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). While the exoskeleton, or shell, is reddish, brown, or blue-green, this is often masked. Young crabs deliberately attach algae, bryozoans, hydroids, sponges, and other creatures to their exoskeletons as a form of camouflage to hide from predators. Animals larger than 8 centimetres (3.1 in) cease to decorate themselves. This species stops molting when it reaches sexual maturity, however, and a layer of algae often develops on the long-lived adult exoskeleton, giving these crabs a green appearance.

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.

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. A male crab grasps a female for mating and delivers a sperm packet. A female can store sperm for later use, if no males are available. Fertilization takes place internally, and the fertilized eggs are then glued to the exterior of the female's abdomen. Females brood their eggs to protect them from predation. Broods may contain 125,000 to 500,000 eggs. The development of the eggs can be discerned by their color. Orange eggs are the freshest, red are of intermediate development, and brown can be seen to have eyes and are ready to be released. Brooding females have been found year-round suggesting that there is no strong seasonal peak in reproduction. The length of the brooding period is unknown. After release from the female, larval crabs are free swimming plankton. …