Philippines, Negros Island, Danjugan. A fiddler crab, sometimes known as a calling crab, may be any of approximately 97 species of semi-terrestrial marine crabs within the genus Uca. Found in mangroves and on sandy or muddy beaches of West Africa, the Western Atlantic, Eastern Pacific and Indo-Pacific, fiddler crabs are easily recognized by their distinctively asymmetric claws. It is the males which boast an oversized claw or cheliped; it plays a role in courtship and signalling among conspecifics.