Unlike other shell-enclosed marine invertebrates, articulate brachiopods are repellent to predators. Fish, sea stars, snails, and crabs all prefer bivalve molluscs such as mussels to articulates. The mussels tested are mobile and out-compete immobile articulates when space is limited. In subtidal field experiments, mussels alone and predators alone each reduced the survivorship of articulates. However, adding mussels to articulates in the presence of ambient predation increased brachiopod survivorship by diverting predation from the brachiopods to the mussels. Competition from mussels (or mussel-like bivalves) is a plausible cause of the post-Paleozoic decline of articulates.