Simultaneous defense against shell entry and shell crushing in a snail faced with the predatory shorecrab Carcinus maenas

Abstract

Prey with versatile predators need diverse defenses. Such an example of a versatile predator is the voracious European green crab Carcinus maenas. Green crabs prey on snails by either crushing shells or, when shells are too tough to break, by extracting flesh through the shell opening (aperture). Among populations in the northwest Atlantic, the claw size of green crabs (an indicator of crushing strength) co-varies with shell mass of the intertidal snail Littorina obtusata (mass is an indi- cator of a shell's crushing resistance); thus shell-crushing predation appears to be an important part of the predator-prey interaction. We report that aperture occlusion and soft tissue withdrawal depth (two shell-entry defenses) of L. obtusata snails co-vary with their shell mass (an anti-crushing trait) among populations. When snails were fed directly to green crabs in the laboratory, populations with smaller shell openings and deeper withdrawal depths were less frequently killed by shell-entry attacks, and these same populations, with more massive shells, were also better at resisting shell- crushing attacks. Results provide compelling evidence that greater shell mass, a smaller shell open- ing, and deeper withdrawal depth are adaptive traits for snails faced with green crab predators. Furthermore, results suggest that American L. obtusata responded to the introduction of green crabs by escalating both anti-crushing and anti-entry defenses.