Interaction between an invasive decapod and a native gastropod: predator foraging tactics and prey architectural defenses

The shell architecture of the intertidal snail Littorina obtusata (L.) is thought to have undergone an adaptive transition in response to invasion of the Gulf of Maine, NW Atlantic, by the European green crab Carcinus maenas (L.). In order to investigate the hypothesis that this morphological transition affects snail fitness, we conducted predation experiments with snail populations showing morphological differences that are hypothesized to have been caused by, and affect resilience to, green crab predation. Our results are consistent with the adaptive-transition hypothesis, but they reveal more varied predator foraging tactics and prey defensive attributes than previously considered. Crabs killed smaller and less heavily-armored snails by breaking their shell, but killed larger and more heavily-armored individuals using a fairly complex 'shell-entry' tactic, which we refer to as 'winkling'. The snail population which suffered lower mortality from green crab predation apparently obtained protection from crushing by having a thicker, more massive shell, and from winkling by having a smaller aperture. Our study provides evidence that the morphological transition undergone by L. obtusata following the green crab's invasion of the NW Atlantic is adaptive, and raises new questions regarding the phenotypic basis of this recent ecological interaction.