Thickclaw Porcelain Crab    Pachycheles rudis

Occurring under floats at the Boston Harbor Marina, this rarely seen, small (body width of 3/4 inch) crab likes to hide under any larger object such as a kelp holdfast or clump of mussels.  There are often both male and female crabs and even young ones huddled together.  While it looks like a normal shore crab in fact it is a relative of a major group of crab-like crustaceans (Anomura) containing hermit crabs, squat lobsters and several other types.   The Anomura have either two pairs of walking legs (hermit crabs) or three pairs of walking legs whereas regular crabs have four.

There are several local species of Porcelain crabs, so named for the ease with which they lose their legs when captured - delicate as porcelain.  As a group they are generally found under things such as rocks and move little.  They feed on plankton using specialized fan-like appendages near their mouth. The large claws are used for defense and protection of their territory.

The Thickclaw Porcelain Crab ranges from Alaska to Baja California where it lives between the low intertidal to around a minus 95 feet in areas with moderate to strong currents.

One good reference to the local crabs and their allies is "Pacific coast Crabs and Shrimp" by Gregory C. Jensen.