Sand Fiddler Crab

The carapace is square in shape, tapering slightly to the rear; it can reach a width of 25 millimetres (0.98 in), but is typically up to 21 mm (0.83 in) wide and 14 mm (0.55 in) long. The space between the eyes is much shorter than the eyestalk. The males have one extremely enlarged chela (claw), which they use to claim their territory and fight with other males. The claw can be much larger than the body, at up to 35 mm (1.4 in) long, or exceptionally up to 41 mm (1.6 in) long. It is common for males to lose claws in the battles. When this happens, the claw regenerates and the opposite side begins to enlarge. Fiddler crabs are right or left clawed. The inside of these claws are also very smooth, unlike its other close relatives Uca pugnax and Uca minax.

http://en.wikipedia.org/wiki/Uca_pugilator