This crab is very common in the intertidal zone in small spaces under and between rocks, and also subtidally in dead barnacles. It emerges at night to feed mainly on small barnacles, but also on snails, bivalves, worms, and some green algae. Is an important predator on small Japanese oysters Crassostrea gigas
. Males have larger chelipeds than do females. Predators include pacific cod, and occasionally river otter and red rock crab Cancer productus
. May be found in "harems" of one male and several females in their crevices, especially during the summer breeding season (photo). Mating takes place after the females molt, and the males often carry females who are preparing to molt, and afterward until she has hardened. Ovigerous females are found in Puget Sound from November to April/May. May be infected by parasitic sacculinid barnacles. When disturbed outside its hole, this crab may fold its legs and roll like a stone.