Large and slow moving, a sheep crab crawls along on long legs, which are segmented by big, knobby joints. Its oval-shaped body (carapace) tapers to spinelike points on its snout (rostrum), which bends sharply downward. Spines and bumps (tubercles) cover its body. Adult male sheep crabs have more formidable claws and longer legs than females. These animals are the largest members of the California spider crabs (Majidae).
To camouflage themselves, juvenile sheep crabs decorate their carapaces with barnacles, bryozoans, hydroids and algae. Adult crabs often stop decorating themselves, and a thin film of fuzzy green algae replaces the masking organisms. Divers have seen sheep crabs walking openly on sandy seafloors.
A market exists for whole live sheep crabs and their large claws. Gillnetters gather claws from adult male crabs caught in their nets, usually killing the crabs in the process. Fishermen use traps to catch live crabs. In Southern California, fisheries for claws and whole live sheep crabs thrived in the 1980s. After a 1990 California State initiative barred the use of gillnets in shallow water, the catch of claws plummeted. At the present, trap fishermen and gillnetters supply the market, which remains relatively low. Large-scale fishing for sheep crabs is no longer economically feasible.
Male sheep crabs spend the winter in deep water. In early spring, both sexes migrate to shallow water. Sheep crabs aren’t social animals but, in spring mating season, divers see adult females in piles surrounded by adult males. Since females can store sperm for multiple broods, they can continue to fertilize eggs even in the absence males. Broods can contain 125,000 to 500,000 eggs.
Like all crustaceans, sheep crabs molt, or shed, their old, too-small shell. They grow a soft shell under the old carapace, back out of it, and then take in water to expand the new soft shell. In a short time the new shell hardens, and the crab has room to grow. Some species of crabs molt their entire lives, but adult sheep crabs have a terminal molt. After this molt, they’re unable to increase in size or regenerate lost limbs.