SPREAD AND POTENTIAL IMPACT OF THE RECENTLY INTRODUCED EUROPEAN GREENCRAB, CARCINUS-MAENAS, IN CENTRAL CALIFORNIA

Our study examines the potential impact of the European green crab Car cinus maenas on communities of coastal embayments of western North Ame rica. We document the current distribution and range expansion of this species beyond San Francisco Bay, where C. maenas first became establ ished along this coast in 1989-1990, and we test the effect of C. maen as predation on different species and sizes of infaunal invertebrates in field and laboratory experiments. In our samples from eight coastal locations in central California collected between June 1993 and May 1 994, we found no green crabs at the two closest embayments south of Sa n Francisco Bay and found the crabs in all four embayments sampled wit hin 120 km north of San Francisco Bay, up to and including Bodega Harb or. C. maenas was not present in samples from sites farther north. Thi s northward range expansion is apparently the result of larval recruit ment by a single cohort, corresponding to the predominant northern tra nsport of surface waters and the approximate distance water moves duri ng larval green crab development. At Bodega Harbor, the current northe rn range limit, the C. maenas population is now well established lishe d and reproducing. Females and males became sexually mature within the ir first year at similar or equal to 40 mm carapace width, molting app roximately monthly from summer through fall, and females were ovigerou s in late fall of their first year at similar or equal to 50 mm. We ex pect larvae from this population to recruit locally and to the north, promoting episodic range extensions as new populations are established and b reproduce. Enclosure experiments conducted during the summer of 1993 at the intertidal sandflats of Bodega Harbor showed that C. maen as significantly reduced densities of the most abundant taxa, includin g the bivalves Transennella confusa and T. tantilla, the cumacean Cume lla vulgaris, and the amphipod Corophium sp. Furthermore, Carcinus mae nas selectively removed larger (>3 mm) rather than smaller (<1 mm) Tra nsennella spp. in both field and laboratory experiments. Based on the available data from this and other studies of green crabs, and our 10 yr study of community dynamics at Bodega Harbor, we predict C. maenas will significantly alter community structure, ecological interactions, and evolutionary processes in embayments of western North America.