Fiddler crab is the name of a group of small crabs that live along temperate and tropical seacoasts. These crabs burrow into sand and mud on beaches, salt marshes, and mangrove swamps. Like all crabs, a fiddler crab has two claws called chelae. In the male, one chela is much larger than the other. The male waves the large chela to threaten other males or to attract females. The waves somewhat resemble the movements of a person playing a violin, and give fiddler crabs their name.
Fiddler crabs eat water organisms called algae. The crabs feed by picking up small balls of sand and mud with their claws. They scrape algae from the sand grains. Males cannot use the large chela for feeding because it is too large to maneuver to their mouth.
Some fiddler crabs change in color from light to dark during the day. This change occurs because of the movement of pigment (coloring matter) within special cells in the skin. See Biological clock (Other rhythms).
