Shell damage, if properly recognized, can provide information about biotic interactions between molluscs and their predators. However, it can be difficult to distinguish predatory damage from mechanical breakage, thus making interpretation of damaged modern and fossil shells problematic. To establish a clear-cut distinction between antemortem predatory crab damage and ante- and postmortem mechanical damage in Mytilus trossulus shells, a combined field and experimental approach was used. Mussels were exposed to predation by crabs, tumbled-live, tumbled-dead, and trampled. After 100 h of tumbling, live-collected mussel shells were abraded and disarticulated but not otherwise damaged. Eight percent of the dead-collected shells were broken during tumbling. There was a proportional (length, width, and thickness) size reduction in both tumbled-live and tumbled-dead shells after 100 h. Breakage caused by crab predation under laboratory conditions was ∼19% of the prey offered. Three types of diagnostic damage were inflicted by crab predation: nibbles, nibbles and chips, and peels. Trampling and tumbling yielded three diagnostic breakage patterns: crescentic chips, angular chips, and slivered chips. Crushed shells and shells with fractured margins were caused by predation and trampling. Only twenty percent of the trampled-shells could be mistaken for preyed-upon shells. Only twenty-seven percent of the preyed-on shells could be mistaken for mechanically-damaged shells. Overall, the source of damage could be correctly identified in 74% of the shells. Proper identification of crab predation in dead shells of this commercially important resource may prove valuable in studies of trophic interactions in modern environments. Inferring levels of crab predation, based on damage in fossil specimens, can be reliable if such analyses are calibrated by experimental studies of living representatives or analogs.