Shell morphological variability of native snails and their vulnerability to introduced crab predators: Can sub-lethal injury provide prey with a reduced risk of lethal predation?

Historical differences in crab abundance and temperature, along with the natural history of specific species, have caused littorinid snail shells to vary in morphology across New England's rocky shores. In New England (USA) the native rough periwinkle, Littorina saxatilis, overlaps with two introduced crab species, Hemigrapsus sanguineus and Carcinus maenas. We examined the shell thickness and frequency of rough periwinkles that manifested crab attack scars (failed predation events that were recorded by morphological repairs of chipped shells) as an indication of selection for thick shells. Where crabs were less common in the north the rough periwinkle had thinner shells that make them more vulnerable to shell-breaking predators than those from the south. Evidence of scars was less frequent in the north likely because there were fewer predators, predators may have been less active, and thin-shelled snails were less likely to survive an attack. Scarred shells may also confer advantage to snails against future attack. Hemigrapsus sanguineus (but not Carcinus maenas) spent a significantly longer time handling scarred Littorina saxatilis over unscarred conspecifics, suggesting that while sub-lethal injuries in the form of shell scars can provide snails with reduced predation risk, the level of risk is species-specific. Introduced predators influencing the morphology of native prey provide a model system to investigate these ecological and evolutionary changes on a rapid time-scale.