Petrolisthes cinctipes (Randall 1840)

How to Distinguish from Similar Species: Petrolisthes eriomerus has a cheliped with a longer carpus with parallel margins, its antennal flagellae are greenish, and it has blue on its mouthparts and on the cheliped.

Under stones and in mussel beds., especially on the outer coast.

Geographical Range: Queen Charlotte Islands, Canada to Point Conception, CA + northern Channel Islands in CA. Most common in southern part of range.

Depth Range: Upper and middle intertidal

Porcelain crabs are Anomuran crabs, as can be seen by the antennae being lateral to the eyes, the reduced last leg, and the fact that the abdomen is not held tightly against the underside of the thorax. The abdomen of Petrolisthes has uropods. The cheliped of Petrolisthes cinctipes has a carpus about 1.5-2x as long as wide, its anterior and posterior margins are not parallel, and it has a distinct lobe on the inner margin. The distal segments of the maxillipeds are reddish. The walking legs are banded. Overall color reddish brown; occasionally blue. Carapace length up to 2.4 cm, width to 1.4 cm. Chelae are usually nearly equal in size and strongly flattened. Antennal flagellum is reddish.

Biology/Natural History: A very common inhabitant of mussel beds in central and northern California. Feeds on plankton and suspended detritus by using the feathery hairs on its maxillipeds, which it waves in the water. Occasionally eats algae or dead animal tissue. Reproduces all year in California, March to July in Puget Sound. Eggs are bright red or maroon when laid, fade to brownish red. As with all porcelain crabs, this species will readily drop its claws if grasped by them. Unlike P. eriomerus, the claw of this crab stops pinching when dropped. The zoea larvae of porcelain crabs have extremely long spines, especially the rostral spine. This species can often be found in the same areas as P. eriomerus, but this species avoids hiding under rocks that have sand or sediments around them so they are usually higher in the intertidal. They don't generally swim, but if forced to do so they often swim with their ventral side up, flapping their abdomen with their uropods extended for propulsion. Individuals are often found with puncture wounds on their claws. This seems to be due to interspecific competition in the form of "shoving matches". The incidence of injuries is similar for males and females, suggesting the competition may be for space rather than for mates. Injuries are most common on intermediate individuals and in more crowded, wave-exposed sites.