 Also, ‘gravel-scrubbing’ is used to feed on detritus and micro-organisms.For this feeding technique, "N. ursus" uses a cheliped to pick up pieces of gravel and move them along the inner mouthpart, thereby taking particles from the gravel.Seaweed crabs require camouflage because they are unable to defend themselves from other sea animals, such as sea otters, sea turtles, lobsters, rock crabs, and octopuses.