East meets west: competitive interactions between green crabCarcinus maenas, and native and introduced shore crabHemigrapsusspp

The recent introduction of the European green crabCarcinus maenasand Asian shore crabHemigrapsus sanguineusto the west and east coasts of North America, respectively, presents a unique opportunity for investigation into competitive dynamics among intertidal crabs. JuvenileC. maenasutilize rocks, shell, and other cover, and their arrival on the west coast places them in potential competition for these resources with an abundant native grapsid,H. oregonensis. Similarly,H. sanguineususe intertidal shelter on the east coast, thus placingC. maenasin possible competition with grapsids both as an invader on the west coast and as ‘resident’ on the east coast, having been established there for more than 150 yr. Field sampling and laboratory experiments testing competition for space betweenC. maenasandHemigrapsusspp. of equal carapace width were conducted on both coasts, as were videotaped feeding trials to examine and quantify agonistic interactions between species. When competing for food (a single damaged, anchored bivalve),C. maenasdominated overH. oregonensis, whileH. sanguineuswere overwhelmingly dominant overC. maenas. Within-quadrat, stratified sampling of rocks and sand revealed striking differences in habitat utilization byC. maenasliving in the presence or absence ofHemigrapsus. Only ~20% of juvenileC. maenasoccurred under rocks in areas occupied by eitherHemigrapsusspecies, while north of the present distribution ofH. sanguineus(in Maine) >97% of theC. maenaswere found beneath rocks. This pattern was reflected in laboratory trials as well, where both species ofHemigrapsusconsistently dominated in contests for shelter. Given the importance of intertidal cover for small crabs, such competitive interactions will likely affect patterns of habitat use byC. maenason the east coast and may have important implications for the ultimate distribution and impact of this species in the northeastern Pacific.