Sheep Crab

The sheep crab has a carapace, four sets of walking legs, and a set of claws, or chelipeds. Males are larger than females. The carapace of a male can be over 9.6 inches (24 cm) across, while females can grow to 6.8 inches (17 cm). The carapace is tear-shaped with a wide, rounded posterior which tapers to a point at its snout, or rostrum. There is an obvious preorbital spine on the tip of the rostrum, which is sharply down-curved and deeply notched at its tip. The carapace is covered in short spines, or tubercles.

The longest legs, the second pair of walking legs, is longer than the carapace is wide, so sheep crabs can reach an overall width of 57 centimetres (22 in).

This crab lives in coastal waters from Cordell Bank, California to Punta San Bartolome, Baja California. It is found in waters between 6 metres (20 ft) and 152 metres (499 ft) deep. Sheep crabs are quite mobile, achieving speeds of 0.4 kilometres per hour (0.25 mph), foraging on soft and rocky bottoms, as well as pilings. This species is migratory, spending the warm months of the year in shallow waters and the cooler months in deeper waters.

The sheep crab is a scavenger-generalist in diet, eating algae, mussels, sea stars, small crabs, including juvenile sheep crabs and northern kelp crabs, and dead fish.

Sheep crabs are gonochoric, which is to say that individuals are either male or female. They form aggregations, piles of dozens or hundreds of crabs. The aggregations which have been studied in detail are composed only of adults and included females and at least one male. Sheep crabs are typically solitary, and it has been hypothesized that the purpose of the aggregations is to increase the concentration of a chemical signal, a pheromone, from the females to attract males for mating.

In the late 1970s a commercial fishery for sheep crabs developed in California. Two markets emerged, one for the large claws of the male crabs, and another for whole crabs. The fishery peaked in 1988 when 96,000 pounds of claws and 108,000 pounds of whole crabs were landed. In 1990, a California ballot initiative banned the use of gill nets and trammel nets that were used to catch sheep crabs, and the fishery shrank significantly. In 2018, California reported commercial landings of 503 pounds of claws and 74,863 pounds of whole crabs.

Sheep crab are taken as bycatch in the California spiny lobster fishery. One study found 1.29% of the animals caught in lobster traps were sheep crabs.