Northeastern Naturalist Vol. 25, No. 2 N.J. O’Connor 2018 197 2018 NORTHEASTERN NATURALIST 25(2):197–201 Changes in Population Sizes of Hemigrapsus sanguineus (Asian Shore Crab) and Resident Crab Species in Southeastern New England (2010–2016) Nancy J. O’Connor* Abstract - The invasion of Hemigrapsus sanguineus (Asian Shore Crab) has been of concern in northeastern North America since the late 1980s. A relatively long-term record (1998–2011) of density estimates in southern New England showed displacement of resident crab species at 3 locations in Massachusetts and Rhode Island. In 2016, I visited the same locations to estimate current crab densities. The springtime Asian Shore Crab density decreased at a coastal location, but increased at an estuarine location to >300 crabs/ m2. Resident crabs Carcinus maenas (European Green Crab) and Mud Crabs in the family Panopeidae increased in abundance at all 3 locations. Introduction Hemigrapsus sanguineus (De Haan) (Asian Shore Crab) colonized eastern North America in the 1980s near Cape May, NJ (Williams and McDermott 1990), likely in ship ballast from Japan (Blakeslee et al. 2017). Its range has since expanded into Maine, and population sizes have increased (Bloch et al. 2015, Kraemer et al. 2007, Lord and Williams 2017, McDermott 1998). Population growth of the Asian Shore Crab was monitored for 12 years at 3 rocky intertidal locations in southern New England (O’Connor 2014). At all 3 locations, Asian Shore Crab population sizes increased rapidly following the invasion and then appeared to stabilize at different densities, with the highest densities (150–200 crabs/m2) at an estuarine location. Concurrent with the increase in Asian Shore Crab populations, resident crab species—Carcinus maenas (L.) (European Green Crab), and Mud Crabs in the family Panopeidae—declined in abundance (O’Connor 2014). I visited all 3 locations 5–6 years later to determine if population sizes of either the Asian Shore Crab or resident crab species had changed. Field-site Description Two sampling sites were located on the open coast of Massachusetts, 1 in Scituate (42o11'47.83''N, 70o42'56.63''W) and the other in Marshfield (42o5'45.32''N, 70o38'53.19''W). A third site was located in the upper reaches of the Narragansett Bay estuary in Bristol, RI (41o38'37.19''N, 71o15'32.93''W). All 3 locations have a substrate of cobbles and boulders on a sandy base. O’Connor (2014) includes a map of study locations. Water salinity varied from 30 to 32 at the coastal locations and from 28 to 31 at the estuarine location. *Department of Biology, University of Massachusetts Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02747; noconnor@umassd.edu. Manuscript Editor: Thomas Trott Northeastern Naturalist 198 N.J. O’Connor 2018 Vol. 25, No. 2 Methods With the help of assistants, I conducted sampling using methods described in O’Connor (2014). In each location, I non-randomly selected for sampling three 2-m2 quadrats low in the intertidal zone with substantial (≥75%) rock cover. We sampled quadrat sites during spring tides in areas exposed to air for ~1 h before and after the predicted time of low tide; 4–6 people sampled each quadrat at each location. We removed, identified, and returned all crabs to the quadrats. We sampled all 3 locations in the spring (early June 2016) and fall (mid-September to mid-October 2016). I employed 2-tailed t-tests to compare Asian Shore Crab densities in the spring of 2016 at each location with those averaged for the last 3 years of sampling reported in O’Connor (2014). No statistical tests were performed on the fall density estimates because of limited data prior to 2016. Results Compared to the most recent previous surveys, Asian Shore Crab densities at the coastal locations decreased (Scituate: P = 0.032) or remained steady (Marshfield: P = 0.533) (Figs. 1, 2). Although Asian Shore Crab densities at the 2 locations Figure 1. Mean densities of Hemigrapsus sanguineus (Asian Shore Crab) and Carcinus maenas (European Green Crab) in Scituate, MA, in the spring (May–June) and fall (September– October). Density estimates prior to 2016 are from O’Connor (2014). Northeastern Naturalist Vol. 25, No. 2 N.J. O’Connor 2018 199 differed in 2011, they were identical (63 crabs/m2) in the spring of 2016 and were even lower in the fall. Meanwhile, European Green Crabs, which had virtually disappeared by the spring of 2011 at both Scituate and Marshfield, increased in density to 6–10 crabs/m2 by spring 2016 and continued to be present in the fall. In contrast, at the estuarine location (Bristol, RI) Asian Shore Crab populations increased in density from ~200/m2 in 2008 to 338/m2 in the spring of 2016 (Fig. 3; P = 0.015, reciprocal transformation of data), which was among the highest densities ever recorded for this species, and this high abundance of crabs continued into the fall. Although native Panopeid Mud Crabs had low density in the spring, they increased to 61/m2 in the fall. Discussion The increase in resident crabs at all locations was not expected, especially the rebound in Mud Crab density concurrent with a large rise in Asian Shore Figure 2. Mean densities of Hemigrapsus sanguineus (Asian Shore Crab) and Carcinus maenas (European Green Crab) in Marshfield, MA, in the spr ing (May– June) and fall (September–October). Density estimates prior to 2016 are from O’Connor (2014). Northeastern Naturalist 200 N.J. O’Connor 2018 Vol. 25, No. 2 Crab numbers. This trend is different from that seen by Schab et al. (2013), who found that the Asian Shore Crab abundance declined at the mouth of Delaware Bay between 2001 and 2013 and the native Panopeus herbstii H. Milne-Edwards (Black-fingered Mud Crab) increased in density. There may be several reasons for the different direction of change in density of Asian Shore Crabs at the coastal versus the estuarine locations. For example, the estuarine location is more anthropogenically impacted and temperatures there are generally warmer than at the coastal locations (N.J. O’Connor, pers. observ.). Warmer temperatures could affect reproduction; female crabs with eggs were found during both spring and fall 2016 sampling at Bristol but only in the spring at Scituate and Marshfield, suggesting a shorter period of reproduction at the coastal locations. Resident crabs appear to be coexisting with the Asian Shore Crab, despite being inferior in competition for food and shelter (Hobbs et al. 2017, Jensen et al 2002). The European Green Crab, a well-established non-native species in North America, Figure 3. Mean densities of Hemigrapsus sanguineus (Asian Shore Crab) and crabs in the family Panopeidae in Bristol, RI, in the spring (May– June) and fall (September–October). Density estimates prior to 2016 are from O’Connor (2014). Northeastern Naturalist Vol. 25, No. 2 N.J. O’Connor 2018 201 continued to recruit to the coastal rocky intertidal zone. Native Mud Crabs increased in abundance in the lower intertidal zone at Bristol, despite the growth of the Asian Shore Crab population to very high densities. The striking changes in Asian Shore Crab abundance over a 5–6 year period suggest that continued monitoring of populations of the Asian Shore Crab in northeastern North America is necessary for a better understanding of population dynamics of this successful non-native species. Acknowledgments I thank the following individuals who participated in crab sampling: M. Judge, L. Moritzen, A. Profetto, G. 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