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Eight-year Record of Hemigrapsus sanguineus (Asian Shore Crab) Invasion in Western Long Island Sound Estuary
Cynthia D. Huebner

Northeastern Naturalist, Volume 14, Issue 2 (2007): 207–224

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2007 NORTHEASTERN NATURALIST 14(2):207–224 Eight-year Record of Hemigrapsus sanguineus (Asian Shore Crab) Invasion in Western Long Island Sound Estuary George P. Kraemer1,*, Monica Sellberg1,2, Alon Gordon1, and Jeff Main3 Abstract - Hemigrapsus sanguineus (Asian shore crab) first arrived at Rye, NY in 1994. The intertidal abundances of H. sanguineus, Carcinus maenas (green crab), and the native crabs Eurypanopeus depressus (flatback mud crab), Cancer irroratus (Atlantic rock crab), and Libinia emarginata (spider crab) were censused from 1998–2005. Asian shore crab densities (estimated in June) increased from 1998–2001 to ca. 120 crabs m-2, and then declined to 80 crabs m-2 from 2002–2005. The flatback mud crab declined in abundance by about 95%. Decreases in the abundances of Atlantic rock crabs, green crabs, and spider crabsmay also have occurred, though these species were uncommon at the outset of the study. The lower intertidal density of the gastropod Littorina littorea (common periwinkle) decreased by about 80%, and the decline was coincident with the expansion of the Asian shore crab population. In June, small Asian shore crabs were disproportionately more abundant in the upper intertidal zone compared with lower zones, where large crabs were more abundant. January intertidal populations were dominated by small Asian shore crabs, and these were restricted to the lower half of the intertidal zone. Introduction Humans have a long history of the transport of organisms into territories outside their natural ranges (Cox 1999, Huston 1994). While introductions have the potential to produce higher local species richness (Huston 1994), non-native species can cause marked declines in biodiversity. In fact, decreases in both diversity and abundance of similar trophic-level organisms often occur following introductions (e.g., Cox 1999, Hill and Lodge 1999, Kimmerer et al. 1994). Many of the world’s busiest seaports are situated on estuaries. The large volume of commercial and recreational ship traffic, coupled with ballastwater transport of non-native organisms, is responsible for much of the high rate of non-native introductions into estuaries (Cohen and Carlton 1998, Ruiz et al. 1997). The Long Island Sound estuary (LIS) is not likely to differ in this respect. In 1998, five species of crabs were observed in the intertidal zone at 1Department of Environmental Sciences, SUNY - Purchase College, 735 Anderson Hill Road, Purchase, NY 10577. 2Dvirka and Bartilucci Consulting Engineers, 330 Crossways Park Drive, Woodbury, NY11797 . 3Conservation Division, Westchester County Division of Parks, Recreation, and Conservation, 25 Moore Avenue, Mt. Kisco, NY 10549. *Corresponding author - george.kraemer@ purchase.edu. 208 Northeastern Naturalist Vol. 14, No. 2 Read Wildlife Sanctuary (Rye, NY) near the western terminus of the Long Island Sound estuary: the natives Eurypanopeus depressus Smith (flatback mud crab) and Cancer irroratus Say (Atlantic rock crab), the established nonnative Carcinus maenas Linnaeus (green crab), introduced from Europe ca. 1817; and the recently arrived Hemigrapsus sanguineus De Haan (Asian shore crab). In addition, Libinia emarginata Leach (spider crab) was sometimes found near the low tide mark. The flatback mud crab is an estuarine crab typically found in the low intertidal to subtidal zones from Massachusetts into the Gulf of Mexico, though it is less common north of Chesapeake Bay (Gosner 1978, Weiss 1995). Atlantic rock crabs and spider crabs are estuarine crabs with a broad latitudinal distribution on the Northeast coast. Though generally found subtidally, this species is occasionally found under rocks in the lower intertidal zone (Weiss 1995). The Asian shore crab was discovered on the southern New Jersey coast in 1988 (McDermott 1991), and first reported in LIS in 1994 (McDermott 1998). The Asian shore crab possesses life-history and autecological characteristics that have helped make it a successful invader. It is a diet generalist; stomach content analysis and laboratory feeding trials reveal that Asian shore crabs consume a variety of invertebrates and seaweed (Bourdeau and O’Connor 2003; Brousseau et al. 2000; Gerard et al. 1999; Ledesma and O’Connor 2001; Lohrer and Whitlatch 1997, 2002a). Additionally, females can produce several broods per year of up to 60,000 eggs over a three-year lifespan (McDermott 1991), and both the larvae and adults show broad salinity (Epifanio et al. 1998; A. Gordon and G.P. Kraemer, unpubl. data) and temperature (cf. Lohrer et al. 2000) tolerances. The current distribution of Asian shore crab extends from Maine to North Carolina on low-energy, rocky intertidal sites. Competition with other crabs is probably occurring since there is evidence of niche overlap (Lohrer et al. 2000), though predation may be more important (Lohrer and Whitlatch 2002b). Littorina littorea Linne (common periwinkle) is an often abundant member of the intertidal community at protected sites, and a potential prey item for Asian shore crabs. This common gastropod generally occurs across a broad band of the intertidal zone. This herbivorous snail was introduced from Europe onto North American shores in the early 1800s (Carlton 1992). Experimental evidence suggests that this species has greatly modified the community structure at protected intertidal locations by preventing the establishment and maintenance of a macroalgal canopy (Bertness 1984). The goal of this study was to document changes in populations of Asian shore crab and several intertidal invertebrates as Asian shore crabs became established at a low-energy, estuarine intertidal site. Specifically, we tracked the changes over eight years in the abundance of Asian shore crabs and resident crabs (the natives flatback mud crab, Atlantic rock crab, and spider crab, and the established non-native green crab), and the common 2007 G.P. Kraemer, M. Sellberg, A. Gordon, and J. Main 209 periwinkle. We also compared the size structure of the Asian shore crab population at different intertidal elevations to examine habitat use. Materials and Methods The study was conducted at Edith Read Wildlife Sanctuary in Rye, NY, near the western end of the LIS estuary. This site is ca. 240 km (linear distance) from the initial discovery near Cape May, NJ, and near the geographic midpoint of the current known range of Asian shore crabs. The substrate at the site consists of cobbles to boulders, on top of a layer of sand and gravel that varies in thickness depending on intertidal elevation and time of year. This low energy site has an average slope of 3°. Crab populations were censused during low tides from 1998–2005. During part of this period, observations were made throughout the year (up to biweekly), although for the sake of brevity and to enable annual comparisons that are not confounded by seasonal effects, June sampling was always conducted each year. On each June sampling date, three intertidal transects were randomly positioned along an 80-m stretch of shoreline. These transects ran from -0.1 to +2.2 m above mean low water (MLW). A 0.49-m2 quadrat was placed at 2-m horizontal intervals along each transect, for a total of 21 quadrats sampled per transect across the intertidal zone. Rocks within each quadrat were overturned individually, and crabs with a carapace width (CW) > 4 mm were captured. The sand-gravel mixture under the rocks was examined by hand for buried crabs. After collection, cobbles and boulders were returned to their previous positions and orientations. Beginning in June 2000, the CW of all captured crabs was measured using either calipers or via digital photography and the ImageTool image analysis software (ddsdx.uthscsa.edu/dig/itdesc.html). Comparison of measurements made using both techniques revealed no difference in the CW estimate (n = 61, slope of regression not significantly different from 1.0; p > 0.85). The relationship between CW and biomass (fresh weight) was determined empirically for Asian shore crabs of both sexes to enable estimation of individual biomass from CW measurements. Biomass was linearly related to the cube of CW, but males of a given CW were heavier than either gravid or non-gravid females. Crab abundance was also pooled within transects across the intertidal zone to record the total number of each crab species captured and species richness for each June transect. From these data, Shannon’s diversity index was calculated (H' = 􀂙piln[pi], where pi = proportion of all crabs constituted by crab species “i”) . For evaluations of seasonal differences in intertidal crab distributions and population size-frequency characteristics, measurements of crab size (CW) from pooled intertidal transects collected in January and June 2005 were compared in detail. Transect data were 210 Northeastern Naturalist Vol. 14, No. 2 subdivided into low (-0.1 to +0.7 m above MLW), mid (+0.7 to +1.5 m), and high (+1.5 to +2.2 m) ranges. Size-frequency information and sex ratios for Asian shore crabs were obtained for these three zones. For crabs 􀂔 8 mm CW, a dissecting microscope (20x magnification) was used to determine sex, since external morphological differences between males and females are slight at this size. Size-frequencies and sex ratios were similar for the mid- and low intertidal ranges; these two zones were pooled for analysis. Sex ratios were examined for deviation from 1:1 using chisquared tests (Sokal and Rohlf 1987). In addition to the intertidal transects from low to high tide, a permanent 50-m transect line was established parallel to the shoreline at +1.2 m above MLW. This elevation corresponded to the maximum Asian shore crab densities at the Rye site. In June and at other times during the year, a 0.49-m2 quadrat was placed at 6–8 randomly chosen points along the transect line. In cases where samples were collected to determine seasonal changes in abundance, no position along the transect was sampled more than once within an eight-week period. Crabs were captured, counted, and measured as above. These data are presented as numerical density (no. m-2) and biomass density (g m-2) across one year (2001). Common periwinkle densities were estimated in late summer (late September– early October). Quadrats (680 cm2; n = 20–60, average = 36) were placed randomly at a low intertidal elevation (+0.2 m above MLW). Common periwinkles were inspected; empty shells and those inhabited by hermit crabs were discarded. Live common periwinkles were counted, though shell size was not recorded. During this eight-year study, personnel turnover occurred. To maintain methodological consistency and ensure comparability among years, two strategies were employed. First, the lead author always sampled the midintertidal (+1.2 m above MLW) June transects, with the help of one assistant trained before collection. A second approach was used for the three June transects, running from the low to high intertidal zones, since these required the most manpower. Each year, the lead author demonstrated the harvest technique to three-person groups of assistants. At least one of the assistants was already experienced, having participated in one or two prior June sampling efforts. This person acted as leader of the three-person groups. The first author monitored the groups during transect sampling for compliance with technique and consistency of effort. Results Crab population dynamics Mid-intertidal densities of Asian shore crabs were similar during the first two years of the study, but increased abruptly after June 1999 (Fig. 1). By June 2000, the mid-intertidal density of Asian shore crabs had increased 2007 G.P. Kraemer, M. Sellberg, A. Gordon, and J. Main 211 almost five-fold over the previous year, a trend mirrored across the rest of the intertidal zone. Average abundances peaked at 120 Asian shore crabs/m2 in 2001 and 2002. Several individual quadrats contained more than 200 Asian shore crabs/m2, and one had a density of 305 individuals/m2. Asian shore crab densities declined slightly to apparent constancy (ca. 80 individuals/ m2) during 2003–2005. The density and biomass of Asian shore crabs at the mid-intertidal elevation (+1.2 m) varied with season (Fig. 2); both were lowest in winter (Jan–Feb) and highest in early fall (Sept). Four crab species were recorded in the intertidal zone in June 1998 and 1999, when Asian shore crabs and flatback mud crabs were most common and equally abundant (maximum density ca. 15 crabs m-2). These two crabs also had the broadest intertidal distributions (Table 1). In 1998 and 1999, green crabs and Table 1. Intertidal distributions of resident crabs at Read Wildlife Sanctuary (Rye, NY) in 1998 and 1999. Full range indicates capture of at least one individual; highest abundance range shows distribution where population densities ranged between 30–100% of the maximum value. Cancer irroratus (Atlantic rock crab) and Libinia emarginata (spider crab) were found in the lower intertidal zone, but were not common enough to allow accurate determination of ranges Highest Full intertidal range abundance range Species (m above MLW) (m above MLW) Hemigrapsus sanguineus (Asian shore crab) -0.1–2.0 0.7–1.7 Eurypanopeus depressus (flatback mud crab) -0.1–1.5 0.4–1.2 Carcinus maenas (green crab) -0.1–1.0 0.2–0.5 Figure 1. June densities (number per square meter) from 1998–2005 of Hemigrapsus sanguineus (Asian shore crab) in the mid-intertidal zone (+1.2 m above mean low water; Rye, NY). Error bars represent standard deviations (n = 8, except for 1999, when n = 6). 212 Northeastern Naturalist Vol. 14, No. 2 Atlantic rock crabs were relatively uncommon (0–1 crabs m-2), and present only in the lower intertidal zone. When the captured crabs were pooled within transects across the intertidal zone, differences across time were notable (Table 2). Over the period during which Asian shore crabs increased (1999–2000), intertidal flatback mud crab abundances decreased by 95%. A broad area of the intertidal zone that once harbored a substantial flatback mud crab population (-0.1 to +1.5 m above MLW) was virtually devoid of these natives by 2000. Neither green crabs nor Atlantic rock crabs were found in any of the June 2000 transects. From 2001–2004, Asian shore crabs comprised > 99% of all crabs captured across the intertidal transects. Green crabs and Atlantic rock crabs were even less common than before, and the Figure 2. Seasonal patterns of density (number per square meter) and biomass (grams fresh weight per square meter) in 2000–2001 of Hemigrapsus sanguineus (Asian shore crab) in the mid-intertidal zone (+1.2 m above mean sea level; Rye, NY). Error bars represent standard deviations (n = 6–8). 2007 G.P. Kraemer, M. Sellberg, A. Gordon, and J. Main 213 Table 2. Abundance and species richness of the intertidal crab community at Read Wildlife Sanctuary (Rye, NY) in western Long Island Sound from 1998–2005. Abundance values (n) represent numbers of crabs (three intertidal transects pooled) captured in early June each year. Species richness values are average numbers of species per transect. Shannon diversity index, incorporating both crab species richness and relative abundance, are average values for the three transects. Hemigrapsus Eurypanopeus Carcinus Cancer sanguineus depressus maenas irroratus Libinia Transect (Asian shore (flatback mud (green (Atlantic emarginata Transect Shannon crab) crab) crab) rock crab) (spider crab) species richness diversity index Year n % total n % total n % total n % total n % total Avg SD Avg SD 1998 351 52.5% 304 45.4% 6 0.9% 8 1.2% 0 0.0% 3.3 1.2 0.75 0.14 1999 558 51.8% 500 46.4% 12 1.1% 8 0.7% 0 0.0% 3.7 0.6 0.74 0.05 2000 2367 99.3% 18 0.7% 0 0.0% 0 0.0% 0 0.0% 1.7 0.6 0.08 0.04 2001 4354 99.2% 27 0.6% 3 0.1% 0 0.0% 3 0.1% 4.0 0.0 0.09 0.01 2002 2751 99.5% 13 0.5% 0 0.0% 0 0.0% 0 0.0% 1.7 0.6 0.05 0.06 2003 1757 99.5% 8 0.5% 0 0.0% 0 0.0% 0 0.0% 1.3 0.6 0.03 0.05 2004 1692 99.6% 6 0.4% 0 0.0% 0 0.0% 3 0.2% 1.7 1.2 0.02 0.04 2005 1733 100.0% 0 0.0% 0 0.0% 0 0.0% 0 0.0% 1.0 0.0 0.00 0.00 214 Northeastern Naturalist Vol. 14, No. 2 already small fraction of all intertidal crabs constituted by flatback mud crabsteadily declined. In June 2005, only Asian shore crabs were recorded in intertidal transect quadrats. Crab diversity in the intertidal community, measured as both species richness and the Shannon diversity index per pooled perpendicular transect, declined during the study, the latter by more than an order of magnitude (Table 2). Asian shore crab demography Summer and winter intertidal populations differed in size structure and distribution. In addition to being much less abundant, Asian shore crabs captured during January were found only in the lower intertidal zone (roughly the bottom half; Fig. 3a). Asian shore crabs, pooled across the Figure 3. Carapace size-frequency distributions of Hemigrapsus sanguineus captured at three intertidal elevation ranges during January (panel a) and June (panel b) at Rye, NY (2005). Low intertidal range = -0.1 to +0.7 m above MLW; mid-intertidal range = +0.7 to +1.5 m; high intertidal range = +1.5 to +2.2 m. 2007 G.P. Kraemer, M. Sellberg, A. Gordon, and J. Main 215 intertidal zone, were also very small in January; 99.8% of Asian shore crabs were 􀂔 15 mm CW (ca. 1.5 g), and 88% were 􀂔 10 mm CW (ca. 0.5 g). The size structure of the Asian shore crab population varied as a function of elevation within the intertidal zone in June (Fig. 3b). In this month, the low intertidal zone harbored the biggest crabs; for example, in 2005, the largest crabs (all males) captured in the low, mid-, and high intertidal zones were 37, 34, and 22 mm CW, respectively. These CW values are equivalent to 27, 21, and 6 g, respectively. The average sizes of Asian shore crabs in the three zones were 18, 15, and 11 mm CW (4.4. 2.7, and 0.9 g), respectively. About 54% of low intertidal Asian shore crab individuals were 16–26 mm CW (2–8 g), and 15% were very large (33–37 mm CW; 16–32 g). The upper end of the intertidal distribution was dominated by Asian shore crabs that were small: 88% of the crabs there were less than 15 mm CW (1.5 g). When June biomass and abundance data for each size class were combined, elevation-based differences in the distribution of population biomass were also dramatic. About 71% of the total Asian shore crab population biomass in the low intertidal consisted of crabs greater than 4 g (21 mm CW; Fig. 4a). This value dropped to 55% and 6% in the mid- and high intertidal ranges. However, the overall biomass profile obscures sex-based differences (Figs. 4b and c). Different growth allometries lead to more massive males at any given CW (e.g., at a CW of 25 mm, non-gravid females weigh 6.9 g, compared with 8.3 g for males). In addition, males grow to larger CW than do females; in June 2005 of this study, the average male weighed ca. 20% more than the average female. Most of the sex-based difference in population biomass occurred in the low and the mid-intertidal zones. Only 5% of biomass from these zones was contributed by females > 8 g (25 mm CW), compared with 39% for the males. Sex ratios for the combined (low + mid) and the high intertidal collections were compared against an expected 1:1 ratio (Fig. 5). Of the juvenile Asian shore crabs found in the (low + mid) intertidal range (juveniles defined operationally as < 1.0 g, 12 mm CW, the approximate limit of sexual maturity in females), males were slightly, though significantly, more common than females. As size increased up to 4.0 g (21 mm CW), the fraction of males decreased. Further increases in size were accompanied by a switch to proportionately more males; almost 80% of Asian shore crab individuals in the > 8 g size class were male, a significant departure from 1:1. Samples sizes from the upper intertidal range were small and limited the strength of the analysis. The results showed only one significant departure from a 1:1 ratio: only 35% of juvenile Asian shore crabs from the high intertidal zone were male. The density of common periwinkles in the low intertidal zone (0.0–0.2 m above MLW) declined between 1999 and 2001, and remained at similar levels from 2002–2005 (Fig. 6). A similar decrease in snail abundance was observed 216 Northeastern Naturalist Vol. 14, No. 2 Figure 4. Biomass class contribution to total Hemigrapsus sanguineus (Asian shore crab) population biomass at three intertidal elevation ranges during June (2005) at Rye, NY. Carapace width converted into mass using sex-specific allometric equations. Panel a: all crabs pooled. Panels b and c: males and females, respectively. Low intertidal range = -0.1 to +0.7 m above MLW; mid-intertidal range = +0.7 to +1.5 m; high intertidal range = +1.5 to +2.2 m. at a slightly higher elevation (+0.5–0.7 m; data not shown); the decline in snail density from 1998 to 2005 current levels was about 80% at both elevations. The 2007 G.P. Kraemer, M. Sellberg, A. Gordon, and J. Main 217 declines were coincident with the increase in Asian shore crab population (Fig. 6 inset). The negative correlation between the mid-intertidal density of Asian shore crabs and that of common periwinkles lower in the intertidal was significant (R2 = 0.72, p = 0.016). Discussion Changes in population levels The Asian shore crab has been present at the study site since 1994, but increased greatly in abundance to ca. 120 individuals m-2 between 1999– 2001. A time lag like this, between introduction of a non-native and its population expansion, is common though poorly understood (Cox 1999). At the Rye site, the increase in Asian shore crab abundance may have been recruitment-driven; a large pulse of Asian shore crab juveniles settling onto both intertidal and floating offshore substrate was reported in northern and western LIS in the summer of 2000 (Ledesma and O’Connor 2001; R.B. Whitlatch, University of Connecticut, pers. comm.). The abundance Figure 5. Percent of Hemigrapsus sanguineus (Asian shore crabs) captured in June 2005 that were male presented as functions of biomass class and intertidal elevation at Rye, NY. Data from low and mid-intertidal ranges (similar pattern) were pooled. Stars indicate statistically significant deviation from 1:1 sex ratio (p < 0.01 in all cases). The number under each bar represents the sample size. 218 Northeastern Naturalist Vol. 14, No. 2 of Asian shore crabs dropped from the 2001–2002 highs, though June 2005 densities are still eight times greater than those measured in 1998, the first year of the study. The dramatic increase in the abundance of the non-native crab likely caused the decline of the flatback mud crab at the western LIS site. Green crabs and Atlantic rock crabs may also have been impacted by Asian shore crabs, but because these were relatively uncommon at the outset of the study, we are less confident of the trend (cf Tyre et al. 2003). However, Lohrer and Whitlatch’s (2002b) study is noteworthy: similar proportional decreases (up to 90%) in green crab abundance were reported at three intertidal sites in Connecticut. Experimental field manipulations led Lohrer and Whitlatch (2002b) to conclude that Asian shore crab predation on young-of-the-year green crabs was responsible for the observed decreases. Predation by Asian shore crabs does not exclude other mechanisms acting in concert to cause the decline of resident crabs. The existence of niche overlap with the other crabs (Lohrer et al. 2000) argues that the Asian shore crab is also competing with natives for resources of food and habitat. Although the intertidal abundance of flatback mud crabs and perhaps other resident crabs have declined greatly, refugia may exist. The Atlantic Figure 6. Densities of gastropod Littorina littorea (common periwinkle) in low intertidal zone (+0.2 m above mean low water) at Rye, NY. Error bars represent standard deviations (navg = 36, range = 20–60). ND = not determined. Inset: Hemigrapsus sanguineus (Asian shore crab) densities from June censuses (see Fig. 1) plotted against common periwinkle densities estimated during September of same year. 2007 G.P. Kraemer, M. Sellberg, A. Gordon, and J. Main 219 rock crab has substantial subtidal populations (Weiss 1995), as does the green crab (Aagard et al. 1995, Novak, 2004, Ross et al. 2002). Whether interactions between Asian shore crabs and other crabs are qualitatively different in the subtidal environment may determine the relative value of the subtidal environment as a refuge. The statistical disappearance of the native flatback mud crab from the 2005 samples across the intertidal zone is significant; the flatback mud crab was co-dominant with Asian shore crabs during the first years of the study, and the most abundant crab at the study site prior to the population increase of Asian shore crabs (G.P. Kraemer and J. Main, pers. observ.). Even if Asian shore crab abundance was to drop, the recovery of the native flatback mud crab might not be certain. Panopeus herbstii Milne Edwards (common mud crab), another xanthid crab, relies on a species-specific chemical cue to induce settlement and metamorphosis of planktonic larvae (Andrews et al. 2001). Lacking an adult population, flatback mud crabs might not receive the information necessary to select suitable habitat. The intertidal crab community at this estuarine site experienced a loss of diversity, seen in terms of species richness and Shannon diversity index, with the rapid expansion of the Asian shore crab population. These diversity declines are particularly significant considering that estuaries generally have low native diversities (Day et al. 1989) and are subject to high rates of non-native introductions (Cohen and Carlton 1998, Ruiz et al. 1997), and because diversity may be inversely correlated with invasion susceptibility (Byers and Noonburg 2003, Stachowicz et al. 1999). Asian shore crab densities at the Rye, NY site were generally high in comparison with other sites on the northeast coast of North America. Taking into account that population maxima at this site occur in September (Fig. 2), the June 2001–2002 densities may translate into September densities of 250 Asian shore crabs/m2 during the same years. Brousseau et al. (2002) and Lohrer and Whitlatch (2002b) reported densities of 60 and 90 crabs/m2 at Bridgeport and New Haven, CT, respectively. O’Connor’s (2001) Bourne, MA study reported 120 Asian shore crabs/m2. These high values were all obtained at sites south of Cape Cod, MA, a biogeographic boundary. Densities from sites north of Cape Cod are much lower; densities of 5–15 crabs m-2 were reported in Cape Cod Bay (O’Connor 2001), and a 2004 study in New Hampshire captured about 13 crabs m-2 (Griffen and Byers 2006), only one-sixth of densities at Rye, NY during the same year. The abundances, coupled with rather generalist diets, indicate that the Asian shore crab is likely exerting broad ecological effects on the structure of low-energy, estuarine intertidal communities. These omnivorous consumers eat a wide variety of meiofauna, juvenile and adult macrofauna, as well as plant material. Predation by Asian shore crabs is implicated in the declines of mussels and other organisms at several 220 Northeastern Naturalist Vol. 14, No. 2 Connecticut sites (Lohrer and Whitlatch 2002a, b). Barnacle densities have also declined since the introduction of Asian shore crabs at Rye (J. Main, pers. observ.). The decrease in common periwinkle densities was coincident with the increase in Asian shore crab abundance. Though the results suggest a cause-and-effect relationship, we are aware that other explanations exist for the decline (e.g., human harvest [Cummins et al. 1999], inverse covariance by Asian shore crabs and common periwinkles to a common environmental factor). However, predation by Asian shore crabs may be connected to the observed littorinid decline. Stomach contents analysis and laboratory feeding trials have shown that Asian shore crabs can consume common periwinkles (Gerard et al. 1999, Ledesma and O’Connor 2000, Lohrer and Whitlatch 1997). Although Bourdeau and O’Connor (2003) reported very few adult common periwinkles consumed by Asian shore crabs in laboratory feeding trials. The smaller juvenilees maybe more readily consumed. Asian shore crab demography The upper intertidal zone can be physically stressful due to desiccation and temperature stresses. For example, temperatures under rocks at the upper limit of the Asian shore crab distribution after 5 h emersion during a warm (28 °C) mid-June day reached 41 °C by 1400 hrs (G.P. Kraemer, unpubl. data). Asian shore crabs inhabiting the upper intertidal zone during summer showed significant differences in size-frequency characteristics compared with crabs from the mid- and low intertidal zones. In June, the average size declined from 16.7 mm CW (ca. 2.1 g) in the low intertidal range to only 10.9 mm (ca. 0.6 g) in the high range. This was driven by the preponderance of the smallest crabs; 35% of the high intertidal crabs were 􀂔 0.25 g (􀂔 8 mm CW, young-of-the-year), compared with 19% and 17% in the mid- and low intertidal ranges. In addition, during winter, virtually all Asian shore crabs inhabited the lower intertidal zone, and all were small; 60% were 􀂔 8 mm CW, and 96% 􀂔 13 mm. We are confident that most Asian shore crabs at the Rye site, and certainly the larger individuals, migrate subtidally in winter; recruitment and growth cannot explain the abrupt appearance of large crabs the following June. The subtidal activities of Asian shore crabs are currently unknown, though anecdotal reports from commercial fishermen suggest that the range of Asian shore crabs extends far into the subtidal environment. The elevated proportion of small individuals high in the intertidal zone during summer and the preponderance of small intertidal Asian shore crabs in winter seem paradoxical given that the higher surface area-tovolume ratio engenders greater risk of death due to temperature and desiccation stresses (cf. Spivak et al. 1994). However, larger crabs are known to prey upon conspecific juveniles (Lohrer and Whitlatch 2002b). Even if cannibalism does not occur (e.g., lacking a large size differential), larger 2007 G.P. Kraemer, M. Sellberg, A. Gordon, and J. Main 221 Asian shore crabs likely out-compete smaller individuals to obtain moister, more thermally favorable habitat (cf. Jensen et al. 2002). Predation pressure and/or competition may drive an ontogenetic shift in habitat use by Asian shore crabs, with smaller crabs inhabiting environmentally sub-optimal, but biologically safer habitats. Similarly, Rochette and Dill (2002) reported that adult Littorina sitkana Nomen Nudum (Sitka periwinkle)gastropods used higher intertidal elevations in response to predation in the low intertidal zone. Juvenile Asian shore crabs from the (low + mid) range were more likely to be male than female. The dependence of the sex ratio on crab size class in the (low + mid) intertidal range may have three non-exclusive explanations. It could result from (i) differential mortality at the larval and/or settlement stages (Wenner 1972), (ii) a sex-based difference in habitat use, or (iii) sex-based sampling bias. As the crabs grow in size to 4 g, males become less abundant relative to females (Fig. 5), possibly due to malebiased mortality (though migration of larger males into the subtidal zone is also possible; Flores and Negreiros-Fransozo 1999, Koga et al. 1998). For crabs greater than 4 g, the proportion shifts again towards male dominance due to a difference in abundance, coupled with strong sexual dimorphism. Few of the largest Asian shore crabs are female; 77% of crabs larger than 8 g (26 mm CW) were male, and no female exceeded 12 g. Captured males weighed up to 24 g (37 mm CW). Also, males of any CW outweighed females by about 20%. 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