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Impact of Predation by the Invasive Crab Hemigrapsus sanguineus on Survival of Juvenile Blue Mussels in Western Long Island Sound
Diane J. Brousseau, Ronald Goldberg, and Corey Garza

Northeastern Naturalist, Volume 21, Issue 1 (2014): 119–133

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Northeastern Naturalist Vol. 21, No. 1 D.J. Brousseau, R. Goldberg, and C. Garza 2014 119 2014 NORTHEASTERN NATURALIST 21(1):119–133 Impact of Predation by the Invasive Crab Hemigrapsus sanguineus on Survival of Juvenile Blue Mussels in Western Long Island Sound Diane J. Brousseau1, Ronald Goldberg2, and Corey Garza3 Abstract - Hemigrapsus sanguineus (Asian Shore Crab) has shown a remarkable ability to colonize rocky intertidal communities along the east coast of the United States since its introduction in the late 1980s and is an important predator of juvenile Mytilus edulis (Blue Mussel) in invaded habitats. In this study, we used two field-caging experiments and the Kaplan- Meier model to assess the impact of predation by Asian Shore Crab on the survival of juvenile Blue Mussels in an intertidal habitat of western Long Island Sound along the Connecticut coastline. Five treatment levels (high-density enclosure, low-density enclosure, exclosure, partial cage, and open plot) were used in the 2007 experiment. The high-density enclosure treatment was omitted in the 2010 experiment since there was no statistically significant difference in the proportion of mussels surviving between low- and high-density crab treatments in 2007. In 2007, we measured a statistically significant difference in mussel mortality between exclosure and crab-enclosure cages, with crabs lowering the median survival time for mussels from 15.4 to 7.6 days. In 2010, we ag ain measured a statistically significant difference in mussel mortality between exclosure and crab-enclosure cages, suggesting a crab effect on mussel survival. In the 2010 experiment, approximately 25% of the mussel mortality was attributable to crab predation, which reduced median survival time for mussels from 12.8 to 5.6 days. The median survival time for mussels exposed to the full complement of factors affecting survival (open plots and partial cages) was only 2–3 days. Our study shows that predation by Asian crabs may account for up to 25% of the Blue Mussel mortality in the intertidal zone at Black Rock Harbor. Further studies focusing on the importance of other biotic and abiotic factors are needed to understand the apparent declines in Blue Mussel populations and the interannual variability in recruitment success in this area. Introduction Invasions of marine habitats by non-indigenous species can have significant ecological and evolutionary consequences for native populations (Ruiz et al. 1997). Invasive species have the potential to impact marine communities either by direct predation or by competition with native species for critical resources (Cohen and Carlton 1998), resulting in altered or impaired ecosystem function. The popular argument that invasive species also pose a major threat to marine biodiversity (Molnar et al. 2008) has only recently been challenged by Briggs (2007). He argues 1Biology Department, Fairfield University, Fairfield, CT 06824. 2National Oceanic and Atmospheric Administration, Northeast Fisheries Science Center,Milford Laboratory, 212 Rogers Avenue, Milford, CT 06460. 3Division of Science and Environmental Policy, California State University, Monterey Bay, 100 Campus Center, Seaside, CA 93955.*Corresponding author - brousseau@fairfield.edu. Manuscript Editor: Melisa Wong Northeastern Naturalist 120 D.J. Brousseau, R. Goldberg, and C. Garza 2014 Vol. 21, No. 1 that major contemporary marine invasions have occurred via the opening of the Suez Canal, in the Wadden Sea, along the European coast, and into the tropical eastern Atlantic from the Indian Ocean, but there has been little evidence of impaired biodiversity or ecosystem function as a result of these invasions (Briggs 2010). Nonetheless, there are many examples in the literature which show that primary interactions between native and invader species most often result in alterations in species abundance or habitat shifts (Galil 2007, Reise et al. 2006, Thieltges 2005). The effect of an invader on a native species, however, may not be the same throughout the entire range of the invasion. Confounding factors such as local anthropogenic stressors (Galil 2007) or improved prey defenses (Freeman and Byers 2006) may alter the impact of a non-indigenous species on the native biota. More extensive experimental field studies which document species impacts in different geographic locations and under a range of environmental conditions are needed to better evaluate the extent of the threat of invasive species to ecosystem function and worldwide marine biodiversity. Hemigrapsus sanguineus (De Haan) (Asian Shore Crab), indigenous to the western Pacific, has been a particularly successful marine invader in the western Atlantic. It was first observed in New Jersey in the late 1980s (Williams and Mc- Dermott 1990) and since then has shown a remarkable ability to colonize rocky intertidal habitats along the east coast of North America (McDermott 1998). It now ranges from Maine to North Carolina and has become the most abundant species of intertidal crab in southern New England and Long Island Sound, reaching densities ≥150 crabs m-2 in some locations (Ahl and Moss 1999, Gerard et al. 1999, Lohrer and Whitlatch 2002). Mytilid bivalves are prominent in the natural diet of the Asian Shore Crab in its native environment (Lohrer et al. 2000), and several studies have identified this crab species as an important predator of juvenile Mytilus edulis L. (Blue Mussel) in invaded environments (Bourdeau and O’Connor 2003; Brousseau et al. 2000; Gerard et al. 1999; Lohrer and Whitlatch 1997; McDermott 1991, 1999; Tyrell and Harris 2000; Tyrell et al. 2006). Laboratory experiments have shown that Asian Shore Crabs can consume large numbers of mussels daily (Brousseau et al. 2001), and that despite the species’ omnivorous diet, it exhibits a strong preference for Blue Mussels over macroalgae (Brousseau and Baglivo 2005). Short-term microcosm experiments run under both laboratory and natural conditions also support the argument that predation by the Asian Shore Crab can cause significant declines in juvenile mussels (Tyrell et al. 2006). In a study assessing the relative impacts of crab predation, Lohrer and Whitlatch (2002) concluded that the predation pressure exerted by Asian Shore Crabs was a significantly greater threat to native mussel populations than that of the co-occuring exotic Carcinus maenas (L.) (European Green Crab). The Blue Mussel is an important prey item in northeast rocky intertidal communities, utilized by a variety of native fauna, including birds, fish, and invertebrates such as crabs and seastars (Lubchenco and Menge 1978). It is also a commercially valuable species, harvested and farmed in coastal New England. The extremely high Northeastern Naturalist Vol. 21, No. 1 D.J. Brousseau, R. Goldberg, and C. Garza 2014 121 densities of Asian Shore Crabs present in these habitats has prompted considerable speculation about the role of this invader in restructuring prey populations and decreasing the refuge value of intertidal areas for recruit and juvenile life-stages of this mussel. Several published studies imply that the Asian Shore Crab may have broad impacts on the native community (Bourdeau and O’Connor 2003, Ledesma and O’Connor 2001, Tyrell and Harris 1999) as well as large species-specific effects on bivalve prey (Lohrer and Whitlatch 2002). Reduction of mussel beds in intertidal areas decreases the amount of complex habitat available for other faunal assemblages, leading to a decrease in diversity of associated invertebrate communities (Koivisto and Westerbom 2010, Seed and Suchanek 1992). Consequently, the addition of another mussel predator, especially one that is so abundant, could potentially have a severe impact on prey population stability and overall community diversity. As an ecosystem engineer species, Blue Mussels are important filter feeders with the ability to process large volumes of water and remove particulate organic matter such as bacteria, phytoplankton, microzooplankton, and detritus from the water column (Commito and Boncavage 1989, Norling and Kautsky 2008). Oystergardening efforts such as those in Chesapeake Bay, use bivalve suspension feeders to reduce the nitrogen load and improve water quality. The decline or loss of intertidal mussel populations in a eutrophic estuary like Long Island Sound could remove an important shoreline “filter” of organic carbon, resulting in the loss of a critical particle-clearance mechanism for the entire ecosystem. In this study, we analyzed data from field-caging experiments with a Kaplan- Meier survivorship model to measure the impact of predation by Asian Shore Crabs on the survival of juvenile Blue Mussels in rock-strewn intertidal mudflats of the type commonly found in western Long Island Sound. Our experiments were designed to determine the contribution of Asian Shore Crab predation to the overall predation pressure exerted on Blue Mussel populations and to assess its role in the limited mussel recruitment observed in this area. Materials and Methods Caging experiments To determine the effects of the invasive Asian Shore Crab on survival of juvenile native Blue Mussels. we conducted two field-caging experiments in the mid-intertidal zone in Black Rock Harbor, Bridgeport, CT (41°08.8'N, 73°13.5'W; Fig. 1).This area is characterized by a gently sloping tidal flat densely covered with rocks of various sizes. At both caging sites, seawalls form the shoreward boundary of the study area. Environmental conditions and tidal immersion times at both sites are similar (approximate tidal range: 2.1 m). We determined natural crab densities in this area (10 ± 1.3 crabs per 0.125 m2) along 2 across-shore transects within the intertidal zone by counting all Asian Shore Crabs present within a quadrat (0.25 m2) placed on the substrate at each haphazardly determined sampling point (n = 15). An ongoing multi-year study of stomach contents in free-ranging Asian Shore Crabs from Black Rock Harbor shows crabs feed on macroalgae, barnacles, cyprid larvae, Northeastern Naturalist 122 D.J. Brousseau, R. Goldberg, and C. Garza 2014 Vol. 21, No. 1 polychaetes, Blue Mussels, amphipods, salt marsh grass, and detritus at this site (D.J. Brousseau, Unpubl. data). Figure 1. Chart of a portion of the northern shore of Long Island Sound indicating the locations of the 2007 and 2010 study sites in Black Rock Harbor , Bridgeport, CT. Northeastern Naturalist Vol. 21, No. 1 D.J. Brousseau, R. Goldberg, and C. Garza 2014 123 We conducted the first experiment in May–June 2007 along a mudflat located near the mouth of the harbor. To determine if the results of the first experiment were repeatable across sites and years, we carried out a second experiment in May 2010 at a location approximately 1.5 km northeast of the first study location. We chose the May–June experimental period because it coincides with the major spawning/ settlement season of Blue Mussels in Long Island Sound (Brousseau 1983, Newell et al. 1982). This choice ensured that our estimates of crab predation were made under environmental conditions (e.g., temperature and length of tidal immersion) similar to those present when crabs are foraging naturally on mussel recruits in the field. In the first caging experiment, we used five different treatments to test the null hypothesis that Asian Shore Crab predation has no effect on juvenile Blue Mussel survival. Enclosure cages with a low and a high crab density (8 and 23 per cage, respectively) measured Asian Shore Crab predation on mussels. Exclosure cages measured background mortality in the absence of Asian Shore Crab predators too large to enter the cages (>3 mm CW). Exclosures also excluded other predators such as fish, birds, and other crabs such as Panopeus herbstii (L.) (Mud Crab) and European Green Crabs. We used open plots (no cages) to measure natural mortality, and partial cages (two opposing sides of cage removed) to test for possible cage artifacts. In the second experiment, we used only a low-density of crabs (9 per cage) since the first experiment showed no statistically significant difference in Blue Mussel survival between low- and high-density crab treatments. We used four replicates of each treatment in the experiment. The cages were constructed of 10-mm vinyl-coated wire mesh with open bottoms and a single, hinged access door on the top. Three-millimeter mesh plastic netting completely lined the cages to prevent washout or emigration of mussels >3 mm SL. We partially buried the cages in the sediment to an approximate depth of 10 cm, resulting in an enclosed area of 0.3 m x 0.4 m x 0.2 m high. Rocks were present in the cages; no additional rocks were added. In 2007, we collected small Blue Mussel recruits from intertidal and shallow subtidal rocks at Fort Weatherill, Jamestown, RI, using SCUBA, and held them in flow-through seawater raceways at the Milford Laboratory (NMFS). In 2010, we collected mussels by hand from the surface of the dam sluiceway at the southern end of Holly Pond at Cove Island Park, Stamford, CT. Before the start of each experiment, we placed 100 randomly selected Blue Mussels of 5–20 mm shell length (SL) on a “conditioned” 3-M® Scotch Brite scrub pad (14 cm x 12 cm in size). Mussels produce byssal threads quickly (in less than 24 hr) when held in the laboratory (Young 1985). In our experiment, we held 100 mussels on each pad for two days in a seawater table to allow for attachment. We considered mussels “attached” if they remained fixed to the pad when it was turned over. At the beginning of each experiment, we anchored pads with their attached mussels to the substrate in the center of each cage and open plot. Experimental crabs were collected at random by hand from the field sites and were representative of the size distribution of the resident population. Median carapace width of all crabs used in enclosures was 20.83 mm. There was no statistiNortheastern Naturalist 124 D.J. Brousseau, R. Goldberg, and C. Garza 2014 Vol. 21, No. 1 cally significant difference in crab size between high- and low-density treatments in 2007 (Mann–Whitney U = 1452.5, n(low) = 32, n(high) = 92, P = 0.914) or across years (Mann-Whitney U = 1873.5, n(2007) = 124, n(2010) = 36, P = 0.143). We brought crabs to the Milford Laboratory for tagging and marked each crab by gluing a small color/ shape-coded plastic tag to the carapace with cyanoacrylate. We used both male and female crabs whose chelae and other appendages were intact. The sex ratio in enclosure cages was 1:1. We held crabs overnight in running seawater and placed them in cages the following day. Prior to release, we cleared caged plots of all visible Asian Shore Crabs. We then added tagged crabs to the enclosure cages. At the end of the experiment, average recovery rate of tagged crabs in low-density enclosures was 70% in 2007 and 63% in 2010. A 58% mean recovery rate was present in the high-density cages. The presence of untagged crabs and carapace fragments in cage enclosures indicated that missing crabs resulted from tag loss due to molting and/or cannibalism. Griffen and Byers (2009) have shown that cannibalism occurs among Asian Shore Crabs when densities are high. To estimate survival, we censused the number of mussels surviving in all caged and open plots eight times in the 2007 experiment and six times during the 2010 experiment. The 2007 experiment ran for 13 days. We terminated the 2010 experiment on day 9 when a storm dislodged three of the experimental cages and the pads with mussels attached were lost. At the conclusion of the experiment, we removed, counted, and measured the remaining uneaten mussels in the enclosure cages. We also counted and measured tagged and untagged Asian Shore Crabs remaining in the cages. We analyzed for content the stomachs of a subsample of the crabs retrieved from enclosure cages (2007: n = 30, 2010: n = 15). On collection day, we removed the stomachs from live crabs, flushed their contents with seawater into a Petri dish, and examined them under a dissecting microscope to determine food type. We noted the presence of crushed mussels (shells). Statistical analysis of survival data In both experiments, we analyzed average survivorship across treatments using a one-way ANOVA. In our model, treatments served as fixed, independent variables. The proportion of mussels surviving (number surviving/original number present) in each treatment served as the dependent variable. We further analyzed significant treatment effects using Tukey tests (α = 0.05). In order to assess the impact of the treatments on mussel survival time, we used a Kaplan-Meier survival model (Lee 1992) to assess the difference in median mussel survival time across each treatment. A Kaplan-Meier survival model is a non-parametric method that can be used to calculate median survival time for one or more groups. The survival model can be described as: S(t) = S(t - 1)pt , where, the survivorship estimate, S(t), is calculated as the number of individuals surviving divided by the total number of individuals at risk at time t. The probability of surviving to a point in time pt is estimated from the cumulative Northeastern Naturalist Vol. 21, No. 1 D.J. Brousseau, R. Goldberg, and C. Garza 2014 125 probability of surviving each of the preceding time intervals S(t - 1). The survival estimate S(t) may therefore be thought of as the product of S(t - 1) and pt, with a standard range of survivorship values of 0 (no survivorship) to 1 (total survivorship). A Weibull, as opposed to normal, distribution is commonly used to describe the distributional shape of data in a Kaplan-Meier model since mortality in ecological systems does not follow a normal distribution across time (Garza 2005, Kalbfleisch and Prentice 1980). Standard analyses of predator impacts on survivorship over time typically involve the use of repeated measures ANOVA. However, one main assumption of repeated measures ANOVA is that over time, data are normally distributed; often survivorship data are not and can exhibit skewed or binomial type distributions (Fox 1993, Garza 2005, Lee 1992, Petraitis 1999). Survival models are robust enough to analyze nonparametric data (Lee 1992). We then used a chi-square analysis to assess differences in the median survival time of mussels across our experimental treatments. Results A majority of the recovered Asian Shore Crabs examined (77% in 2007, 60% in 2010) had consumed food before capture; the rest had empty stomachs when dissected. Food items included green algae, detritus, polychaete worms, and crushed mussels (shells). Mussel shell fragments were found in approximately one-third of the crab stomachs that contained food. Mean size of mussels not eaten by crabs in enclosure cages was 14.5 ± 2.6 mm SL (mean ± SE, n = 205). No crabs less than 9 mm SL were recovered in enclosure cages. The results of both experiments show a decline in the proportion of mussels surviving over the course of the experimental period (Fig. 2). The 2007 results reveal significant differences among the five treatments (Table 1). A Tukey HSD post-hoc analysis across the five treatments indicates that the exclosure treatment had the highest average survivorship, which was significantly different from survivorship in the low- and high-density enclosure treatments (Q = 3.0892, P < 0.05). The lowest survival occurred in the partial cage and open plots (Fig. 3). The 2010 results also reveal significant differences among treatments (Table 2). Mussel survivorship in the exclosure treatement was again the highest and significantly different from survivorship in the crab enclosure plots (Q = 2.9688, P < 0.05). The lowest survival occurred in the partial cage and open plots, and mortality in these treatments differed significantly from the crab-enclosure and crab-exclosure treatments (Fig. 3). In both years of the study, mussels in the exclosure treatments experienced an aver- Table 1. Result of one-way ANOVA of final mean number of mussels surviving (n = 4) across the five experimental treatments in 2007, comparing exclosures, two crab densities, a partial-cage control, and an open plot. Source of variation df MS F P Treatment 4 1733.88 7.514 0.0016* Error 15 230.77 Total 19 Northeastern Naturalist 126 D.J. Brousseau, R. Goldberg, and C. Garza 2014 Vol. 21, No. 1 Figure 2. The plotted curves indicate the mean proportion of mussels surviving (n = 4, ± SE) across treatments over time during the course of experiments in 2007 and 2010. All curves are plotted assuming a Weibull distribution in the data. Northeastern Naturalist Vol. 21, No. 1 D.J. Brousseau, R. Goldberg, and C. Garza 2014 127 Figure 3. Final average number of mussels surviving (n = 4, ± SE) among the five treatments in 2007 and four treatments in 2010. Groups with a different letter are significantly different according to Tukey’s HSD. Northeastern Naturalist 128 D.J. Brousseau, R. Goldberg, and C. Garza 2014 Vol. 21, No. 1 age mortality of approximately 46%, Analysis of median survival time across the five treatments in the 2007 experiment revealed a significant difference among the five treatments (χ2 = 94.429, df =1, P < 0.001). The shortest median survival time for mussels was observed within the open and partial-cage control treatments (Table 3). The second longest median survival times were observed in the high- and low-density crab treatments (Table 3). The longest median survival time for mussels was observed in the absence of Asian Shore Crab predators (Table 3). In the 2010 experiment, median survival time was again longest in the exclosure and shortest in the open plot (χ2 = 498.987, df = 3, P < 0.001). Survival times in the crab enclosure and partial-cage control were equal, but significantly shorter than in the exclosure cage and longer than the open plot (Table 3). Discussion The higher survival of Blue Mussels protected from Asian Shore Crab predators in our cage experiment supports the conclusion that some mussel losses were, in fact, due to Asian Shore Crab predation. Statistical comparisons of mortality in exclosure and enclosure treatments showed that Asian Shore Crab predation measured in enclosure cages accounted for about 25% of the total mortality measured in the 2010 experiment. In the 2007 experiment, a statistically significant difference was also observed that suggested crab predation in the crab-enclosure treatments accounted for 20% of the observed mussel mortality. Although these results do not support the view that Asian Shore Crab predation is the major cause of juvenile Blue Mussel mortality at our study site, they do support the conclusion of a minor but measurable role for this predator. Unlike other similar mussel caging studies in which the reported background Table 3. Median survival time (n = 4, ± SE) in days of mussels across experimental treatments in 2007 and 2010. Median survival (days) Treatment 2007 2010 Exclosures 13.0 ± 1.20 12.8 ± 0.09 High crab density 7.0 ± 0.29 No treatment Low crab density 7.0 ± .033 5.6 ± 0.10 Open control 2.0 ± 0.10 2.7 ± 0.07 Partial-cage control 2.0 ± 0.11 4.5 ± 0.10 Table 2. Result of one-way ANOVA of final mean number of mussels survivng (n = 4) across the four experimental treatments in 2010, comparing exclosures, one crab density, a partial-cage control and an open plot. Source of variation df MS F P Treatment 3 2153.06 7.953 0.0035 Error 12 270.73 Total 15 Northeastern Naturalist Vol. 21, No. 1 D.J. Brousseau, R. Goldberg, and C. Garza 2014 129 mortality rates were less than 10% (Carroll and Highsmith 1996, Lohrer and Whitlatch 2002), mortality in our predator exclosure cages was high. It is not likely that mussel emigration or washout by waves was a confounding problem since even the smallest mussels could not pass through the cage mesh. The high mortality probably resulted largely from non-predatory sources such as dessication and high temperature. Habitat characteristics of intertidal areas vary from west to east in the Sound. Rocky intertidal flats in the western end are mainly sedimentary, with small rocks and cobbles in some areas. There is little algal cover; most rocks are covered by dense barnacle sets. East of New Haven, large rocks and boulders are more common, and rocky intertidal areas have abundant algal cover keeping them moister and cooler. These site differences in environmental conditions likely affect the relative importance of various biotic and abiotic factors in causing mussel mortality. It may be the reason for the different background mussel mortality rates reported in other caging experiments. More importantly, it emphasizes the need to assess the impact of invaders within different habitat types and geographic regions to fully understand native-nonnative species interactions. Since the aim of our study was to measure the effect of Asian Shore Crab predation on mussel survival, we did not attempt to quantify other potential sources of mortality. As discussed above, abiotic factors such as dessication and elevated temperatures likely play a significant role in areas similar to our study site where habitat complexity is low. Additionally, a number of predator groups including birds, fish (Tautoga onitis (L.) [Tautog], Fundulus heteroclitus (L.) [Mummichog]), native crab species (European Green Crab, Mud Crab) and polychaete worms can be found foraging during both high and low tide. Together, these factors were likely the most important causes of overall mortality, resulting in the nearly 100% mussel loss from open and partial-cage treatments by the end of the experiment in both years. Initially, we had hypothesized a predator-density effect, predicting lower mussel mortality in the low-density crab treatment than in the high-density crab treatment. There is a trend, but no statistically significant difference in final average mussel survivorship between those treatments. This result may be due to caging artifacts influencing crab behavior. Increased crab density can lead to increased agonistic interactions among crabs resulting in less time spent foraging and lowered consumption rates, or to eating less-preferred prey to minimize competition for limited food resources (Clark et al. 1999, 2000). Studies have shown that increasing conspecific densities in the laboratory leads to increased diet breadth in Asian Shore Crabs (Brousseau and Baglivo 2005), suggesting foraging behavior may be influenced by crab density in the cages. Feeding patterns of Asian Shore Crabs on mytilids under field conditions differ from those observed in the laboratory. High consumption rates of 10–18 mussels day -1 are typical under laboratory conditions where crabs are allowed to feed continuously (Boudreau and O’Connor, 2003, Brousseau et al. 2001, DeGraaf and Tyrrell 2004), whereas crabs ate fewer than one mussel day -1 over the course of this field study. Moreover, in a study of the diet of Asian Shore Crabs from Odiorne Point, NH, Griffen et al. (2012) found that during April–October, mussels were present in a Northeastern Naturalist 130 D.J. Brousseau, R. Goldberg, and C. Garza 2014 Vol. 21, No. 1 minority of the crabs guts sampled (25% in June, 11% in August), supporting the conclusion that this food item is not the major component of its diet. These differences between laboratory and field results may be due to increased food choice in the field, limited foraging opportunities because of a semidiurnal tidal cycle (crabs forage primarily during high tide at night) and/or the presence of occasional high wave action hampering the ability of these small crabs to handle and open mussel prey. Other factors such as water temperature and prey densities may also play a role. Although Asian Shore Crabs prefer mussels ≤10 mm SL, they will feed on mussels up to 20 mm SL in the lab (Brousseau et al. 2001, Gerard et al. 1999). Our data indicate that smaller mussels (less than 9.0 mm) may have been preyed upon preferentially by Asian Shore Crabs, suggesting that as mussels grow larger, they will experience a size refuge from predation. This conclusion is supported by the argument that in nature rapidly growing bivalves such as Blue Mussels can quickly move out of the size range most vulnerable to predation from many predator groups including the Asian Shore Crab (Suchanek 1978). Moreover, the recent work by Freeman and Byers (2006) shows that mussels from locations in southern New England (including Long Island Sound) have evolved an inducible shell-thickening response to waterborne cues from Asian Shore Crabs, and that this presumed anti-predator response has evolved rapidly, within approximately 20 years of its introduction. This work suggests that there is considerable potential for interactions between native and invasive species to vary temporally and geographically due to local selection pressures. Although there are no historical records documenting mussel abundance and distribution for this harbor, there have been many anecdotal reports over the past few decades of mussel declines in this part of Long Island Sound. Many factors may be responsible, and more research is needed to fully understand the role of local physical and biological factors—such as wave action, temperature, recruitment success, and the impact of all predator groups—in regulating abundance and stability of intertidal mussel populations in this area. However, our study clearly demonstrates that predation by Asian Shore Crabs plays a minor but measurable role in Blue Mussel mortality in the intertidal zone at Black Rock Harbor. This finding emphasizes the need to assess the impact of invaders within different habitat types and geographic regions to fully understand the ways native-nonnative species interactions can vary over the entire range of selection pressures and ecological conditions in which they occur. 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