fiddler crab

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various burrowing crabs of the genus Uca of coastal areas, the male of which has one greatly enlarged claw.

from The Century Dictionary.

• noun A small crab of the genus Gelasimus, as G. vocans or G. pugilator; a calling-crab: so called from the waving or brandishing of the odd large claw, as if fiddling.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a genus of crab in which the male has one hugely enlarged claw (genus Uca).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun burrowing crab of American coastal regions having one claw much enlarged in the male