Mud crabs (scylla serrata) are large brownish-green crabs that grow up to about 10 inches (25cm) long and may weigh up to as much as eight pounds. This weight can sometimes be flesh and shell or disproportionately mud. Look for mud crabs that have some algae or barnacles on the shell or even some wear and tear on the claws. This helps to insure the crab has not molted recently which suggests little edible flesh.
The mud crab is found in Australia, the Philippines as well as the Pacific Islands. This type of crab is less popular in the U.S. due to lack of availability.