A fiddler crab.
fiddler crab definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a small, burrowing crab (genus Uca), the male of which has one claw much larger than the other
fiddler crab definition by American Heritage Dictionary
Any of various burrowing crabs of the genus Uca of coastal areas, the male of which has a greatly enlarged anterior claw.