Northeastern Naturalist Vol. 26, No. 1 G.P. Kraemer 2019 81 2019 NORTHEASTERN NATURALIST 26(1):81–94 Changes in Population Demography and Reproductive Output of the Invasive Hemigrapsus sanguineus (Asian Shore Crab) in the Long Island Sound from 2005 to 2017 George P. Kraemer* Abstract - Cross-intertidal transects at a western Long Island Sound estuary site provided estimates of the density of the non-native Hemigrapsus sanguineus (Asian Shore Crab) from 1998 to 2017, and measurements of crab size (carapace width; CW) from 2005 to 2017. Since 2001, average intertidal density declined by ~5% per year. This decline was driven by decreases in the density of larger crabs, with consequent reductions in average and maximum sizes of both males and females. The proportion of the largest crabs (>24 mm CW) dropped from 10.1% of the population in 2005 to 1.4% in 2017. Individual reproductive output scales with size; thus, I estimate the loss of the largest females to have reduced population reproductive output by half between 2005 and 2017. Also, the frequency of ovigerous females in the smallest reproductively mature classes (12–14 mm CW) increased. Though the density and average size of Asian Shore Crab have declined significantly, resident and native crab populations have still not recovered. Introduction Non-native species are an increasingly important aspect of the marine biological landscape, the result of global trade transporting novel biota via ballast water and substrate fouling (Molnar et al. 2008, Seebens et al. 2013). Ecological and economic impacts of non-natives are generally negative, though the degree of harm reported varies from moderate (Sargassum muticum [Yendo] Fensholt [Japanese Wireweed] on the Pacific Coast; Smith 2016) to severe (e.g., Dreissena polymorpha [Pallas] [Zebra Mussel]; Karatayev et al. 2015). This variability may represent real ecological differences in invader–host community interactions. Alternately, because invasions are complex, dynamic processes, the variability in perceived impact could stem from examination of invasions at different times after the initial introduction (Campbell and Echternacht 2003). Most ecological investigations are limited in duration (Jenkins and Uyà 2016); thus, time may be insufficient for the full integration of the invader into the receiving community and the re-equilibration of community structure. Many invasion studies may be snapshots of current conditions from within a variable, successional process. Much of recent non-native species research has focused on understanding the mechanisms underlying the success of invaders. Though not uniformly the case, arrival of non-natives without the predators, parasites, and competitors of the home environment plays a role in success (Prior et al. 2015), as does the ability to produce *Department of Environmental Studies, Purchase College (SUNY), 735 Anderson Hill Road, Purchase, NY 10577; George.Kraemer@purchase.edu. Manuscript Editor: Thomas Trott Northeastern Naturalist 82 G.P. Kraemer 2019 Vol. 26, No. 1 copious, widely dispersing offspring (Brousseau and McSweeney 2016). The enemy release hypothesis has been invoked to explain the growth of the non-native Carcinus maenas (L.) (Green Crab), to larger size in the invaded environment than in the home range (Grosholz and Ruiz 2003). Size is an ecologically important life-history characteristic (Peters 1983); organism size determines diet composition (Costa 2009), metabolic requirements (Gillooly et al. 2001), and population density (LaBarbera 1989, White et al. 2007). Accordingly, the importance of size in understanding the invasion process and ecological impacts cannot be underestimated. Predators act as agents of natural selection. Preferential selection of largest prey has ecological and evolutionary consequences, since this behavior removes individuals with a high degree of fitness from the breeding stock. Removal of the largest individuals can shift the genotype profile of the population to one of slower growth, earlier age and smaller size at maturity, and altered reproductive output and recruitment (Conover and Munch 2002, Enberg et al. 2012). Hemigrapsus sanguineus (de Haan) (Asian Shore Crab) is a western Pacific grapsid crab introduced onto the coast of northeastern North America in the mid- to late 1980s likely via transport of ballast water (McDermott 1998). The species has spread to inhabit rocky coastlines from North Carolina to Maine. In the Long Island Sound estuary, the Asian Shore Crab grew from low densities in 1998, similar to those of the most abundant native crab, to become extremely abundant by 2001 (Kraemer et al. 2007). Monitoring of populations of Asian Shore Crab and other crabs in the intertidal zone of a western Long Island Sound site has continued through 2017. This long-term data set provides an opportunity to examine the Asian Shore Crab population in detail and detect changes over many generations. The objectives of this retrospective analysis were to (1) examine the intertidal Asian Shore Crab population for changes in density over time, (2) investigate whether this successful non-native crab exhibits the increase in size reported for other estuarine invaders, and (3) determine whether demographic shifts may be reflected in changes in reproductive o utput. Materials and Methods The study site at Read Wildlife Sanctuary (Rye, NY; 40°57'58.85"N, 73°40'7.07"W) is ~240 km (linear) from the site of initial discovery (Cape May, NJ), south of the geographic midpoint of the current range of the Asian Shore Crab. The site is located at the western end of the Long Island Sound estuary (LIS), and is a low-energy, gradually sloping, rocky intertidal site. The surface of the intertidal site consists of pebbles, cobbles, and boulders overlying a sandgravel- mud substrate. At the outset of the study, I observed 5 crab species within the intertidal zone: Asian Shore Crab, Panopeus herbstii H. Milne-Edwards (Chocolate-fingered Mud Crab), Green Crab, Cancer irroratus Say (Atlantic Rock Crab), and Libinia emarginata Leach (Common Spider Crab) (Kraemer et al. 2007). Densities of the intertidal crabs at this site have been recorded during the first set of spring low tides in June each year since 1998. Intertidal abundance of the Asian Shore Crab is highest from June to September (Kraemer et al. 2007). Northeastern Naturalist Vol. 26, No. 1 G.P. Kraemer 2019 83 During each sampling event, I positioned 2–4 (average = 2.9) 50-m transects across the intertidal zone from mean lower low water toward the dunes. I haphazardly chose locations of the starting points. I placed a 0.49-m2 PVC quadrat alongside the transect tape at 2-m intervals along each cross-intertidal transect, (i.e., during most years, 26 quadrats were sampled per transect, totaling 78 quadrats per sampling event). Teams of 3 people captured crabs under rocks within each quadrat. We bagged the crabs, which we placed on ice, returned to the laboratory, and counted. Beginning in 2005, we recorded sex and used calipers to measure the carapace width (to nearest 0.1 mm; CW) of each crab. Morphological differences between males and females are evident for CW > ~10 mm. I used a dissecting microscope (120x) to determine the sex of smaller crabs by examination for gonopods (males only). I estimated the average intertidal density of each crab species by summing data from all quadrats within each transect. I also recorded the number of species of brachyuran crabs discovered in each collection within quadrats and pooled the data for each transect. Population reproductive output was modeled for each year from 2005. In 2010, 2012, 2015, and 2017, I measured the total egg mass for females that spanned much of the size variability (12–30 mm CW) found during the study. I removed eggs from ovigerous females (n = 264) and dried them overnight at 60 °C. Year did not influence the total egg mass; hence, I pooled data. I modeled reproductive output as total dry-egg mass as a power function of the size (CW) of the female. In addition, I placed small aliquots of freshly collected eggs on pre-weighed microscope slides (n = 102 crabs). I photographed the slides at 63x using a Leica S6D digital microscope before drying at 60 °C overnight, and employed ImageJ software (https:// imagej.nih.gov/ij/) to count the eggs in the photos. I combined the relationship between dry mass of eggs (1–14 mg dry weight) and number of eggs (50–1300) with the relationship between CW and total dry-egg mass to estimate the potential reproductive output of each reproductively mature female captured in the population census (≥12 mm CW; though see Brousseau and McSweeney 2016) from 2005 to 2017. The number of reproductive events was not determined and depends on crab size and temperature (Fukui 1988); thus, estimates are presented as output relative to the starting 2005 value. I plotted and fitted average population densities of Asian Shore Crab (integrated across the intertidal zone) and maximum and average CW for both sexes against year in Sigmaplot (Systat Software, Inc. San Jose, CA) using a 2-parameter exponential model. The R2 value from an exponential model fit to the data was greater than the R2 value from a linear model fit to the data. Results The number of different crab species at the study site declined by ~56% over the course the study, from an average of 3.2 species per transect from 1998 to 2001 (median = 4; n = 12) to only 1.4 species per transect from 2002 to 2017 (median = 1; n = 47). The loss of resident, non-Hemigrapsus crabs was dramatic, with virtual extirpation of other crab species after 1999 (Fig. 1). Before 2000, densities of the Northeastern Naturalist 84 G.P. Kraemer 2019 Vol. 26, No. 1 Asian Shore Crab (7.5 crabs m-2) and all other crabs pooled (7.2 m-2) were similar, while the same metrics calculated over the 2000–2017 period were significantly different (51.6 vs. 0.15 crabs m-2, respectively, MannnWhitney U = 0.000, P < 0.001). Overall densities of the Asian Shore Crab, integrated across the intertidal zone, peaked at ~74 crabs m-2 in 2001, and then declined ~5% yr-1 from 2005 to 2017 (Table 1, Fig. 2). The exponential decline was statistically significant (P = 0.013). Figure 1. Asian Shore Crab dominance (Rye, NY). The relative abundance of invader and resident crab species has changed across the invasion record. See the text for a summary of the changes in the number of brachyuran crab species. No crab species other than Asian Shore Crab were captured in 2005, 2011–2014, and 2016–2017. Table 1. Results of regression analyses of Asian Shore Crab size dynamics (all metrics regressed against year from 2005 to 2017). Data were obtained by pooling quadrats within cross-intertidal transects. I used a two-parameter exponential model. ns = non-significant result. Metric Regression analysis outcome P Rate of change (% y-1) Overall density (2001–2017) F1,52 = 6.64 0.013 -4.6 Adult density (≥12 mm; 2005–2017) Males F1,36 = 24.5 less than 0.0001 -6.7 Females F1,36 = 25.2 less than 0.0001 -8.8 Size of largest ♂ crab F1,11 = 8.58 0.0137 -1.6 Size of largest ♀ crab ns Average size of ♂ F1,11 = 7.01 0.023 -2.2 Average size of ♀ F1,11 = 5.47 0.039 -2.1 Density less than 12 mm ns Density 12–20 mm F1,36 = 11.8 0.0015 -4.2 Density 20–24 mm F1,36 = 24.8 less than 0.0001 -12.8 Density >24 mm F1,36 = 46.7 less than 0.0001 -15.6 Northeastern Naturalist Vol. 26, No. 1 G.P. Kraemer 2019 85 The endpoints derived from the regression indicate a 38% drop in density from 2001 to 2017. Similarly, the density of adult male and female crabs (≥12 mm CW) crabs declined significantly from 2005 to 2017; average intertidal densities of females dropped by 8.8% y-1 and that of males declined by 6.7% y-1. The size (CW) of the largest crab captured each year, invariably male due to the marked sexual dimorphism in this species (Payne and Kraemer 2013), shrank by 1.6% y-1 from 2005 to 2017, for a total decline of ~15% (Fig. 3). The regression also fits well the single field-measurement of a very large male (43 mm CW) discovered outside of quadrats during 2001 sampling. The size of the largest female crab did not vary significantly with time (Fig. 3), even though the maximum size of males and females was significantly correlated (F1,11 = 4.896, P = 0.0489; data not shown). The average size of male and female Asian Shore Crabs also declined significantly by 2.2% y-1 ((F1,11 = 7.01, P = 0.023) and 2.1% y-1 (F1,11 = 5.47, P = 0.039), respectively. The changing demographics of the Asian Shore Crab population at the Rye, NY site were size-dependent. Densities of young of the year through small juveniles, defined as less than 12 mm CW, did not vary from 2005 to 2017 (Table 1; Fig. 4). Densities of crabs with 12–20-mm CW (numerically the most abundant class after the less than 12 mm class) declined significantly, as did the densities of crabs with 20–24-mm CW and those with >24-mm CW (Fig. 4). In 2005, the largest crabs (>24 mm) Figure 2. Population density estimates for Asian Shore Crab during the course of the study during which morphometric data was collected. Upper panel : overal l density (all sizes). Lower panel: male and female crabs ≥12 mm CW. Each symbol represents the average density for a cross-intertidal transect. In both cases, regressions are significant (see Table 1 for details). Northeastern Naturalist 86 G.P. Kraemer 2019 Vol. 26, No. 1 constituted 10.1% of the population, while in 2017 the same size-range accounted for only 1.4% of crabs. The rate at which densities declined was greater for larger size classes (Table 1). Smaller crabs were roughly 50% female, while largest crabs (≥24 mm) were mostly male (85%; Fig. 5). Though the largest Asian Shore Crabs disappeared from the population, the proportion of females remained similar from 2005 to 2017. Larger variability in the relative abundance of males and females is seen in larger size classes because sample sizes are smaller. For example, crabs >24 mm CW captured in 2005 numbered 27–58 per transect, while those captured in 2017 numbered 1–5 per transect. At the level of the population, the relative abundance of males and females did not differ in 2005 and 2017 (χ 2 = 1.49, P > 0.05). Sexual maturity in female Asian Shore Crab was well defined at this site; of the 10,988 crabs examined from 2005 to 2017, no crab smaller than 12 mm CW was found brooding eggs. Ovigerous female crabs provided the data to relate size (CW) to production of egg biomass. The power model relating CW and total egg mass was highly significant (F1,256 = 922, P much less than 0.0001, R2 = 0.67), with an exponent close to 3 (Fig. 6, top panel). The number of eggs was linearly related to dry-egg mass (Fig. 6, middle panel). This relationship was also highly significant (F1,101 = 474, P much less than 0.0001, R2 = 0.81). Combining these 2 relationships predicted the reproductive output by individual crabs: number of eggs = 4.08 * CW2.90. The model, applied to females ≥12 mm CW collected in cross-intertidal transects each year in June, suggests that areal reproductive output (eggs m-2) dropped significantly between 2005 and 2017 (Fig. 6, bottom panel). Figure 3. Largest male (circles) and female (triangles) Asian Shore Crab captured in transects each year. The unfilled symbol (upper panel) was a single, field-measured crab discovered under intertidal rocks during a random search. Year significantly influenced the size of the largest male from 2005 to 2017 (P = 0.0150), but year did not significantly influence the size of the largest female (P = 0.12; see Table 1 for details). Northeastern Naturalist Vol. 26, No. 1 G.P. Kraemer 2019 87 Of the smallest females, the fraction brooding embryos increased over the course of the study (Fig. 7). Before 2009, none of the 12–13-mm CW females captured, and only 4% of the 13–14-mm CW females were ovigerous. When we examined three 4–5-y periods during the study, a similar pattern of increasing proportion of small females brooding eggs was evident for the 2 size-classes of female crabs. Average intertidal densities of small ovigerous females (12–14 mm CW) in 2005 did not differ from those in 2017 (t-test, P value = 0.22), nor did densities of Figure 4. Densities of Asian Shore Crab from 2005 to 2017 by size class. Declines in density over time for crabs of 12–20 mm carapace width (CW), 20–24 mm CW, and >24 mm CW were statistically significant (in all cases, P < 0.01; see Table 1 for details). Northeastern Naturalist 88 G.P. Kraemer 2019 Vol. 26, No. 1 ovigerous females 12–15 mm and 12–16 mm CW differ between 2005 and 2017 (P values = 0.23 and 0.96, respectively). Discussion Data from this long-term study provide detailed evidence of population-level change at this estuarine rocky intertidal site. The declines in overall population density and in maximum and average crab sizes (via the disappearance of the largest crabs), suggest a change in ecological interactions among the Asian Shore Crab and other intertidal organisms. The fraction of crabs that were female remained essentially unchanged from 2005 to 2017; thus, the data suggest a mechanism unconnected to sex (i.e., the reduction in density affected both males and females similarly). The generality of the patterns reported here is not known; the present study sacrificed geographic range for temporal extent. This approach was required to reveal patterns in an inherently variable system. Most ecological investigations are of limited duration; 85% of marine benthic ecological investigations last 2 y or less (Jenkins and Uyà 2016), perhaps representing snapshots from within a dynamic equilibration. Two exceptions to this generalization are the studies of O’Connor (2014) and Bloch et al. (2015). The former study also reported a slight shift to smaller size over the course of the invasion, while the latter demonstrated rising Asian Shore Crab density from 2003–2012, though at a slower rate than seen at the Rye, NY, site (Kraemer et al. 2007). Figure 5. Average percent male Asian Shore Crab by size class for collections representing early (2005) and late (2017) invasion. A total of 1733 crabs were captured in 2005, and 1046 crabs in 2017. The small number above each bar represents the number of males captured. Error bars are standard deviation of transect averages (absence of error bars indicates equal replicates). Northeastern Naturalist Vol. 26, No. 1 G.P. Kraemer 2019 89 The observation that marine and estuarine invertebrates in novel habitats increase in size compared with their size in native habitat (Grosholz and Ruiz 2003, McGaw et al. 2011) was not supported by the present study’s data. In fact, Grosholz and Ruiz (2003) reported a slight, but non-significant reduction in the maximum size of the Asian Shore Crab from 4 populations in the introduced range. The lack of significance may derive from the need for a long-term, fine-grained investigation to reveal the change in a variable ecological system. Larger individuals are disproportionately important from the standpoint of ecology (Birkeland and Dayton 2005, Peters 1983). Claw strength, a determinant of Figure 6. Reproductive output by Asian Shore Crab at Rye, NY. Upper panel: determination of total egg mass brooded as a function of crab size (n = 264). Middle panel: number eggs as a function of mass of eggs (n = 102). Lower panel: estimated areal reproductive output (eggs m-2, integrated over intertidal zone), standardized to average 2005 estimate (472,000 eggs per m2 per reproductive event). Northeastern Naturalist 90 G.P. Kraemer 2019 Vol. 26, No. 1 prey identity and size, scales as a power function of crab size (Payne and Kraemer 2013). The disappearance of the largest (i.e., strongest) crabs influences population diet composition, with consequences for local prey, and possibly also for the predator (Belgrad and Griffen 2016). The loss of the largest female size classes suggests a reduction of the population’s reproductive output, since the data presented here predicts that the number of eggs brooded per reproductive event is roughly a function of the cube of crab size (CW). This estimate assumes no compensatory increase in individual reproductive output, via either an increase in the number of reproductive events per year or a change in the number of eggs brooded. Were the decline in population reproductive output (measured as total mass of eggs) compensated for by an increased number of (smaller) eggs, principles of geometry require a population-wide reduction in egg radius by almost half (42%). Non-compensation is supported by the observation that, while the frequency of brooding by the smallest Asian Shore Crabs has increased, the average intertidal densities of these smallest ovigerous females did not differ between 2005 and 2017. The life histories of prey populations exposed to predators that focus on large-prey individuals often adapt; earlier sexual maturity and increased reproductive effort are commonly seen (e.g., Conover and Munch 2002, Kindsvater and Palkovacs 2017). The former possibility may have occurred in this system; the fraction of females of 12–13 mm CW and 13–14 mm CW that were captured brooding eggs increased over time (Fig. 7), even though the densities of these size classes were not significantly different at the start (2005) and 12 y later (2017). The latter possibility (increased reproductive effort) is also possible; the smallest females may produce more broods per season than in the past. Fukui (1988) reported that year-1 females produce only 1 brood per season within the native range. Taken together, the data demonstrate significant demographic changes in this non-native grapsid crab population at the western LIS site. Over the 16 y, from Figure 7. Changing frequency across the study of ovigerous female crabs with 12–13- mm and 13–14-mm CW. Values under X axis labels represent the number of females examined during each phase of the invasion. Northeastern Naturalist Vol. 26, No. 1 G.P. Kraemer 2019 91 2001 to 2017, the number of different crab species has remained low, and populations of other crabs were essentially non-existent during the ~63% decline in the density of Asian Shore Crab adults. These data suggest at least a demographic lag in recovery (e.g., Borja et al. 2010, Esler et al. 2017), if not a state change (e.g., Hughes et al. 2005). Similarly, Schab et al. (2013) reported a reduction in the Asian Shore Crab population density (2001 vs. 2011, 2012) with no clear, accompanying increase in the native Chocolate-fingered Mud Crab. The lack of evidence for population rebounds after invasion may derive from the paucity of studies of a duration sufficient to detect change in a “noisy” system. The pattern of change at the site in Rye, NY, follows a gradual boom–bust cycle (Strayer et al. 2017). However, the dynamics of the Asian Shore Crab invasion may be context-dependent. O’Connor (2018) described boom–bust dynamics at 2 sites, but not at a low-energy estuarine site (Narragansett Bay), where densities continued to increase from 1999 to 2016. The reason(s) for the changes observed over the course of 12 y of morphometric measurements and 20 y of density records cannot be ascertained from this data set. At least 2 ecological mechanisms could have influenced Asian Shore Crab demography: changes in predator–prey interactions and altered pathogen and/ or parasite impact. Asian Shore Crabs within the invaded range exhibited low incidence of parasitism (Blakeslee et al. 2009, McDermott 2011). Asian Shore Crabs from parasite-naïve populations were almost twice as likely to become infected by rhizocephalan parasites as individuals from highly parasitized populations (Keogh et al. 2017). Genetic evidence points to repeated introductions into LSI (Blakeslee et al. 2017); thus, the possibility of parasites accompanying recent introductions and reducing fitness of local populations cannot be ruled out. Size-selective predation is common (e.g., Torres et al. 2012). Although Tautoga onitis (L.) (Tautog) in LIS consume Asian Shore Crabs (Clark et al. 2006), the largest Asian Shore Crabs may reach a refuge in size, rendering them immune to predation. The loss of larger crabs could have occurred via a mechanism similar to that proposed for the Green Crab decline (Lohrer and Whitlatch 2002); though large crabs are relatively safe from predation, smaller ones are not. Large Asian Shore Crabs may not have been replaced due to predation on smaller-size classes. Loss from the population through predation by humans (i.e., harvest) can be cautiously ruled out. Fishermen who might use the Asian Shore Crab as bait (McDermott 1998) are periodically seen at the study site (Read Wildlife Sanctuary), though they not numerous. In addition, the numerical dominance of Asian Shore Crabs at the site, and connectivity with other sub-populations would require intense, widespread, and continued harvest to bring about the changes observed here. Crabs captured during this study (2005–2017) were estimated at ~0.1% of the crabs present in the Read Sanctuary rocky intertidal zone at the time of collection (data not presented here). In addition, the present study did not target the largest individuals. Overall, the study demonstrated a ~40% reduction in overall density of Asian Shore Crabs. The reduction in density occurred at the expense of the largest crabs, both male and female. The loss of the largest female crabs may have reduced population output. Notwithstanding reductions in the population density and Northeastern Naturalist 92 G.P. 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