Intertidal Organisms EZ-ID GUIDES

 

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Petrolisthes eriomerus (Porcelain crab)

photo of porcelain crab
Copyright © 2005 Mary Jo Adams

 

Look for this species low in the intertidal on beaches where rocks rest on sand or fine gravel and there is a fair amount of wave action.   It is also sometimes seen in mussel beds or among kelp holdfasts.    Petrolisthes eriomerus is a small crab with a carapace reaching a diameter of only about ¾ inch.   The carapace is round and compared to other crabs, the claws appear huge in relation to the rest of the crab.   If the claws are open, you may see that the base of the moveable "finger" has a blue spot.   This species has been found to depths of 280 feet.

 

A similar porcelain crab, Petrolisthes cinctipes tends to be more of an outer coast species and is often found in clumps of the California mussel ( Mytilus californianus ).   It has a red-orange spot at the base of its "finger", can be found somewhat higher in the intertidal, and avoids areas where the rocks are set in gravel or sand.

 

These species are filter feeders, collecting diatoms and other nutrients from the water.   They also have tufts of hair on their claws that they use to brush bits of food up from rock surfaces.   Porcelain crabs have a unique defense mechanism in which if they are threatened or disturbed, they may drop one of their appendages.   The lost limb will eventually grow back.   

 

Porcelain crabs are not true crabs, but belong to a group called anomurans, making them more closely related to hermit crabs and king crabs.   One easily observed distinction between true cabs and anomurans is that in anomurans, only 4 pairs of legs are normally visible where as in true crabs, 5 pairs can be seen.     

 

This page was created by Mary Jo Adams on 12/5/05.

 

 

photo of a porcelain crab

photo of a porcelain crab