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

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 snail Littorina littorea and the omnivorous crab Carcinus maenas leading to morphological and spatial distribution changes in two native gastropod grazers (Littorina saxatilis and Littorina obtusata). Subsequently, another intertidal omnivorous crab Hemigrapsus sanguineus has become abundant in the intertidal, occurring in areas which were once refuges from crab predation. In order to quantify the potential impact of H. sanguineus on native snails, we conducted both in and ex situ experiments, 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 saxatilis was the most vulnerable to predation, with 73 and 64% of the population susceptible to large H. sanguineus and small C. maenas, respectively, while more than 89% of the L. littorea population was resistant to predation. Littorina saxatilis has been relegated to the high intertidal where there is high abiotic stress and poor-quality food, but until the invasion of H. sanguineus, there was little predation risk. Now, it seems that L. saxatilis is at the most risk of all three snail species, and may be at threat of local extirpation at locations with high populations of H. sanguineus.