Most crabs are scavengers, such as the typical hermit crab . Some are herviborous, and some are filter-feeders, either by filtering scooped mud and detritus or by extending their specially adapted appendages called maxillipeds into the current. Typically most crabs move in a sideways motion, though they can also crawl forward slowly. A crab with a specialized large pair of rear legs may use them for holding a sponge, as does the sponge crab . Some crabs can swim, such as the common edible blue crab Callinectes sapidus .
Blue crabs have large, flat, paddle-shaped rear legs for swimming.