Found a pair of pea crabs nestled in a tube under a rock. I'm not sure what built the tube, but it wasn't the pea crabs. These odd, double-wide crabs insert their soft bodies into tube worm tubes, clam shells, shrimp burrows, anywhere that will allow them shelter and a flow of plankton and detritus. They're freeloaders, but not exactly parasites. It appears the scientific term is "commensal indwellers."