The little reading I've done on the Pea Crab (if you are 100% sure that's what it is) says that it is a 'parasite' to the scallop and not a commensal one as the host may be harmed by the crabs feeding activities. They also can live in a variety of other animals like sea urchins and sand dollars in the rectum of sea cucumbers, in the tubes of parchment worms, in the burrows of mud shrimp, or in the gills of sea squirts.