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O’Connor, N.J., 2001. The Asian Shore Crab Hemigrapsus sanguineus in New England: Changes in Resident Crab Populations, University of Massachusetts Dartmouth. Proceedings of the Second International Conference on Marine Bioinvasions, New Orleans, La., April 9-11, 2001, pp. 105-106. The Asian Shore Crab Hemigrapsus sanguineus in New England: Changes in Resident Crab PopulationsDocumenting ecosystem changes resulting from the establishment of biological invaders requires an understanding of the system before, during, and after colonization of the invader. Biological invasions that are currently in progress provide the best opportunity for this type of assessment. However, such comparative data exist for few species. The Asian shore crab, Hemigrapsus sanguineus, is currently becoming established along the rocky intertidal coastline of New England. First noticed in New Jersey in 1988 (McDermott 1991), the crab was well established in southern New England by 1996 (Lohrer and Whitlatch 1997, Ledesma and O'Connor in press). Currently, the species' range extends to New Hampshire (McDermott, pers. comm.), with populations along the Massachusetts coastline continuing to grow in size. H. sanguineus occurs throughout the rocky intertidal zone, reaching maximum densities in the mid to lower intertidal zone. Resident crab species, such as green crabs (Carcinus maenas, which is also nonindigenous), mud crabs in the family Xanthidae, and rock crabs (Cancer spp.) also inhabit the lower rocky intertidal zone. The objective of the study was to determine whether populations of these crabs are affected as H. sanguineus becomes established. Specifically, I asked whether densities of other crab species tend to decrease as densities of H. sanguineus increase. Crab populations in several localities were sampled repeatedly, usually in the spring (May to early June) and fall (September to October) to examine temporal changes in crab populations and to determine whether any changes observed were similar at different locations. Sites sampled include Bristol, Rhode Island, in Narragansett Bay, and several sites along the Massachusetts coast: Washburn Island, Falmouth, in Vineyard Sound; Bourne, near the east end of the Cape Cod Canal; Sandwich and Dennis in Cape Cod Bay; and Marshfield and Scituate on the shore south of Boston. Three to five replicate 2m2 square quadrats were randomly placed on rocky (³ 75% rock cover) areas low in the intertidal zone during low tide. All crabs were removed from the quadrats, identified, counted, measured in most cases, and then returned to the sampling site. Sampling began in 1996 at Sandwich and Washburn Island, and from 1997-1999 at the other locations. The most abundant species at the sites were xanthid crabs, C. maenas, and H. sanguineus. Cancer spp., when present, usually occurred in very low densities (< 1 crab/m2 ). During initial sampling in Bristol and Washburn Island, xanthids were the most abundant and H. sanguineus occurred at low densities (< 5/m2 ). However, within two years the dominance pattern switched, with H. sanguineus reaching densities of 30-50/m2 and xanthids falling to < 2/m2 . At Sandwich and Bourne, H. sanguineus abundance increased over 2-4 years, reaching densities of 76/m2 in Sandwich and 120/m2 in Bourne by fall 2000. C. maenas densities remained low (< 10/m2 ). At Dennis, C. maenas was the most abundant species in fall of 1997, although its density was low (5/m2 ). By fall of 1998, H. sanguineus became (and remained) the most abundant species, with densities fluctuating around 15/m2. North of Cape Cod, in Marshfield and Scituate, C.maenas remained the dominant crab species. However, C. maenas densities declined in Marshfield between 1999 and2000 whereas H. sanguineus densities increased. Sampling planned in 2001 should determine whether this trend continues. Size-frequency distributions of C. maenas and H. sanguineus showed temporal patterns. For both species, especially C. maenas, the populations were dominated by small crabs (< 8 mm carapace width) in the fall, indicating a recent recruitment period. H. sanguineus populations also contained several small crabs in May-early June, suggesting a spring as well as late summer-fall period of recruitment. In summary, the establishment of H. sanguineus has negatively affected the abundance of xanthid crabs, although the mechanism is unknown. H. sanguineus might be impacting C. maenas, because densities are low where H. sanguineus is abundant. However, future sampling at Scituate and Marshfield, where C. maenas currently dominates, is necessary to determine whether C. maenas densities will decline if H. sanguineus populations grow in size. References: Ledesma, M.E. and N.J. O'Connor. 2001. Habitat and diet of the non-native crab Hemigrapsus sanguineus in south-eastern New England. Northeastern Naturalist (in press) Lohrer, A.M. and R.B. Whitlatch. 1997. Ecological studies on the recently introduced Japanese shore crab (Hemigrapsus sanguineus) in eastern Long Island Sound. Pp. 49-60 in N. Balcom (ed.). Proceedings of the Second Northeast Conference on Nonindigenous Aquatic Nuisance Species. Connecticut Sea Grant College Program CTSG-97-02, 68 pp. McDermott, J.J. 1991. A breeding population of the Western Pacific crab Hemigrapsus sanguineus (Crustacea: Decapoda: Grapsidae) established on the Atlantic coast of North America. Biological Bulletin 181: 195-198 Contact: Nancy O’Connor, Department of Biology and School for Marine
Science and Technology, University of Massachusetts Dartmouth, 285 Old Westport
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