Abstract
Invasive species have become an important concern for marine ecosystem function and
conservation. Many studies have shown the negative effects of an exotic species on native
species through increased competition or other behavioral changes. My study examined three
mussels of the genus Mytilus found along the central coast of California, each with a unique life
history and ecological characteristics. M. californianus, the ribbed mussel, is a native species that
forms dense beds in rocky intertidal habitats; the native blue mussel, M. trossulus, and the
invasive blue mussel, M. galloprovincialis, are found in bays and estuaries. Mytilus
galloprovincialis, which entered California waters some time in the middle of the 20" century,
has replaced the native blue mussel from Southern California up to the latitude of Monterey Bay,
where both blue mussels co-occur. I sought to determine how the three congeners differed (1) in
aggregation behavior, which can influence growth and reproductive output; and (ii) vulnerability
to predatory whelks of the genus Nucella. Aggregation behavior was compared in the absence
and presence of whelks. Clumping behavior was analyzed to identify species preferences in
neighboring mussels. All aggregations in whelk-free seawater, except for M. galloprovincialis
when grouped with M. californianus, showed significant preference for mixed species
assemblages. Under predation-pressure conditions, none of the mussels in combination with M.
trossulus demonstrated a significant preference for a specific neighbor in the aggregation. The
presence of whelks also yielded slightly larger aggregation sizes. To extend the predation portion
of this experiment, two species of predatory whelk, Nucella emarginata and Nucella ostrina,
were given the choice between the native and invasive blue mussels. Nucella ostrina
demonstrated a preference for the native mussel but N. emarginata did not, suggesting that native
predators could potentially feed upon and thereby contribute to the regulation of the invasive
population. However, the invasive has not yet been deterred by native predators. Overall, results
of this study provide preliminary data for further work on mussel aggregation and predation and
the dynamics of invasive and native populations on the western coast of the United States.
1. Introduction
Marine environments are hot spots for invasion, interconnected not only geographically
but also anthropogenically through travel and trade. Larval forms are brought in ships' ballast
water, exotics are released from aquaria, and commercial aquaculture experiences the occasional
escapee. As exotic species increase, they invariably affect the natives housed in a particular
coastal environment. Marine invertebrates present a particularly interesting case in that they
provide a vital food source for many other organisms, including larger vertebrates. While in
many cases humans find related species hard to distinguish, differences in physiology and
behavior can act to the detriment of other species. According to Geller et al. (1994), 367 species
of marine invertebrates and plants are regularly brought from Japan to the west coast of the
United States, often with a substantial amount of Mytilus mussel larvae in the ballast water. The
Californian coast experiences introductions on a regular basis, including a species of blue
mussel, Mytilus galloprovincialis.
Mytilus species are often the most abundant intertidal invertebrate and play a pivotal role
in the ecosystem (Braby and Somero 2006). Three species of mytilid mussels inhabit the coast of
California, Mytilus californianus (a ribbed mussel), M. trossulus (native blue mussel), and M.
galloprovincialis. Since the 1940's, when the first introduction of M. galloprovincialis is
suspected to have occurred, the geographic range of M. trossulus has declined substantially
(Geller 1999). Mytilus galloprovincialis has successfully invaded many new locations globally
and has subsequently developed several hybrid zones with M. trossulus in the North Pacific,
including Japan, Washington, and central California (Braby and Somero 2006). In these zones,
M. galloprovincialis tends to dominate in areas of warmer water and more constant salinity,
harkening back to its Mediterranean ancestry. On the other hand, the native M. trossulus
flourishes in the cold, seasonally variable waters of the North Pacific, and commonly is found in
estuaries and other locations with lower, fluctuating salinity. Yet, the invasive species continues
to move up the coast to cooler waters. Physiologically, its success can be attributed its ability to
grow much larger than the native, but it appears to have other competitive advantages as well. It
has been shown to suffer less snail predation, parasite loads, and lower mortality rates in Europe
(Geller 1999). It has even been known to climb over and smother the other native Californian
mussel, M. californianus (Geller 1999).
Mytilus galloprovincialis appears to possess an ideal set of behavioral and physiological
characters that promote a successful invasion. These adaptations and, essentially, survival
mechanisms are critical in coping with environmental conditions. Aggregation behavior in
mussels is one of the most often-cited responses to both abiotic and biotic obstacles. Past studies
involving a closely related blue mussel species, M. edulis, have discussed clumping behavior at
length. Topics include the presence of small groups within a larger matrix of mussels, the
negative correlation between growth of an individual and clump size, and the attractive pull by
other mussels (Okamura 1986; de Vooys 2003). Studies have also been conducted on mixed
mussel beds involving invasive M. galloprovincialis. Within a hybrid zone in Southwest
England, M. edulis in the presence of M. galloprovincialis exhibited weaker byssal thread
attachments and a tendency to move to the edge of an aggregation (Schneider et al. 2005). In
South Africa, the native mussel Perna perna showed less movement and increased clumping in
the presence of a predatory lobster, whereas the invasive M. galloprovincialis did not (Nicastro
et al. 2007).
Predators exert considerable pressures on marine organisms, influencing dispersion,
aggregation, and density. In a previous study, Leonard et al. (1999) showed an inverse
relationship between growth rate and shell thickness of M. edulis. In the presence of a predatory
crab, the mussel shells either grew large and thin or small and thick. In addition the strength of
byssal thread attachment increased. Furthermore, mussels stationed in the center of an
aggregation suffered lower mortality; however, they also experienced reduced growth and
reproductive output (Côté and Jelnikar 1999). Although larger clumps decrease the likelihood of
predation, a tradeoff with fitness exists.
Mytilus species along the Californian coast experience predation pressures from many
species, particularly from seastars (Pisaster spp.) and whelks (Nucella spp.). Nucella abundance,
in particular, has been cited as a key factor in determining M. trossulus distributions within the
intertidal zone (Noda 1999). West (1986) demonstrated a level of individual prey selection
among Nucella when provided a choice between a variety of prey including barnacles and
mussels. This study showed that selection does not reflect abundance of the prey, but is
potentially related to ingestive conditioning. Further studies of Nucella focused on the location-
specific preference for Mytilus species (Sanford et al. 2003). Because Nucella lack a dispersal
planktonic stage, populations are relatively isolated and have had time to diverge in prey choice.
Californian whelks were substantially more likely to drill M. californianus than either the whelks
from Oregon or Washington, even when raised under identical circumstances. Findings such as
these lead to questions about prey choice between closely related invasive and native species.
They also point to potential implications for the associated ecosystem. Particularly in the central
California hybrid zone of M. trossulus and M. galloprovincialis, the interaction of these mussels
both in relation to aggregation and predators is noteworthy.
This study addresses three questions related to aggregation behavior and predation
pressures. It seeks to investigate the influence of M. californianus and M. galloprovincialis on
the aggregation behavior of M. trossulus, both in the absence and presence of a predator. To
further the knowledge of predation impacts, it also looks at the preference of native predatory
whelks, N. emarginata and N. ostrina, between the native and invasive blue mussel.
2. Materials and Methods
2.1. Experimental Design
I observed the patterns of aggregation of different Mytilus species in laboratory
conditions. In a set of sequential experiments, I looked at how different combinations of M.
trossulus, M. galloprovincialus, and M. californianus sorted out into aggregations, under ambient
sea-water conditions and in the presence of a predator. During the months of May and June 2007,
each individual trial took place over five days, during which aggregations were mapped daily. As
a side experiment, I conducted a feeding preference experiment with two species of whelks,
Nucella emarginata and N. ostrina. Consisting of three replicates of each species, this study
occurred over four weeks in May.
Nucella emarginata were collected at Hopkins Marine Station and N. ostrina were
collected at Bodega Bay, CA. M. californianus were collected at Hopkins Marine Station, CA,
M. trossulus were supplied from the South Jetty in Florence, OR, and M. galloprovincialus were
brought up from Santa Barbara harbor, CA. All of the mussels collected were of comparable size
and measured between 28 and 42 mm. Both the N. ostrina and M. californianus were marked
with Orly Salon Nails“ Orange Flash nail polish and a coat of Scotch Super Glue"" Liquid,
whereas the M. galloprovincialus were painted with Wet 'n' Wild Flirty Rose Crème nail polish
and the same overcoat. M. trossulus remained unpainted.
2.2. Mytilus Aggregation Experiment
The aggregation experiments consisted of several combinations of the different species of
mussels as well as a single species trial. For the two-species trials, ten individuals of each species
were randomly assigned to one of five 5.7-L containers (Holiday Housewares, Inc; Visuals"M
Deluxe Clear Storage), each with nine holes (approximate diameter of 1 cm) drilled into either
side to facilitate water flow. I placed individuals onto a four by five square grid marked on the
bottom of the container, alternating species by square. I also ran a trial with two of the containers
containing only M. trossulus, two containing only M. californianus, and one a mix of the two
species set up in the same manner as the other mixed species trials. Situated in a running
seawater table, the containers were provided with a constant supply of fresh, filtered sea-water.
I mapped the position of each mussel daily for five days, taking careful note of which
mussels touched which. For the purpose of analysis, I recorded how many individuals of each
species every individual mussel touched. In theory, from the point of view of the mussel, if a
particular species of mussel has no preference of aggregation partners, then it will be touching an
equal number of each species. I also documented the clump-size distribution. A clump was
defined as a cluster of two or more mussels in which each mussel was touching at least one other
mussel in the cluster (Côté and Jelnikar 1999).
2.3. Predator-Influenced Aggregation Experiment
The second set of experiments looked at aggregation in the presence of a predator. The
set-up used five 4.5-L containers (Rubbermaid“ Serve n SaverrM) with the same four by five
square grid. Three experimental units had fresh sea water running into the top right corner of the
container from a two-compartment tank filled with 30 individuals of N. emarginata (Fig. 1). A
procedural control had a constant supply of water running through an identical, but empty tank.
The control simply had a small hose running into the center of the container and supplying fresh
sea water. The containers were elevated above an aquarium table so that water could spill over.
As with the other aggregation experiments, I mapped the position of each mussel relative to the
others. I also recorded the frequency of different clump sizes.
2.4. Nucella Feeding Experiment
Although not an aggregation experiment, the predation study complemented the
predation aggregation experiment. Six 5.7-L containers (Holiday Housewares, Inc; Visuals'M
Deluxe Clear Storage), each with nine -1 cm holes drilled into either side to facilitate water
flow, acted as the environment to house the species under scrutiny. Also placed on an aquarium
table filled with running sea water, three tanks held N. ostrina and three held N. emarginata. The
sets of three containers were separated so that all three of one species of whelk were grouped
together. Then two small Petri dishes each containing five randomly selected M. trossulus and
five M. galloprovincialis were placed in each container along with four whelks of the appropriate
species.
Daily observations consisted of noting any whelks positioned on top of a mussel, which
species of mussel had been targeted and/or eaten, and where on the shell the whelk drilled. I
began the study at the beginning of May. After two weeks, I replaced any eaten mussels so that
the whelks had access to equal numbers of each species for the final two weeks of the
experiment. At the end of each two-week period, I recorded how many of each mussel species
were eaten by each species of whelk.
2.5. Data Analysis
For the aggregation experiments, mussel preference was measured by taking the
difference between the number of conspecifics and congeners that each mussel was touching. I
calculated this value for each mussel, so that a negative value indicated a preference for a
different species and a positive value represented conspecific associations. Values for each
species in all containers (five for no predation and three for predation) of a combination were
pooled, a mean value was calculated, and then a p-value was generated using a Student's t-test.
Solitary mussels constituted only a small proportion of the observed data (average of 1-2 per
trial), and thus were not included in the analysis. For clump size, I conducted a two-way
ANOVA with the orthogonal factors of species composition and predation/no predation. To
detect a difference in Nucella prey preferences, I ran another two-way ANOVA with mussel
species and whelk species as the two factors.
3. Results
3.1. Mytilus Aggregation Experiment
For the aggregation trials under natural (predator-free) seawater conditions, all the
species, except for M. galloprovincialis when paired with M. californianus, yielded values
significantly different from zero after 120 hours. This translates into each mussel species
touching a disproportionate number of one species (Fig. 2; t-test, p£0.01). Looking at the average
divergence from zero (being no preference), the data are skewed in the negative direction,
indicating a preference for the congener.
3.2. Predator-Influenced Aggregation Experiment
Under predator-influenced conditions, only the M. californianus and M. galloprovincialis
combination demonstrated a significant preference for the other species (Fig. 3; t-test, p£0.01).
This can be seen clearly from the distribution of the means around zero. Clump size, however,
differed in the presence or absence of a predator (Fig. 4; two-way ANOVA, Fi, 14 = 11.91, p -
0.0039). A Studentized-Neuman-Keuls (SNK) post-hoc test revealed that the mixed species
10
combinations with M. trossulus all exhibited larger clump size in the presence of a predator than
without one (CV = 2.60, p«0.05, n = 3).
3.3. Nucella Feeding Experiment
In the feeding experiment, a significant interaction between whelk and mussel species
was identified (Fig. 5; two-way ANOVA, Fi,8 = 12.5, p = 0.008). Using an SNK post-hoc test, N.
emarginata did not demonstrate any identifiable preference between the invasive and the native
blue mussel. However, they also did not consume considerable amounts of mussels, potentially
due to mating requirements (Martel et al. 1986). Nucella ostrina demonstrated a preference for
M. trossulus over M. galloprovincialis (CV = 1.54, p50.05, n =3).
4. Discussion
4.1. Aggregation Experiment
Aggregation behavior in the Mytilus mussels can have far-reaching implications for
survival and fitness of individual species. Preference in neighbor is an interesting topic, due to
the different life history characteristics of different mussel species. Of the three species that
inhabit the central California coast, M. californianus, M. trossulus, and M. galloprovincialis all
exhibited a significant preference towards the opposite species. This result may have been an
artifact of my set-up, as each individual’s closest neighbors were of the opposite species;
however, the mussels were observed to move a great deal over the course of the 120 hours of
each trial. Behavior of the individual species of mussels within these mixed clumps may provide
insight into the success of the M. galloprovincialis invasion in the western United States and the
absence of M. trossulus in beds of M. californianus. These mixed species beds may result in
skewed ratios of species, one species potentially out-competing the other. In a study done along
the Israeli Mediterranean coast, mixed mussel beds of invasive Brachydontes pharaonis and
Mytilaster minimus revealed a higher biomass of the invasive species in each clump (Riloy et al.
2004).
That study also exhibited a trend similar to that seen with the mussels of this study, where
the invasive species is usually larger than the native (Braby and Somero 2006). Mytilus
californianus is also a large, longer-lived, and thicker-shelled mussel with strong byssal thread
attachments, a potential obstacle for M. trossulus in any mixed-species beds (Hunt and
Schiebling 2001). For M. trossulus this could result in crowding out or smothering, limited
access to food, and difficulty in recruitment. As shown in a study done with M. edulis and M.
trossulus by Dolmer (1998), faster growing, larger mussels occupied the top of the three-
dimensional aggregations found in the rocky intertidal. An additional consideration is provided
by a study done in a hybrid zone in Southwester England, which showed that M. edulis sought
the edges of clumps and exhibited weaker byssal thread attachment than M. galloprovincialis,
(Schneider et al. 2005). This superficial positioning, combined with weaker attachment
facilitates dislodgement by waves and, therefore, higher mortality. Mytilus edulis also showed
higher mortality rate in the center of the bed than M. galloprovincialis and their hybrids. This
resonates with studies conducted in the hybrid zone of California and the intermediate characters
of the hybrid mussels (Braby and Somero 2006). Further studies should be carried out to look at
the within patch location of each mussel species.
4.2. Predator-Influenced Aggregation Experiment
Mussels under predator-influenced conditions demonstrated no preference for neighbors
(except in the M. californianus/M. galloprovincialis combination); however, there was a trend
towards larger clump sizes. Okamura (1986) noted that M. edulis in larger groups experienced
decreased growth and reproductive output. The mussels on the edges of the bed were also subject
to disproportionately higher predation rates by a predatory crab. Hunt and Schiebling (2001) also
recognized the negative correlation between shell length and clump size, so in areas with high
predation such as Cranberry Cove, Canada, there was a higher proportion of small mussels. So
patch size influenced by predation levels may have considerable impact on the fitness of the
mussels in the clumps. Mytilus trossulus appears to have a lower tolerance of harsh conditions in
general, reaching a smaller size (size and fecundity are linked), prospering more in protected
estuarine environments, and being driven northward in its distribution by the invasive M.
galloprovincialis (Braby and Somero 2006). Larger-sized and mixed species clumps could
negatively affect the survival of this native species.
4.3. Nucella Feeding Experiment
The observation that N. ostrina prefer M. trossulus over M. galloprovincialis has
potential implications for the structuring of native mussel communities on the California coast.
Navarrete and Menge (1996) demonstrated that Nucella species have a significant ecological role
in controlling mussel population size. In light of the data shown herein, Nucella predation could
be a factor contributing to the success of the invasive M. galloprovincialis over the native M.
trossulus. Since Nucella species seem to have genetically programmed prey preferences (Sanford
et al. 2003), it is important to note that N. emarginata showed no preference for one blue mussel
species over the other. The preference for M. trossulus by N. ostrina might be because this
species was collected farther north where M. trossulus is more abundant than M.
galloprovincialis, and the whelks may have been conditioned to feed upon the native mussel
(Braby and Somero 2006). These results suggest the need for further studies.
Mytilus galloprovincialis continues to work its way up the California coast (Braby and
Somero 2006). Since variation in predation intensity depends on wave exposure, recruitment of
prey, and growth of the mussel, location will affect which mussels thrive and which predators are
present (Dolmer 1998). Therefore, because blue mussels tend to be excluded from the high-
energy zones of the California coast, they may not experience the intense predation pressures
that were present in this study. Further study is necessary to determine the effect of whelk
predation on natural populations of blue mussels. There are probably additional behavioral and
physiological factors influencing the success of the invasive species as a competitor of the
native, and these should also be researched in future studies to obtain a clearer picture of the
consequences of interspecific interactions.
Acknowledgments
I would like to my advisor, George Somero, for all of his advice, guidance, and wealth of
information. His passion for science and the marine system and ideas for various studies made
my project more interesting and kept me motivated. All the members of the Somero lab were
equally supportive - most notably Brent Lockwood for collecting N. emarginata for me and
offering his mussel expertise and encouragement, and Jon Sanders for assistance in technical
aspects of my experiment and for collecting M. galloprovincialis. Also, Eric Sanford played a
pivotal role in the project, providing N. ostrina and M. trossulus and acting as another source of
mussel information. Finally, I want to recognize Jim Watanabe for his extensive statistical
knowledge and help with my data analysis.
14
Literature Cited
Braby, C.E. and Somero, G.N. 2006. Ecological Gradients and Relative Abundance of Native
(Mytilus trossulus) and Invasive (Mytilus galloprovincialis) Blue Mussels in the
California Hybrid Zone. Marine Biology. 148: 1249 - 1262.
Côté, I.M. and Jelnikar, E. 1999. Predator-Induced Clumping Behaviour in Mussels (Mytilus
edulis Linnaeus). Journal of Experiment Marine Biology and Ecology. 235:201 -211.
de Vooys, C.G.N. 2003. Effect of a Tripeptide on the Aggregational Behaviour of the Blue
Mussel Mytilus edulis. Marine Biology. 142:1119-1123.
Dolmer, P. 1998. The Interaction Between Bed Structure of Mytilus edulis L. and the Predator
Asterias rubens L. The Journal of Experimental Marine Biology and Ecology. 228: 137-
150.
Geller, J.B. 1999. Decline of a Native Mussel Masked by Sibling Species Invasion. Conservation
Biology. 13: 661 - 664.
Geller, J.B., Carlton, J.T., and Powers, D.A. 1994. PCR-Based Detection of mtDNA Haplotypes
of Native and Invading Mussels on the Northeastern Pacific Coast: Latitudinal Pattern of
Invasion. Marine Biology. 119: 243 -249.
Hunt, H.L. and Scheibling, R.E. 2001. Patch Dynamics of Mussels on Rocky Shores: Integrating
Process to Understand Pattern. Ecology. 82: 3213 -3231.
Leonard, G.H., Bertness, M.D., and Yund, P.O. 1999. Crab Predation, Waterborne Cues, and
Inducible Defenses in the Blue Mussel, Mytilus edulis. Ecology. 80: 1 - 14.
Martel, A., Larrivée, D.H., Klein, K.R., and Himmelman, J.H. 1986. Reproductive Cycle and
Seasonal Feeding Activity of the Neogastropod Buccinum undatum. Marine Biology. 92:
211-221.
Navarrete, S.A. and Menge, B.A. 1996. Keystone Predation and Interaction Strength: Interactive
Effects of Predators on their Main Prey. Ecological Monographs. 66: 409 -429.
Nicastro, K.R., Zardi, G.I., and McQuaid, C.D. 2007. Behavioural Response of Invasive Mytilus
galloprovincialis and Indigenous Perna perna Mussels Exposed to Risk of Predation.
Marine Ecology Progress Series. 336: 169- 175.
Noda, T. 1999. Within- and Between-Patch Variation of Predation Intensity on the Mussel
Mytilus trossulus Gould on a Rocky Intertidal Shore in Oregon, USA. Ecological
Research. 14: 193 -203.
Okamura, B. 1986. Group Living and the Effects of Spatial Position in Aggregations of Mytilus
edulis. Oecologia. 69: 341 -347.
Rilov, G., Benayahu, Y., and Gasith, A. 2004. Prolonged Lag in Population Outbreak of an
Invasive Mussel: A Shifting-Habitat Model. Biological Invasions. 6: 347 -364.
Sanford, E., Roth, M.S., Johns, G.C., Wares, J.P., and Somero, G.N. 2003. Local Selection and
Latitudinal Variation in a Marine Predator-Prey Interaction. Science. 300: 1135- 1137.
Schneider, K.R., Wethey, D.S., Helmuth, B.S.T., and Hilbish, T.J. 2005. Implications of
Movement Behavior on Mussel Dislodgement: Exogeneous Selection in a Mytilus spp.
Hybrid Zone. Marine Biology. 146: 333 -343.
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emarginata. Ecology. 67: 789-908
Figure Legend:
Fig. 1. Predation aggregation experimental set-up. A two-compartment tank had water flowing
into and through the animal-holding area and out through a hole in the second compartment. The
predator-conditioned water then flowed into the container with the mussels.
Fig. 2. Predator-free aggregation. Data are the average difference between neighbors of the same
species and neighbors of a different species in each of the five replicate containers. Zero
indicates no preference between species and the difference has been taken to show divergence
from the null hypothesis. The three graphs represent the mean divergence from zero and *
denotes significant results of t-test, P+ 0.01.
(A) M. californianus and M. trossulus ("M.c. X =-0.477 + 0.158,p = 0.004, n = 44; *M.t. X
=-0.622 + 0.15, p = 0.0002, n = 45).
(B) M. galloprovincialis and M. trossulus ("M.g. X =-0.442 + 0.157, p = 0.007, n = 43; *M.t.
X =-0.5 +0.152,p = 0.002, n =40)
(C) M. californianus and M. galloprovincialis ("M.c. X =-0.854 + 0.138, p = 2.5E-07, n =41;
M.g. X =-0.196 + 0.217,p = 0.371, n= 46)
Fig. 3. Predator-induced aggregation. Data are the average difference between neighbors of the
same species and neighbors of a different species in each of the five replicate containers. Zero
indicates no preference between species and the difference has been taken to show divergence
from the null hypothesis. * denotes significant results of t-test, P+ 0.05.
(A) M. californianus and M. trossulus (M.c. X =-0.478 + 0.25, p = 0.07, n = 23; M.t. X =0+
0.236,p = 1, n = 28),
17
(B) M. galloprovincialis and M. trossulus (M.g. X =-0.379 + 0.283,p - 0.19,n - 29; M.t. X
=-0.241 + 0.22, p = 0. 28, n = 29)
(C) M. californianus and M. galloprovincialis ("M.c. X =-0.667 + 0.2,p = 0.003, n = 27; *M.g.
X =-0.633 + 0.195,p = 0.003, n = 30)
Fig. 4 Average clump size for each species combination, comparing the predator-free and
predator-present treatments. A two-way ANOVA showed significant effects of predator presence
on clump size (Fi.14 = 11.90794, p = 0.003899). An SNK post-hoc test showed significant
differences in the two combinations with M. trossulus. * denotes significant differences in clump
size in the presence of a predator (Qos,214, CV = 2.6, p20.05, n =3) (A) ’M. californianus and M.
trossulus (no predation: X = 3.12 + 0.072; predation: X = 6.32 + 1.44), (B) "M.
galloprovincialis and M. trossulus (no predation: X = 4.23 + 0.89; predation: X = 7.5 + 1),
(C) M. californianus and M. galloprovincialis (no predation: X =3.43 + 0.39; predation: X
4.12 +0.32)
Fig. 5. The Nucella feeding experiment results, split into whelk species and total mussels of each
species consumed. A two-way ANÖVA showed significant whelk and mussel interactions (Fi8:
12.5, p = 0.008). * denotes a significant preference of M. trossulus as prey for N. ostrina shown
in an SNK post-hoc test showed (Qos2,8, CV = 1.54, p50.05, n = 3)(N. ostrina: M.g. X - 1.33 +
0.67, M.t. X = 4 +0; N. ostrina: M.g. X = 1.67 +0.33, M.t. X = 1 +0.58)
18
Fig. 1.
0


L
Fig. 2.
0 +
-0.1
-0.2 -
§ 0.3
504.
05
5 06
-0.7
-0.8 -
-0.9 -
-O.1
-0.2
-0.3
-0.4
80
-0.6 -
-O.7 -
M. califomianus
M. galloprovincialis
M. trossulus
M. trossulus
20
0 +
-0.2-
0.4
0.6
-0.8
1
-1.2
M. galloprovncialis
M. califomianus
Fig. 3.
-0.1
-0.2 -
-0.3
5-0.4
-0.5
-0.6
-O.7 -
-0.8 -
0 -
-0.1
-0.2
-0.3
-0.4
5 -0.5
-0.6
-0.7
M californianus
M. galloprovincialis
M trossulus
M. trossulus
22
-0.1
-0.2
° -0.3
-0.4
-0.5
06
-0.7
-0.8
-0.9
M. galloprovincialis
M. californianus
Fig. 4.
5
4.


calif ornianus/ trossulus
E No Predation
Predation

galloprovincialis/ trossulus galloprovincialis/ californianus
24
5
Fig. 5.
4.5
4
3.5
9 2.5
1.5
0.5
Nucella Predation

+tross
L
+ gallo
N. ostrina

+tross
+ gallo
N. emarginata