor. A univariate ANOVA
test was used to compare the mean number of mussels consumed in the presence of only
whelk predators, only green crab predators, and when both whelk and green crab predators
were present in foraging areas. The mean number of mussels consumed was normally
distributed according to the Shapiro-Wilk test of normality (p-value: 0.091). Post-Hoc
comparisons between the three treatments were performed using Tukey’s HSD test. A 2-factor
ANOVA could not be used to test the interaction between crabs and whelks due to limited
degrees of freedom.
The mean number of mussels consumed by whelks in each treatment was not normally
distributed according to the Shapiro-Wilk test of normality (treatment 1 p-value: 0.04,
treatment 2 p-value: 0.00). Thus, a non-parametric Kruskal-Wallis test was performed to
examine the impact on dogwhelk foraging rate in single and multiple-predator treatments.
To examine the relationship between crab consumption and predator environment, a t-
test was performed to compare the mean number of mussels consumed by crabs when
dogwhelks were present (treatment 2) in comparison to crab-only predators (treatment 3). A
parametric t-test was used because mussels consumed by crabs in treatment 2 and 3 passed
the Shapiro-Wilk test of normality (treatment 2 p-value: 0.792, treatment 3 p-value: 0.162).
Comparison between the sizes of mussels consumed by each predator in all treatments
was examined using a univariate ANOVA test, since the data was normally distributed according

9. 8 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
to the Shapiro-Wilk test of normality (treatment 1 p = 0.740, treatment 2 p = 0.954, treatment 3
p = 0.380). A t-test was used to specifically analyze the prey size selection of crabs between
treatment 2 and 3.
3. Results
Four main questions were examined in this study. The first question examined the
effect on prey risk when treatments included only conspecific predators in comparison to when
predators experienced interspecific interactions. The difference between overall consumption
of mussels when both predators are present, in comparison to single predator treatments is
shown in figure A1. It is observed that when dogwhelks and green crabs are both present, prey
risk is enhanced and therefore the number of mussels consumed is significantly greater than
when only one predator species is present (Figure A1a). A Post-Hoc Tukey HSD test was
performed, in which the mean number of mussels consumed for the interspecific predator pair
(treatment 2) was significantly higher than the conspecific predator treatments (1 and 3)
(Tukey’s HSD test, p=0.003, Figure A1a). When each species is examined separately, crab
foraging rates appear to be higher than foraging rates of whelks. Whenboth predatorsare
presentintreatment2,crab consumptionof musselsisgreaterthanthe amountof musselsconsumed
by whelks(Figure A1b).
The second question asked in this study focuses on the feeding rate of dogwhelks. The
impact on foraging rate of dogwhelks was examined when only conspecifics were present in
comparison to the consumption of dogwhelks when green crabs were also present in the
foraging area. Figure A2 depicts the significant difference found by the Kruskal-Wallis test,

10. 9 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
where feeding rates were significantly lower for whelks which had crabs present (x2
= 9.734, df =
1, p-value = 0.002).
The third question asked by this study was specific to green crab feeding rates. Figure
A3 shows the comparison between the mean number of mussels consumed by crabs
experiencing intra-and interspecific competition (treatment 2) in comparison to consumption
by crabs only competing for food resources with conspecifics (treatment 3). As observed in
figure A3, the mean number of mussels consumed by crabs in the presence of whelks was
higher than crabs without whelks present, and this difference is approaching significance (t =
2.093, df = 13.217, p-value = 0.056).
The final question that was explored in the present study was the matter of mussel size
consumed by each species in different predator systems. Figure A4 depicts the mean size of
mussels consumed for each treatment. The mean size of mussels consumed by whelks in
treatment 1 is higher than all other treatments, but this is not statistically significant (F= 1.664,
p-value = 0.215). Therefore, predator treatment did not significantly affect the mean size of
mussels consumed by green crabs and dogwhelks in this study. Figure A5 depicts the mean size
of mussels eaten by crabs only in treatment 2 and treatment 3. The difference between prey
size selection by crabs was not significantly different for the two types of predator systems
(t=0.627, p-value=0.543).
4. Discussion
4.1 Prey Consumption – Dogwhelk Predators
Mussel consumption was significantly lower for whelks in mixed-predator treatments
relative to the whelk-only treatments. Previous research supports the diminished response of

11. 10 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
whelk feeding rates in the presence of crabs. Lab studies conducted by d’Entremont (2005) and
Trussel et al. (2003) observed a reduction in feeding activity by dogwhelks when they were
exposed to crab risk cues. d’Entremont (2005) also tested whelk feeding rates in the field and
similarly found a decrease in consumption when predatory crabs are within the same
environment. He explains that physical disturbance by foraging crabs and potential for
intraguild predation serves to intensify feeding inhibition of whelks, as they often seek shelter
when crab risk cues are detected from the surrounding seawater (d’Entremont, 2005).
In whelk-only treatments, the mean consumption of mussels was significantly higher.
Previous research on the effects of whelk feeding by intraspecific competitive have been
inconsistent. Hughes and Dunkin (1984) proposed that whelks are stimulated to increase
consumption rates due to the scent of feeding conspecifics. Within the lab, d’Entremont (2005)
also found a slight positive stimulatory effect of foraging conspecifics on whelk feeding activity.
Intraspecific competition can also be negative between whelks, where they can engage in
interference competition by displacing each other from mussel prey or kleptoparasitism
(Hughes and Dunkin, 1984).
Both interspecific and intraspecific interactions have the potential to decrease feeding
activity of whelks. Consumption was significantly higher in whelk-only treatments, therefore it
is suggested that multiple-predator interactions and crab risk cues have a stronger influence on
whelk foraging rate.
4.2 Prey Consumption – Crab Predators
Crab consumption of mussels was higher in multiple-predator systems where both crabs
and whelks were present. Previous studies have found a similar positive interaction between

12. 11 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
green crabs and dogwhelks. An increase in biomass consumed by crabs when competing for
food with dogwhelks was found by Quinn (2010), suggesting that whelks have the ability to
facilitate crab feeding rates possibly through kleptoparasitism. Kleptoparasitism is a form of
exploitative competition that involves the theft of a food item already acquired by another
organism (Smalegange et al., 2006). In the present study, out of 8 experimental trials
amounting to a total of 96 hours, only one mussel was consumed by whelks when both
predators were present in the foraging area. On the contrary, in 8 trials a total of 31 mussels
were consumed by green crabs when both predators were present. Although no video
evidence was recorded, it is possible that the high number of mussels consumed by crabs were
due to kleptoparasitic activities on whelks. Crabs can detect chemical cues in the water to
initiate kleptoparasitic attacks, since the drilling of a dogwhelk cause a surge of chemicals and
other stimuli to be released from the prey (Smalegange et al., 2006). The process of whelk
consumption of mussels also weakens the structure of the shell and strength of the adductor
muscle, which allows crabs to exploit the weakened prey and reduce handling time (Crothers,
1985).
In addition to crabs stealing food from whelks, crabs also occasionally consume whelks
in order to decrease interspecific competition for limited food resources (d’Entremont, 2005).
Three whelks were consumed by crabs in treatment 2 of this study out of a possible 56 whelk
individuals in all 8 trials. Green crab consumption of dogwhelks is a form on intraguild
predation, which renders the net effect of two predators to be less than additive (d’Entremont,
2005; Sih et al., 1998). From analysis of treatment 2 results, it is evident that the presence of

13. 12 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
whelks had a positive effect on crab foraging. The ability for whelks to facilitate feeding of
crabs may have moderated the negative effects of intraspecific competition between crabs.
Lower mussel consumption was found for green crabs experiencing only intraspecific
competition. Foraging depression of crabs was found in previous studies in which aggressive
behaviour and interference competition between conspecifics inhibited feeding (Griffen and
Williamson 2008; Rovero et al., 2000; Quinn et al., 2012). Only 19 mussels were consumed by
crabs in treatment 3, experiencing only intraspecific competition for prey. In a study conducted
by Quinn (2010), crabs were found to consume less biomass when found in high crab densities,
which was suggested to be a result of antagonistic interactions. Smallegange et al. (2006) also
suggests that green crabs in high densities have a reduction in feeding rates due to interference
competition. They observed an increase in handling time due to intraspecific competition,
possibly due to the crabs being more vigilant and aggressive.
4.3 Size Selection of Prey
Green crabs and dogwhelks have a preferred size range of prey, which is related to crab
carapace width and whelk shell height (Hughes and Dunkin, 1984). In stressful environments,
such as high species density, times of starvation, or competitive interactions between
predators, species can adjust their preferred range of mussel prey (Rovero et al., 2000).
Crab sizes were within a range of 48-68mm in carapace width. Dogwhelks ranged from
25-40mm in height. Mussels were collected and ranged in size from 30-50mm. Smaller
mussels are usually preferred by crabs because it requires less energy to acquire food, however
preferred size classes often become depleted and crabs must be flexible in selecting larger prey
(Hughes and Dunkin, 1984). Larger crabs were used in this study and according to Quinn

14. 13 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
(2010); prey size flexibility is more possible for larger crabs because the maximum size those
individuals can consume increases with body size. As a result, crabs in this study did not have
to be selective about which mussels they consumed because all mussels used were within the
range of possible consumption. Dogwhelks can consume mussels up to 50mm in length (Quinn
2010). Whelks used in this study were not-size selective because they could consume all
mussels provided in the tanks. Prey size selection was not significantly different for green crabs
and dogwhelks across all treatments. This may be due to the fact that crab risk cues nearly
eliminated mussel consumption by whelks in multiple-predator treatments, and therefore
whelks or crabs did not pressure each other to alter their prey size selection.
Sizes of mussels consumed by crab predators were not statistically different for multiple
and single predator treatments. However, the average size of mussels consumed by crabs in
the presence of whelks was higher than those consumed in crab-only treatments. Larger
mussels require longer handling times, so kleptoparasitism may be beneficial as it lowers
handling time and increases the biomass returned (Iyengar, 2008). In mixed-predator
treatments, any kleptoparasitism of mussels being eaten by whelks that occurred may have
facilitated consumption of larger mussels by crabs.
4.4 Conclusions
The effects of intra- and interspecific interactions between predators of blue mussels
appear to play an important role in community dynamics. Whelks appear to engage in positive
intraspecific interactions in which feeding by conspecifics stimulates whelk foraging activities.
Both intraspecific and interspecific interactions have the potential to reduce feeding activity of
whelks. Consumption was significantly higher in whelk-only treatments, therefore it is

15. 14 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
suggested that interspecific competition and crab risk cues have a stronger influence on
decreasing whelk foraging rates. Crabs consumed more mussels when whelks were present,
suggesting that whelks have the ability to facilitate crab feeding rates possibly through
kleptoparasitism. Future research should focus on the importance and frequency of
kleptoparasitism between different sizes of predators, other predators influencing the
interactions between whelks and crabs, and the long-term consequences of decreased feeding
of dogwhelks in the presence of green crabs.

16. 15 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
References
Bertness, M.D., Trussell, G.C., Ewanchuk, P.J. & Silliman, B.R. (2002). Do alternate stable states
exist in the gulf of Maine rocky intertidal zone? Ecology, 83, 3434-3448.
Boudreau, M.R., 2011. Interactions between predators in an intertidal mussel bed community.
MS Thesis, Mount Allison University, Sackville, NB.
Crothers, J.H. (1968) The biology of the shore crab Carcinus maenas (L.) 2. The life of the adult
crab. Field Studies, 2, 579-614.
Crothers, J.H. (1985). Dog-whelks: an introduction to the biology of Nucella lapillus (L.). Field
Studies. 6, 291-360.
Dunkin, S. & Hughes, R.N. (1984). Behavioural components of pre-selection by dogwhelks
Nucella lapillus, feeding on barnacles Semibalanus balanoides, in the laboratory. Journal
of Experimental Marine Biology and Ecology, 77, 45-68.
Griffen, B.D., Byers, J.E. (2006). Partitioning mechanisms of predator interference in different
habitats. Oecologia 146, 608–614.
Griffen, B.D., Williamson, T. (2008). Influence of predator density on nonindependent effects of
multiple predator species. Oecologia 155, 151-159.
Hamilton, D.J. (2000). Direct and indirect effects of predation by common eiders and
abiotic disturbance in an intertidal community. Ecol. Monogr. 70, 21–43.
Iyengar, E.V. (2008). Kleptoparasitic interactions throughout the animal kingdom and a
re-evaluation, based on participant mobility, of the conditions promoting the evolution
of kleptoparasitism. Biological Journal of the Linnean Society, 93, 745-762.
Jensen, G.C., P.S. McDonald, D.A. Armstrong. (2007). Biotic resistance to green crab, Carcinus
maenas, in California Bays. Mar Biol 151: 2231-2243.
Mansour, R.A., Lipcius, R.N. (1991). Density-dependent foraging andmutual interference in
blue crabs preying upon infaunal clams. Mar. Ecol. Prog. Ser. 72, 239–246.
McQueen, D.J., Johannes, M.R.S., Post, J.R., Stewart, T.J., Lean, D.R.S. (1989). Bottom-up
and top-down impacts on freshwater pelagic community structure. Ecol. Monogr.
59, 289–309.
Menge, B.A., Branch, G.M. (2001). Rocky intertidal communities. Marine community ecology.
Sinauer, Sunderland, MA, pp. 221-251.

17. 16 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
Quinn, B.K., Boudreau, M.R., Hamilton, D.J. (2012). Inter-and Intraspecific interactions among
green crabs and whelks foraging on blue mussels. Journal of Experimental Marine
Biology and Ecology 412: 117-125.
Ropes, J.W. (1968). The feeding habits of the green crab, Carcinus maenas (L.). Fishery Bulletin,
67, 183-203.
Rovero, F., Hughes, R.N., Chalazzi, G. (2000). When time is of the essence; choosing a
currency for prey handling costs. J. Anim. Ecol. 69, 683–689.
Sih, A., Englund, G., Wooster, D. (1998). Emergent impacts of multiple predators on prey.
Trends Ecol. Evol. 13, 350–355.
Smallegange, I.M., van der Meer J., & Kurvers R.H.J.M. (2006). Disentangling interference
competition from exploitative competition in a crab-bivalve systemusing a novel
experimental approach. Oikos, 113, 157-167.
Soluk, D.A.. (1993). Multiple predator effects: predicting combined functional response of
stream fish and invertebrate predators. Ecology 74, 219–225.
Trussell, G.C., Ewanchuck, P.J., Bertness, M.D., 2003. Trait-mediated effects in rocky intertidal
food chains: predator risk cues alter prey feeding rates. Ecology 84, 629–640.
Vance-Chalcraft, H.D., Soluk, D.A., Ozburn, N. (2004). Is predation risk influenced more
by increasing predator density or predator species richness in streamenclosures?
Oecologia 139, 117–122.

18. 17 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
Appendix A
a)
b)
Figure A1. Mean consumption of mussels over a 12 hour period by a) all predators collectively
in the foraging environment and b) each individual species. Treatment 1 consists of dogwhelks
predators, treatment 2 consists of crab and whelk predators, and treatment 3 has crab
predators. In figure A1a) A significant difference is observed for treatment 2, where a higher
number of mussels are consumed when both predators are present in the same environment (F
= 7.982, MS = 12.875, df = 2, Tukey’s HSD test p-value = 0.003). Different letters on the graph
A1a) indicate significant differences between the treatments. All species tested were collected
from tide pools found in the intertidal zone of Indian Point, New Brunswick.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
1 2 3
Treatment
a
b
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
1 2 3
Treatment
Consumed by Whelk
Consumed by Crab
a
MeanNumberofMusselsConsumedovera12hourperiod

19. 18 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
Figure A2. Mean number of mussels consumed by whelks when no crabs are present
(treatment 1) in comparison to consumption by whelks in the presence of crabs (treatment 2).
Averages were taken from 8 different trials, each with duration of 12 hours. Mussels consumed
by whelks in treatment 1 is significantly higher than mussels eaten by whelks in treatment 2 (x2
= 9.734, df = 1, p-value = 0.002). All species tested were collected from tide pools found in the
intertidal zone of Indian Point, New Brunswick.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
1 2
MeanNumberofMusselsConsumedovera12hour
period
Treatment
a
b

20. 19 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
Figure A3. Mean number of mussels consumed by crabs when whelks are present (treatment
2) in comparison to consumption by crabs when whelks are not present (treatment 3).
Averages were taken from 8 different trials, each with duration of 12 hours. The difference
between mussels consumed by crabs in the presence of whelks and absence of whelks is
approaching significance (t = 2.093, df = 13.217, p-value = 0.056). All species tested were
collected from tide pools found in the intertidal zone of Indian Point, New Brunswick.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
2 3
MeanNumberofMusselsConsumedovera12hour
period
Treatment
a
ab

21. 20 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
Figure A4. Mean size of mussels consumed (mm) over a 12 hour period for each treatment.
Treatment 1 consists of conspecific dogwhelk predators, treatment 2 consists of crab and whelk
predators, and treatment 3 has conspecific crab predators. The mean size of mussels
consumed by whelks in treatment 1 is higher than all other treatments, but this is not
statistically significant (F= 1.664, df = 2, p-value = 0.215). All species tested were collected from
tide pools found in the intertidal zone of Indian Point, New Brunswick.
38
40
42
44
46
48
50
1 2 3
Meansizeofmusselsconsumed(mm)
Treatment
a
a
a

22. 21 | R . B r o d i e / P r e d a t o r i n t e r a c t i o n s b e t w e e n g r e e n c r a b s
a n d d o g w h e l k s i n t h e p r e s e n c e o f b l u e m u s s e l p r e y
Figure A5. Mean size of mussels consumed (mm) over a 12 hour period by crabs when whelks
are present (treatment 2) in comparison to consumption by crabs when whelks are not present
(treatment 3). Crabs in treatment 2 consumed mussels that were of a larger mean size, but this
is not statistically significant (t = 0.627, df = 11.318, p-value = 0.543). All species tested were
collected from tide pools found in the intertidal zone of Indian Point, New Brunswick.
39
40
41
42
43
44
45
46
47
2 3
MeanSizeofMusselsConsumed(mm)
Treatment
a
a