Relative impacts of two exotic brachyuran species on blue mussel populations in Long Island Sound

Green crabsCarcinus maenaswere introduced to North America in the early 1800s and breeding populations were subsequently established in southern New England. Green crabs became dominant predators in rocky intertidal habitats of the region and caused ecological and economic harm in the ensuing decades. During the mid-1990s, the rocky intertidal crab fauna of southern New England began to change once again. A reduction in green crab abundance was coincident with the introduction and spread of another exotic crab species, the Asian shore crabHemigrapsus sanguineus. Less than 10 yr after its appearance in Long Island Sound,H. sanguineusbecame the numerically dominant brachyuran at several rocky intertidal sites in the region (e.g. 90+% of the crabs wereH. sanguineus; densities ofH. sanguineusoften exceeded 70 crabs m–2). This study examined consequences of the invasion byH. sanguineusand the decline ofC. maenason a shared prey population in the rocky intertidal zone. Both crab species readily consumed native blue mussels (Mytilus edulis) and rates of mussel loss (per predator, per species) were quantified during a series of field experiments. WhileC. maenas consumedmore mussels thanH. sanguineuson an individual basis, the greater densities ofH. sanguineusmade it an important predator of juvenile mussels from a population standpoint. In isolation, the decline ofC. maenas(a voracious alien predator) would have benefited mussel populations. However, the replacement ofC. maenasbyH. sanguineushas apparently had a net negative influence on intertidal blue mussel populations.