A genomic comparison finds that snails from crab-rich and crab-poor shores differ in the frequency of several variants associated with shell formation.

Snails from both shore types increase shell thickness by the same proportion when exposed to crab cues after hatching.

In field trials, thicker-shelled snails survive crab attacks more often, whereas thinner-shelled snails grow faster where crabs are scarce.

After two generations bred separately by origin in the same cue-free conditions, the shell-thickness difference disappears, while first-generation thickness tracks nutrient levels in the original egg capsules.