Possible temperature limits to range expansion of non-native Asian shore crabs in Maine

The intertidal Asian shore crab, Hemigrapsus sanguineus (De Haan, 1835), recently (2001) appeared in very low densities along the southern Maine (USA) coast. Indigenous to Russia and Japan, this species arrived in New Jersey around 1988 and rapidly expanded south to the Carolinas and north to southern New England (CT, RI, MA) by the early 1990s. We examined the characteristics of a non-native species invasion by surveying over 30 intertidal sites in Maine from 2002 to 2005. We found relatively low population densities, slow rates of geographic expansion and virtually no colonization northeast of Penobscot Bay on the central coast of Maine. We hypothesized that further geographic expansion of sustaining populations of H. sanguineus in Maine may be limited by coastal temperatures colder than in its native range. To examine this, we deployed recording thermistors at 11 strategic sites and integrated those data with three oceanographic observation buoys and with satellite thermal images of the coastal zone for the period between 2003 and 2005. We found that Maine's H. sanguineus population densities in the intertidal peaked during the warmest months (July-September) and were lowest during the coldest months (January-March) when the crabs retreated to the subtidal zone. Densities were also greater in warmer localities (southern Maine) than in cooler localities (central and eastern Maine). In fact, populations were absent from areas with mean summer temperatures cooler than 13 degrees C. In southern Maine, seasonal population densities and reproductive periodicity corresponded to periods with mean temperatures warmer than 15 degrees and 12 degrees C, respectively. There are many physical and biological factors that could limit this invasion. However, our temperature and demographic data are consistent with the thesis that the H. sanguineus invasion has stalled at the terminus of the Gulf of Maine's cold, Eastern Maine Coastal Current. We compared published mean summer and winter SST data for coastal waters in the western North Atlantic and in the native Asian range of H. sanguineus, and we quantified and compared H. sanguineus abundance in Maine with thermal microenvironments at these sites. From this, we speculate that future distribution of this crab may be limited to the warmer areas of the Maine coast (south and west of Penobscot Bay) and up estuaries that warm every summer. Thus, it may be possible for managers to gauge future risks of marine species invasion from vectors such as ships based on the thermal biogeographic match or mismatch between their ports of origin and their destinations. Such information could be useful for focused monitoring and enforcement of existing laws designed to prevent future introductions of non-native marine species. As coastal waters warm, invasion opportunities could increase from cold-limited species. (C) 2009 Elsevier B.V. All rights reserved.