They are often hard to spot in the daytime, hiding in crevices and under coral heads. Their primary protection comes from hiding in the complex tree-like structures of hard coral branches, such as those of genus Acropora . Other small crabs seek shelter by the hundreds between the branches of hard corals. At night, many of the larger crab species emerge for feeding, though they are often hard to detect owing to their low profile and slow movement. The forward-most legs on a crab, called chelipeds, are not used for crawling.
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.