Normally, common periwinkles produce thicker shells in the presence of crab predators. This study found that they did not do so when living in acidified seawater for 15 days. The snails apparently compensated for their lack of defensive shell-building by moving more to avoid the crabs. In a natural setting, this change in behavior could affect their interactions with other species. The findings suggest that ocean acidification could have complex ecological effects that extend beyond direct effects on physiology. (Laboratory study)