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

Abstract

Green crabs Carcinus maenas were 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 crab Hemigrapsus san- guineus. Less than 10 yr after its appearance in Long Island Sound, H. sanguineus became the numerically dominant brachyuran at several rocky intertidal sites in the region (e.g. 90+% of the crabs were H. sanguineus; densities of H. sanguineus often exceeded 70 crabs m -2 ). This study exam- ined consequences of the invasion by H. sanguineus and the decline of C. maenas on 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. While C. maenas consumed more mussels than H. sanguineus on an individual basis, the greater densities of H. sanguineus made it an important predator of juvenile mussels from a population standpoint. In isolation, the decline of C. maenas (a voracious alien predator) would have benefited mussel populations. However, the replacement of C. maenas by H. sanguineus has apparently had a net negative influence on intertidal blue mussel populations.