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%.
With only one recorded eradication of a non-native marine organism
(Culver and Kuris 2002), The Asian shore crab appears here to stay
(although see Lohrer and Whitlatch 2002b). The drastic (50–95%) declines
that we have observed in the populations of resident crabs and
common periwinkles, temporally coincident with increase of the Asian
shore crab population, strongly suggest that the invader has had significant
ecological impacts.
Acknowledgments
The work presented here was supported by the Heineman Foundation and the
Purchase College Foundation. Special thanks go to J.A. Coyer for comments on an
early draft. Field assistance was provided by A.M. Eversley, D. Kohtio, S. Metzger,
E. Mignone, I. Sen, and R. Wallace. Some of the work presented here constituted
Senior Project research for A. Gordon and M. Sellberg.
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