Investigating the impact of introduced crabs on the distribution and morphology of littorinid snails: Implications for the survival of the snailLittorina saxatilis

Abstract

Introduced species can have profound impacts on communities by displacing and consuming native species. The intertidal communities in the Gulf of Maine have been radically altered through a suite of invasions including the herbivorous snailLittorina littoreaand the omnivorous crabCarcinus maenasleading to morphological and spatial distribution changes in two native gastropod grazers (Littorina saxatilisandLittorina obtusata). Subsequently, another intertidal omnivorous crabHemigrapsus sanguineushas become abundant in the intertidal, occurring in areas which were once refuges from crab predation. In order to quantify the potential impact ofH. sanguineuson native snails, we conducted bothinandex situexperiments, testing the susceptibility of different sized snails to predation by introduced crabs. Additionally, we measured the distribution, abundance, and morphology of intertidal snails and crabs. Smaller snails of all species were the most susceptible to predation, although susceptibility differed among snail species.Littorina saxatiliswas the most vulnerable to predation, with 73 and 64% of the population susceptible to largeH. sanguineusand smallC. maenas, respectively, while more than 89% of theL. littoreapopulation was resistant to predation.Littorina saxatilishas been relegated to the high intertidal where there is high abiotic stress and poor-quality food, but until the invasion ofH. sanguineus, there was little predation risk. Now, it seems thatL. saxatilisis at the most risk of all three snail species, and may be at threat of local extirpation at locations with high populations ofH. sanguineus.