Northeastern Naturalist: Status of the nonindigenous crab, Hemigrapusus sanguineus, at Greenwich Poi

ABSTRACT - The purpose of this study was to assess the status of the newly introduced Japanese shore crab (Hemigrapsus sanguineus) at Greenwich Point, Connecticut and consider its population size relative to the crab populations already established at that location. A survey was conducted during the summer of 1997, and consisted of randomly sampling 0.5 m^sup 2^ plots along the exposed rocky intertidal zone. A total of 1,103 crabs was captured, and more Japanese shore crabs were found per sample than all other crab species combined. The data suggest that H. sanguineus is well established at Greenwich Point.

INTRODUCTION

The Japanese shore crab (Hemigrapsus sanguineus de Haan 1853) is indigenous to Japan and the western Indo-Pacific ocean and was discovered along the New Jersey shoreline in 1988 (Williams and McDermott 1990). Since then, this newly established population has flourished and rapidly expanded its range north and south along the eastern United States coast. Although the current distribution of the Japanese shore crab in North America is still not clear, the species is now thought to be established throughout Long Island Sound, and to range from North Carolina to the north shore of Cape Cod (Lohrer and Whitlatch 1997; McDermott 1991; this study). It is believed that the introduction of H. sanguineus resulted from the release of ballast water from ships, which is a common practice with potentially dangerous ecological consequences (Carlton 1996; Carlton and Geller 1993).

Along the New England coast, H. sanguineus shares the spaces under or among intertidal zone cobble and boulders with other crabs such as mud crabs (mainly Eurypanopeus depressus Smith 1869), rock crabs (Cancer irroratus Say 1817), and green crabs (Carcinus maenas Linnaeus 1758). The green crab is also nonindigenous and can be found along both coasts of the United States (Grosholz and Ruiz 1996). Our study was conducted in 1997, and the main objective was to assess the relative abundance of rocky intertidal zone crab species at Greenwich Point, Connecticut (approx. 41 deg N, 73 deg 35' W). Located on the northwestern shore of Long Island Sound, Greenwich Point is a private park owned and managed by the Town of Greenwich. Our study site encompassed a strip of exposed rocky intertidal shoreline about 100 meters in length and 20 to 30 meters in width, located on the south side of the Greenwich Point peninsula.

METHODS

Between 3 July and 13 August 1997, our study site was sampled on 11 different days by ourselves and a team of volunteers, with 6 to 10 samples taken per day. Data were collected during low tides when intertidal zone exposure was at its maximum. Sampling days included a wide range of weather, from very warm and humid to windy, overcast and cool. Sampling sites were chosen at random by tossing a plastic hoop, with an area of 0.5 m^sup 2^, several meters over the exposed rocky shoreline. Wherever the hoop landed, rocks were removed from within the circle, and the crabs hiding beneath or around the rocks and in the sediment were caught and examined. The crabs were mainly captured by hand. However, when specimens escaped to narrow crevices between adjacent rocks, long, thin forceps were used to extract them. Crab collection was generally conducted by 2 or 3 persons wherein one member would quickly lift the cobble and small boulders within each 0.5 M^sup 2^ sample area, while the others rapidly collected the exposed crabs and placed them in a water-filled holding bin. Data collected for each crab were species, sex, and carapace width (CW) measured in mm. We also noted whether the females were carrying eggs (ovigerous) or not. Enough sample sites were selected to ensure a representative distribution of sites between the low and high water margins and to encompass the range of intertidal microhabitats along the exposed, rocky shore.

RESULTS

From 73 samples, 1,103 crabs of the following species were collected: H. sanguineus (n = 830), E. depressus (n = 264), C. maenas (n = 8), and C. irroratus (n = 1). On average, we found 11.4 Japanese crabs, 3.6 mud crabs, and 0.1 green crabs per 0.5 M^sup 2^ sample area (Table 1). Overall, an average of 15 crabs were collected per sample and significantly more Japanese crabs were found per sample than all other crabs combined (paired sign test, p

Male Japanese shore crabs were more numerous than females and displayed a wider size range, including a greater maximum size (Table 1). For either sex, however, the largest crabs we found (males, 36 mm CW; females, 25 mm CW) do not reflect the maximum sizes known for this species, which for males and females are 42.4 mm and 39.0 mm, respectively (Fukui 1988). The smallest ovigerous H. sanguineus we collected was 12 mm, which supports the conclusion of Lohrer and Whitlatch (1997) that females reach sexual maturity at that size. The percentage of ovigerous Japanese shore crabs captured each day relative to the total number of female Japanese shore crabs tended to increase over the course of the summer with the peak reaching 62% in early August.

DISCUSSION

Assuming our samples are representative of the study site and the study site is representative of the rocky shoreline at Greenwich Point, we conclude that H. sanguineus is well established and now dominates the crab community at that location. Because of their diverse, omnivorous diet (Lohrer and Whitlatch 1997), their high fecundity (Fukui 1998), their high terrestrial mobility, and pugnacious nature (pers. obs.), it is not surprising that H. sanguineus has successfully established itself at Greenwich Point and other sites along the New England coast. It would appear that this species has the potential to displace other resident crab populations and alter the food web already in place. The resulting displacement may have a greater impact than replacing existing crab populations due to the Japanese crab's life history and reproductive strategies. Further investigations are needed to properly assess the effects of the new resident on the intertidal communities.

ACKNOWLEDGMENTS

We thank the town of Greenwich for granting us permission to access Greenwich Point, and are grateful to the staff and participants of National Audubon Society's 1997 Ecology Workshop, particularly intern Angie Knapp, for assisting with the data collection. We also thank Dr. V. Watson and two anonymous reviewers for their helpful comments.

LITERATURE CITED

CARLTON, J.T. 1996. Pattern, process, and prediction in marine invasion ecology. Biol. Conserv. 78: 97-106.

CARLTON, U., and J.B. GELLER. 1993. Ecological roulette: the global transport of nonindigenous marine organisms. Science 261: 78-82.

FUKUI, Y. 1988. Comparative studies on the life history of the grapsid crabs (Crustacea, Brachyura) inhabiting intertidal cobble and boulder shores. Pub. Seto Mar. Biol. Lab. 33: 121-162.

GROSHOLZ, E.D., and G.M. RUIZ. 1996. Predicting the impact of introduced marine species: Lessons from the multiple invasions of the European green crab, Carcinus maenas. Biol. Conserv. 78:59-66.

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. 45-56, In N. Balcom (Ed.). Proceedings of the Second Northeast Conference on Nonindigenous Aquatic Nuisance Species. 68 pp.

McDERMOTT, JT 1991. A breeding population of the western Pacific crab Hemigrapsus sanguineus (Crustacea: Decapoda: Grapsidae) established on the Atlantic coast of North America. Biol. Bull. 181: 195-198.

WILLIAMS, A.B., and JT McDERMOTT. 1990. An eastern United States record for the western Indo-Pacific crab, Hemigrapsus sanguineus (Crustacea: Decapoda: Grapsidae). Proc. Biol. Soc. Wash. 103(l): 108-109.

ROBERT S. AHL 1 AND SUSAN P. Moss 2

1 Department of Environmental Studies, The University of Montana, Missoula, NIT 59812, ahlr@selway.umt.edu ; 2 1015 SW 9th Street, #E2, Gainesville, FL 32601, susan.moss@santafe.cc.fl.us