This large bivalve clam, which occurs only in the wave-swept areas of the midtide zone, is usually associated with the stalked goose barnacle. A special gland in the modified foot of the mussel produces hair-like structures called byssal threads that anchor the animal to the rock. A filter-feeder by habit, this mollusk can be poisonous during conditions of red tides because of its ability to concentrate hundreds of the toxic dinoflagellates in its body.