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ORIGINAL ARTICLE
Distribution of cryptic mussel species (Mytilus edulis and M. trossulus)
along wave exposure gradients on northwest Atlantic rocky shores
JAMIE C. TAM
1,2
& RICARDO A. SCROSATI
1
*
1
Saint Francis Xavier University, Department of Biology, Antigonish, Nova Scotia, Canada, and
2
Present address: Victoria
University of Wellington, School of Biological Sciences, Wellington, New Zealand
Abstract
We determined the distribution of two cryptic mussel species (Mytilus edulis and M. trossulus) along full gradients of wave
exposure in northwest Atlantic rocky intertidal habitats. Research was conducted in Nova Scotia (Canada) and Maine
(United States) and species were determined using genetic analyses. In very sheltered habitats, only M. edulis occurred. In
sheltered, exposed, and very exposed habitats, both species co-existed, M. edulis predominating in Maine and M. trossulus in
Nova Scotia. Hybrids were absent or rare. The distribution of mussels regardless of species (Mytilus spp.) was remarkably
consistent across levels of wave exposure in both regions. In very sheltered habitats, organisms were large (45 cm long, on
average) and old (78 years, on average) and occurred in low densities. In sheltered, exposed, and very exposed habitats,
organisms were small ( B1 cm, on average) and young (12 years, on average); density was low in sheltered and very exposed
habitats, but high in exposed habitats. Abundance data for predatory snails (dogwhelks, Nucella lapillus) and canopy-forming
algae (Ascophyllum nodosum and Fucus spp.) suggest that predation and facilitation may explain some of the observed changes
in mussel population traits along the wave exposure gradients. Our results could be useful as baseline information to predict
the effects of the progressive increase in wave action caused by climate change on intertidal mussel populations.
Key words: intertidal,mussel, Mytilus, wave exposure
Introduction
Mussels are common organisms in rocky intertidal
habitats on temperate shores, where they often play
important ecological roles. For example, when neither
abiotic stress nor predation pressure is particularly
strong, mussels can dominate the rocky substrate,
excluding other sessile species through competition
(Paine 1974; Menge & Sutherland 1987). However, by
increasing habitat complexity with their shells, mussels
can increase overall species richness by facilitating the
occurrence of many small species that thrive on, or
among, the mussels (Borthagaray & Carranza 2007;
Norling & Kautsky 2007;Palomoetal.2007). Also,
through filter-feeding, mussels convert pelagic food
sources into benthic biomass that can sustain intertidal
food webs through predators (Menge et al. 1997).
Thus, several studies have been interested in predicting
ecologically relevant population traits of mussels such
as abundance and size. Through direct and indirect
effects, environmental habitat conditions have often
shown to be good predictors of such traits.
Wave action is an important environmental factor
influencing the performance of intertidal species,
mainly through biomechanical effects on growth and
survival (Denny & Wethey 2001). In wave-exposed
habitats, waves detach mussels that become too
large for byssal threads to hold them attached
(Carrington et al. 2009). Under intermediate de-
grees of wave exposure, mussels can thrive and
competitively exclude other sessile species, resulting
in high mussel abundance (Paine 1974; Menge
1976). In wave-sheltered habitats, benthic predators
often find optimal foraging conditions and limit
mussel abundance (Menge 1976).
The northwest Atlantic coast exhibits a coldtem-
perate biota (Adey & Hayek 2005). Blue mussels
(Mytilus spp.) are common organisms in rocky inter-
tidal habitats under all degrees of wave exposure
(Riginos & Cunningham 2005). The population
*Correspondence: Ricardo A. Scrosati, Saint Francis Xavier University, Department of Biology, Antigonish, Nova Scotia B2G 2W5,
Canada. E-mail: rscrosat@stfx.ca
Published in collaboration with the Institute of Marine Research, Norway
Marine Biology Research, 2014
Vol. 10, No. 1, 5160, http://dx.doi.org/10.1080/17451000.2013.793809
(Accepted 2 February 2013; Published online 18 September 2013)
#2014 Taylor & Francis
Downloaded by [St Francis Xavier University] at 06:50 23 September 2013
patterns described above broadly apply to New Eng-
land (USA) shores. However, on more northern shores
(Canada), changes in the timing of low tides suggest
that intertidal cold stress could be higher because of a
longer aerial exposure during early morning hours in
winter (Tam & Scrosati 2011), when very low tem-
peratures occur (Scrosati 2011). Additionally, Cana-
dian waters on the open Atlantic coast generally have
lower phytoplankton (mussel food) abundance than
New England waters (Cole et al. 2011). Such a
combination of increased abiotic stress and decreased
food supply might determine regional changes in
mussel population structure. Studies on the open
Atlantic coast of Nova Scotia have shown that intertidal
mussel cover in wave-exposed habitats decreases from
south to north (Hunt & Scheibling 2001;Scrosati&
Heaven 2007). However, patterns of mussel cover,
density, size and age along full gradients of wave
exposure on Canadian shores remain unknown.
There is an additional knowledge gap in northwest
Atlantic mussel ecology. Two species occur in
rocky intertidal habitats on this coast: Mytilus edulis
Linnaeus, 1758 and Mytilus trossulus Gould, 1850
(Rawson & Harper 2009). Unfortunately, they are
difficult to distinguish because of morphological
similarities and the existence of hybrids (Comesan
˜a
et al. 1999; Innes & Bates 1999). Thus, field studies
on mussels from this coast have often been unable to
generate species-specific data (Hunt & Scheibling
2001; Bertness et al. 2002; Penney & Hart 2002;
Cusson & Bourget 2005; Lemaire et al. 2006; Smith
et al. 2009). Recent genetic analyses of samples from
wave-exposed habitats between Newfoundland and
New York (a range of 1800 km) have indicated that
both species coexist along most of this coastal range
but exhibit different latitudinal distributions (Tam &
Scrosati 2011). Thus, ecological data generated for
mussels as a whole (Mytilus spp.) may not necessarily
apply to each species.
There is a need, then, to investigate population
structure for both mussel species along full gradients
of wave exposure on New England and Canadian
shores. Therefore, we conducted a study to evaluate
the influence of wave exposure on mussel density,
cover, size, and age (identifying both species through
genetic analyses) in Maine (New England, USA) and
Nova Scotia (Canada). Because of the differences in
abiotic stress and pelagic food supply between Maine
and Nova Scotia, we predicted that mussels would
be more abundant in Maine. However, because of
the scarcity of species-specific ecological informa-
tion, it was unclear how the two mussel species
would differ in abundance between Maine and Nova
Scotia or along the wave exposure gradients. None-
theless, we predicted distribution differences be-
tween both species between Maine and Nova
Scotia and along the wave exposure gradients,
because even closely related species exhibit different
habitat preferences to some extent (Suatoni et al.
2006; Sattler et al. 2007; Pepper et al. 2011).
To evaluate possible biotic influences on mussel
distribution along wave exposure gradients, we also
determined the abundance of relevant invertebrates
and seaweeds along such gradients. We surveyed
dogwhelks (Nucella lapillus (Linnaeus, 1758)), be-
cause these snails are the main predators of mussels
at the intertidal elevations where we conducted our
study (Menge 1976; Hunt & Scheibling 2001). Other
mussel predators (sea stars and crabs) mostly prey at
lower elevations than the ones we surveyed (Menge
1976,1978; see Methods), so such organisms
were not considered for this study. We also deter-
mined the abundance of canopy-forming seaweeds
(Ascophyllum nodosum (Linnaeus) Le Jolis and Fucus
spp.), because these algae can facilitate predator
activity when their cover is high enough to ameliorate
conditions during low tides (Menge 1978).
Material and methods
We surveyed rocky intertidal habitats in Nova Scotia
(Canada) and Maine (USA). The two studied
regions are approximately 700 km apart (Figure 1).
In each region, we surveyed four levels of wave
exposure, which we determined by using a fetch-
based index that measured the percentage of a 1808
angle (parallel to the shoreline) for which land was
visible from the shore, often at distances between tens
and hundreds of metres (Boizard & DeWreede
2006). The wave exposure levels were: very sheltered
(75100% of land cover; very protected sites),
sheltered (5075% of land cover), exposed (25
50% of land cover) and very exposed (025% of
land cover; sites facing the open ocean). For con-
sistency, we did not sample narrows where strong
currents resulting from tidal flow were obvious. For
each exposure level, we surveyed 10 random sites, at
each of which we laid a 10-m transect line following
the coastline at an elevation of one-third (1/3) of the
full intertidal range (where mussels thrive; Bertness
2007). At each site, we established 15 quadrats
(50 cm50 cm) at random along the transect line.
For each quadrat, we measured the density and
percent cover of all mussels (Mytilus spp.), the density
of dogwhelks, and the percent cover of Ascophyllum
nodosum and Fucus spp. (mostly Fucus vesiculosus
Linnaeus). In total for the study, we sampled 80 sites.
To determine mussel size and age, we collected
40 random individuals from each site. We deter-
mined size by measuring the length from the umbo
to the distal end of the shell, and age by counting
the number of annual growth rings on shells
52 J. C. Tam and R. A. Scrosati
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(Tam & Scrosati 2011). All field work was done during
low tides from May to July 2007. To generate species-
specific values of density and cover, we performed
genetic analyses of field samples. For this purpose, we
randomly collected 20 mussels from each exposure
level (two individuals per site) separately for the two
studied regions. Then, we determined the species ratio
(Mytilus edulis :M. trossulus : hybrids) for each
exposure level and region using the ITS marker
following the method described by Heath et al.
(1995). This marker identifies species of blue mussel
reliably and yields similar results to those based on
other genetic markers (Hayhurst & Rawson 2009). We
applied the resulting species ratios to the correspond-
ing quadrat values of density and cover of Mytilus spp.
to generate density and cover data for each species for
each exposure level and region.
We evaluated the effects of wave exposure and
region on mussel density, cover, size and age through
analyses of variance (ANOVA) done separately for
each trait. ‘Region’ was considered as a fixed factor
with two levels (Nova Scotia and Maine), while
‘wave exposure’ was considered as a fixed factor with
four levels (very sheltered, sheltered, exposed, and
very exposed). ‘Site’ was considered as a random
factor nested within the exposure region interac-
tion. The nested nature of our design accounted for
the spatial segregation of the two studied regions
(Underwood 1997; Wikstro
¨m & Kautsky 2007).
When necessary, we evaluated pairwise differences
between treatments with Tukey HSD (Honestly
Significant Difference) tests (Quinn & Keough
2002). We carried out the analyses with JMP 5.1
for Macintosh.
Figure 1. Map of the northwest Atlantic coast showing the two surveyed regions with asterisks: Maine, USA (shown in detail below, to the
left), and Nova Scotia, Canada (shown in detail below, to the right).
Mussel distribution along wave exposure gradients 53
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Results
The density and cover of both mussel species
combined (Mytilus spp.) were higher in the exposed
habitats of Maine than in those of Nova Scotia,
showing similarly low values in both regions at the
other three exposure levels (Figure 2). Mytilus edulis
was the only species in very sheltered habitats, while
the two species (M. edulis and M. trossulus) coexisted
in sheltered, exposed, and very exposed habitats.
In exposed and very exposed habitats, M. trossulus
predominated in Nova Scotia, while M. edulis pre-
Figure 2. Density of (A) Mytilus edulis, (B) Mytilus trossulus, and (C) hybrid mussels, and percent cover of (D) M. edulis, (E) M. trossulus,
and (F) hybrid mussels for four degrees of wave exposure in two regions on the northwest Atlantic coast (Nova Scotia and Maine). Values
are means9standard error (10 sites per treatment). In any given graph, significant differences between means within any given region are
indicated by using different letters above the corresponding means.
54 J. C. Tam and R. A. Scrosati
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dominated in Maine (Figure 2A,B). Hybrids were
absent in Maine and in very sheltered habitats in
Nova Scotia, occurring in low frequencies in shel-
tered, exposed, and very exposed habitats in Nova
Scotia (Table I).
Wave exposure and region had significant effects
on density, cover, age, and size (only wave exposure
for size), but the exposureregion interaction was
always significant (Tables IIIV). Thus, the effects of
each of these two factors need to be viewed
separately at each level of the other factor.
Mussel (Mytilus spp.) density peaked markedly in
exposed habitats in both regions, showing much
lower values in very sheltered, sheltered, and very
exposed habitats. Each mussel species also peaked in
abundance in exposed habitats (Figure 2A,B),
except M. edulis in Nova Scotia, where its abun-
dance was relatively low across all exposures. Where
hybrids occurred, they always showed low densities
(Figure 2C). The highest density of mussels (Mytilus
spp.) recorded in exposed habitats was 8320 indivi-
duals m
2
. Mussels were small (less than 1 cm long,
on average) in very exposed, exposed, and sheltered
habitats, but considerably larger in very sheltered
habitats (up to 8.1 cm long), which was a pattern
common to both studied regions (Figure 3A).
Similarly, mussels were young (12 years old, on
average) in very exposed, exposed, and sheltered
habitats, but much older in very sheltered habitats
(up to 15 years old), which was also a pattern
common to both regions (Figure 3B). Because of
the large size of individuals in very sheltered habitats,
mussel cover was greater than in sheltered and very
exposed habitats, although cover was still less than in
exposed habitats because of locally high mussel
densities there. The pattern of mussel cover was
also common to both studied regions (Figure 2DF).
The highest value of mussel cover found in exposed
habitats was 80%.
Dogwhelks were much less numerous than mus-
sels. A significant exposureregion interaction also
occurred for dogwhelk density (Table V). Dogwhelks
occurred generally in higher densities in Maine than
in Nova Scotia, particularly in sheltered and exposed
habitats (Figure 4A). In Maine, dogwhelks peaked in
density in exposed habitats, showed lower values in
sheltered and very exposed habitats, and were rare
in very sheltered habitats. In Nova Scotia, dogwhelk
density was more similar among sheltered, exposed,
and very exposed habitats, although it was also very
low in very sheltered habitats (Figure 4A).
The cover of Ascophyllum nodosum varied with
wave exposure (Table V), showing high values in
very sheltered habitats and decreasing towards
higher exposures, reaching very low values in very
exposed habitats. This trend was similar in both
studied regions (Figure 4B). Although a significant
exposureregion interaction occurred for the cover
of Fucus spp. (Table V), the trend in cover across
exposures was also similar between both regions:
cover was lowest in very sheltered habitats, highest in
Table I. Proportion of Mytilus edulis,Mytilus trossulus, and hybrid
mussels in rocky intertidal habitats spanning four degrees of wave
exposure in Nova Scotia (Canada) and Maine (USA).
Region
Degree of wave
exposure
Mytilus
edulis
Mytilus
trossulus Hybrids
Nova
Scotia
Very sheltered 1.00 0 0
Sheltered 0.20 0.60 0.20
Exposed 0.10 0.80 0.10
Very exposed 0.05 0.75 0.20
Maine Very sheltered 1.00 0 0
Sheltered 0.46 0.54 0
Exposed 0.73 0.27 0
Very exposed 0.65 0.35 0
Table II. ANOVA results for the density of Mytilus edulis,Mytilus trossulus, and hybrid mussels recorded for four levels of wave exposure in
two regions on the northwest Atlantic coast (Nova Scotia and Maine).
Species Source of variation df MS Fp
M. edulis Region 1 105.29 78.23 B0.001
Exposure 3 146.06 108.52 B0.001
RegionExposure 3 49.65 36.90 B0.001
Site (RegionExposure) 72 1.35 9.52 B0.001
Error 1120 0.14
M. trossulus Region 1 8.74 8.84 0.004
Exposure 3 368.98 373.17 B0.001
RegionExposure 3 8.74 8.84 B0.001
Site (RegionExposure) 72 0.99 10.18 B0.001
Error 1120 0.10
Hybrids Region 1 275.92 484.05 B0.001
Exposure 3 38.25 67.10 B0.001
RegionExposure 3 38.25 67.10 B0.001
Site (RegionExposure) 72 0.57 16.14 B0.001
Error 1120 0.04
Mussel distribution along wave exposure gradients 55
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sheltered habitats, and intermediate in exposed and
very exposed habitats (Figure 4C).
Discussion
Because of the ongoing environmental changes
due to anthropogenic factors, there is a need to
understand how species distribution is shaped by the
environment (Brooks et al. 2012). Our research has
revealed that the two cryptic species of intertidal
mussel from the northwest Atlantic coast, often
assumed to be ecologically equivalent, actually
exhibit different distributions across natural gradi-
ents. While Mytilus trossulus predominated in Nova
Scotia, Mytilus edulis did so in Maine. Our results
agree with those from a latitudinal survey restricted
to wave-exposed habitats between Newfoundland
(Canada) and New York (USA), which found that
M. edulis switched from being dominant in the south
to rare in the north, whereas M. trossulus exhibited
the opposite pattern (Tam & Scrosati 2011). The
switch in dominance seems to occur between north-
ern Maine and southern Nova Scotia (Hayhurst &
Rawson 2009; Tam & Scrosati 2011). Our research
has also revealed distribution differences along wave
exposure gradients, since both species coexisted in
sheltered, exposed, and very exposed habitats, but
M. edulis was the only species in very sheltered
habitats. A previous study in exposed habitats in
central Nova Scotia also showed predominance
of M. trossulus (Hunt & Scheibling 1996), while a
survey of two very sheltered sites in central Maine
(near Darling Marine Center, in the Damariscotta
area) showed almost complete dominance by
M. edulis (Rawson et al. 2001).
Explaining such interspecific differences in distri-
bution is difficult because of the scarcity of species-
specific ecological information. The predominance
of M. trossulus on northern shores suggests that this
species could be more tolerant to low temperatures
during winter low tides (Tam & Scrosati 2011) and
could perform better under a limited planktonic
food supply (Bertness 2007; Cole et al. 2011) than
M. edulis. In turn, because of milder thermal
conditions and higher food supply levels in central
Maine, M. edulis might predominate in dense mussel
Table IV. ANOVA results for the size and age of Mytilus spp. recorded for four levels of wave exposure in two regions on the northwest
Atlantic coast (Nova Scotia and Maine).
Trait Source of variation df MS Fp
Size Region 1 0.11 1.10 0.297
Exposure 3 163.12 1617.10 B0.001
RegionExposure 3 0.93 9.25 B0.001
Site (RegionExposure) 72 0.10 5.11 B0.001
Error 3120 0.02
Age Region 1 112.17 9.99 0.002
Exposure 3 6323.17 562.92 B0.001
RegionExposure 3 130.16 11.59 B0.001
Site (RegionExposure) 72 11.23 8.45 B0.001
Error 3119 1.33
Table III. ANOVA results for the percent cover of Mytilus edulis,Mytilus trossulus, and hybrid mussels recorded for four levels of wave
exposure in two regions on the northwest Atlantic coast (Nova Scotia and Maine).
Species Source of variation df MS Fp
M. edulis Region 1 17824219 106.40 B0.001
Exposure 3 27890965 166.49 B0.001
RegionExposure 3 2883559 17.21 B0.001
Site (RegionExposure) 72 167519.40 8.83 B0.001
Error 1120 21244570
M. trossulus Region 1 4153163 68.51 B0.001
Exposure 3 38868079 641.20 B0.001
RegionExposure 3 811208.60 13.38 B0.001
Site (RegionExposure) 72 60617.87 5.27 B0.001
Error 1120 12890335
Hybrids Region 1 58344300 2227.49 B0.001
Exposure 3 7238841 276.37 B0.001
RegionExposure 3 7238841 276.37 B0.001
Site (RegionExposure) 72 26192.90 9.25 B0.001
Error 1120 3170935
56 J. C. Tam and R. A. Scrosati
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stands in that region by competitively displacing M.
trossulus. It is also plausible that the disappearance of
M. trossulus south of Maine (Tam & Scrosati 2011)
may be caused by high water temperature (Rawson
et al. 2001) or high air temperature during summer
low tides (Jones et al. 2010; Tam & Scrosati 2011).
The colonization history of both species might also
have influenced their current distribution, but this
possibility seems unlikely because both species have
coexisted on the northwest Atlantic coast for several
thousands of years (Rawson & Harper 2009).
Along wave exposure gradients, different resis-
tance to hydrodynamic stress might affect the
species ratio. Studies have suggested that M. trossulus
has a thicker, more elongated shell than M. edulis
(McDonald et al. 1991; Innes & Bates 1999; Penney
et al. 2007), but the implications for species perfor-
mance under different wave exposures remain
unknown. On the other hand, in very sheltered
habitats, seawater exchange with open waters is
limited, so salinity is often lower (to varied levels)
than in more exposed habitats because of local
freshwater inputs. Thus, the lack of M. trossulus
found in very sheltered habitats in our Nova Scotia
and Maine sites could result from a different
tolerance to salinity changes between both mussel
species. However, experiments testing this possibility
have yielded contradictory results (Gardner &
Thompson 2001; Qiu et al. 2002), so further
experimentation is clearly needed.
The distribution of mussels as a whole (Mytilus
spp.) along the wave exposure gradient was remark-
ably consistent in both studied regions. In very
sheltered habitats, mussels occurred in low densities
but were large and old, exhibiting moderate cover
values. At the intertidal elevations that we surveyed,
dogwhelks are the main predators of mussels in
New England, crabs and sea stars playing a minor
role at such elevations (Menge 1976,1978). As in
our study, a previous study in other New England
sites found that dogwhelk densities were lowest in
very sheltered habitats; however, because of the
limited wave action and high fucoid algal cover (which
limits understory abiotic stress during low tides),
dogwhelk predation was intense (Menge 1978).
Thus, given the extensive fucoid algal cover that we
found in very sheltered sites, dogwhelk predation
could be limiting mussel density there. The surviving
Table V. ANOVA results for the density of Nucella lapillus and
percent cover of Ascophyllum nodosum and Fucus spp. recorded for
four levels of wave exposure in two regions on the northwest
Atlantic coast (Nova Scotia and Maine).
Species
Source of
variation df MS Fp
Nucella lapillus Region 1 86.71 48.69 B0.001
Exposure 3 37.52 21.07 B0.001
Region
Exposure
3 27.42 15.39 B0.001
Site (Region
Exposure)
72 1.78 13.54 B0.001
Error 1120 0.13
Ascophyllum Region 1 0.03 0.02 0.89
nodosum Exposure 3 189.44 128.72 B0.001
Region
Exposure
3 2.64 1.80 0.156
Site (Region
Exposure)
72 1.47 10.36 B0.001
Error 1120 0.14
Fucus spp. Region 1 14.40 7.42 0.008
Exposure 3 90.51 46.63 B0.001
Region
Exposure
3 18.02 9.28 B0.001
Site (Region
Exposure)
72 1.94 11.68 B0.001
Error 1120 0.17
Figure 3. (A) Size and (B) age of Mytilus spp. for four degrees of
wave exposure in two regions on the northwest Atlantic coast
(Nova Scotia and Maine). Values are means9standard error (10
sites per treatment). In any given graph, significant differences
between means within any given region are indicated by using
different letters above the corresponding means.
Mussel distribution along wave exposure gradients 57
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mussels could reach large sizes and advanced ages
through access to a good supply of suspended organic
matter, as water exchange with the open ocean is
limited. Seaweed canopies could also be favouring
mussel growth by limiting physiological stress during
aerial exposures at low tide (Bertness et al. 1999, Watt
& Scrosati 2013). Thus, the combination of large
mussel sizes and low densities might be reached
through self-thinning (Guin
˜ez 2005) after years of
growth, together with the predatory action of dog-
whelks as suggested above. Similar size and density
patterns occur in Mytilus populations from similar
habitats on the northeast Atlantic coast (O’Connor
2010).
In our sheltered habitats, dogwhelks were 10 times
more abundant than in very sheltered habitats, while
fucoid canopies were also extensive (although domi-
nated by Fucus instead of by Ascophyllum). Thus,
predation intensity may have been even higher than in
very sheltered habitats, which would explain the rarity
of mussels and their reduced size and age in sheltered
habitats. Interestingly, reduced mussel density, size,
and age also occurred in very exposed habitats,
although the mechanisms behind such patterns were
likely different. On coasts facing the open ocean, the
intense physical stress imposed by waves is the main
factor limiting the survival of most intertidal species,
especially mussels (Lubchenco & Menge 1978; Hunt
& Scheibling 2001; Carrington et al. 2009). Max-
imum water velocity may reach 12 m s
1
in such
habitats in Nova Scotia (Hunt & Scheibling 2001).
Mussel density peaked in our exposed habitats,
although organisms were small and young. In Nova
Scotia, the combination of very low dogwhelk
density, limited fucoid algal cover, and high wave
action suggests that predation intensity may have
been weak in exposed habitats. In Maine, dogwhelks
peaked in abundance in exposed habitats, but the
ratio between the average density of mussels and
dogwhelks was 51.1 in exposed habitats and just 0.6
in sheltered habitats, suggesting that predation
pressure by dogwhelks may also have been weak in
exposed habitats, as found by Menge (1978). Thus,
high mussel densities in exposed habitats could result
from neither too intense predation nor too intense
hydrodynamic stress (lower than in very exposed
habitats). Mussel size and age would be limited in
exposed habitats because of wave action, which
imposes limits to the maximum size that organisms
can reach (Denny et al. 1985). Our findings are
consistent with those for exposed habitats in Rhode
Island, south of Maine (Carrington et al. 2009).
In summary, the two cryptic mussel species from
northwest Atlantic rocky intertidal habitats exhibit
distribution differences across latitudes (Tam &
Scrosati 2011) and wave exposures (this study).
Thus, the possibility that the mussel species ratio
affects community structure and function would add
another level of complexity to ecological studies on
Figure 4. Density of (A) Nucella lapillus and percent cover of
(B) Ascophyllum nodosum and (C) Fucus spp. for four degrees of
wave exposure in two regions on the northwest Atlantic coast
(Nova Scotia and Maine). Values are means9standard error
(10 sites per treatment). In any given graph, significant differences
between means within any given region are indicated by using
different letters above the corresponding means.
58 J. C. Tam and R. A. Scrosati
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this coast. However, since patterns for mussels as a
whole (Mytilus spp.) were consistent along wave
exposure gradients, the ecological role of mussels
might not be highly species-specific. Extensive men-
surative studies like ours should continue, because
our understanding of species distribution along wave
exposure gradients is observation-limited. In this
sense, given that climate change is increasing levels
of wave action globally (Young et al. 2011), our
study offers baseline information that could be used
to predict the effects of the progressive increase in
wave action on intertidal mussel populations.
Acknowledgements
We thank Ian Grace, Chris Georgeson, and Gab-
rielle Beaulieu for field assistance, Ljiljiana Stanton
for laboratory assistance, Roy Kropp, Martin Thiel,
and two anonymous reviewers for helpful comments
on the manuscript, and Darling Marine Center
(University of Maine) for logistical support. Re-
search was funded by grants awarded to RAS by the
Natural Sciences and Engineering Research Council
of Canada (NSERC; Discovery Grant), NSERC’s
Research Capacity Development (RCD) program,
the Canada Research Chairs (CRC) program, and
the Canada Foundation for Innovation (CFI), and
by a Graduate Student Scholarship awarded to JCT
by Darling Marine Center.
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