Description:
Oyster pea crabs are one of a group of small, commensal crabs that live semiparastically inside or upon bivalve shells.  They grow 0.4 - 0.6 inches in carapace length, and have smooth, cylindrical bodies.  Color is generally white to light tan.  The legs are thin, and the chelae are equal.  Females are significantly larger than males and have lightly calcified, flexible carapaces, which are so thin that the internal organs often show through.  Carapaces of the tiny males are stronger, but also semitransparent.  The eyes of females are typically concealed by the margin of the carapace, but males have large eyes that are easily visible.   

Habitat:
Oyster pea crabs live inside oyster, mussel, clam, and other bivalve shells, where they take up residence on the gills.  They feed by collecting particulates and plankton collected on the gills of their bivalve hosts.  Over prolonged periods of time, pea crabs do permanent damage the gills of their hosts, and may eventually erode them.

Similar Species:
The mussel pea crab,  P. maculatus, is similar in appearance to the oyster pea crab but is brown in color, has white spots on the carapace, and is quite hairy in appearance.  Mussel pea crabs inhabit the shells of scallops, mussels, pen shells, and are even known to live on sea stars.

Range:  
Oyster pea crabs range from Massachusetts south to Brazil.