A black-fingered mud crab, Panopeus herbstii, photographed in Shark River Inlet in Avon, New Jersey, USA. Mud crabs are powerful predators on young oysters and clams; their powerful claws can crush 1/2" long hard clams; also attack barnacles and larger clams by chipping their shells. They are often abundant in sponge colonies, among bushy bryzoans and hydroids on pilings, and intertidally under rocks or other debris on protected shores.