fiddler crab, any of the approximately 65species of thegenusUca (orderDecapoda of the subphylum Crustacea). They are named “fiddler” because the male holds one claw, always much larger than the other, somewhat like a violin. Both claws in the female are relatively small. In males, claws can be regenerated if they are lost.

Fiddler crabs often occur in large numbers on beaches in temperate to tropical regions of the world. They live in water-covered burrows up to 30 cm (about 1 foot) deep and feed on algae and other organic matter. Common North American species include themarsh fiddler crab (Uca pugnax), thechina-back fiddler (U. pugilator), and thered-jointed fiddler (U. minax). These species, which range in body size from about 2.5 to 3 cm (1 to 1.2 inches), occur all along the Atlantic coast of theUnited States. The males of all species are more brightly coloured than the females. Colours range from coral red, bright green, and yellow to light blue.