Predators include sand and kelp bass and sculpin (on juveniles) and seabirds such as gulls and pigeon guillemots. Prey include barnacles and smaller crabs, amphipods, sea cucumbers, polychaetes, many other intertidal invertebrates, as well as dead fish. At least 42 prey species have been noted. Are an important threat to commercial oyster beds. Crabs raised on thick-shelled species such as Mytilus californianus
developed even stronger claws. Mating occurs in summer after a female has molted. Males will often guard a female who is preparing to molt, by holding her under his abdomen. This may last for several weeks until she molts. He then guards her until her exoskeleton hardens again. Gravid females may be found from October to June. Females may carry from 172,000 to 597,000 eggs on the pleopods of the abdomen. Males overwinter in shallow areas, while females seem to overwinter in deeper water. Red rock crabs cannot osmoregulate and so are not found in areas of low salinity. Near Vancouver Island, adults have more epibionts than do juveniles (McGraw, 2006). Common epibionts include barnacles (especially Balanus crenatus
) on the dorsal surface, green, red, and brown algae (especially on the antennae), tube-dwelling polychaetes (mainly on the ventral surfaces), hydrozoans (mainly on ventral surfaces and limbs), bryozoans (especially Membranipora membranacea
) on any region of the carapace. A few have sponge, tunicate, or mollusk epibionts.