fiddler crab, common name for small, amphibious crabs belonging to the genus Uca. They are characterized by a rectangular carapace (shell) and a narrow abdomen, which is flexed under the body. They are called fiddler crabs because the males have one enormous claw, held in front of the body like a fiddle. This claw often contrasts in color with the rest of the body, and is used by the male at certain times of the year to attract females in a complicated courtship display procedure, characteristically different in each species of Uca. The claw also functions to warn off intruders and to establish territories. The female Uca has two small claws on the first appendages. Some species of fiddler crabs live on sandy beaches that are somewhat protected from extreme wave action. Others live in muddy marshes and estuaries. The Uca species living on sandy beaches, such as the common Atlantic fiddler, U. pugilator, make burrows about 1 ft (30 cm) deep, just below the high tide line. The sand is carried to the surface by specialized legs of the crab, and pushed away from the entrance. Fiddler crabs are poor swimmers and rarely enter the water during their adult lives. During the spring and summer, the fiddlers remain in their burrows only during high tide periods. The entrances of the burrows are covered with sand, and the burrows contain a bubble of air, which the crabs use for respiration. When the tide ebbs, the fiddlers emerge and scurry about, collecting food in the drift lines left by the ebbing water. Both claws of the female and the smaller claw of the male are used to scoop up sand and pass the grains to the mouthparts. Certain specialized appendages (the first and second maxillipeds) have spoon-shaped setae, used to scour organic matter from the sand grains and pass it to the mouth. The sand grains are then rejected in the form of small sand balls. After mating, the female fiddler crab carries the fertilized eggs under her flexed abdomen. Certain cyclic changes occur in Uca (as well as in some other crustaceans), such as changes in pigmentation. During the day the crabs are dark; at night they are pale. Fiddler crabs are classified in the phylum Arthropoda, subphylum Crustacea, order Decapoda.
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