This here's a fiddler crab. A small fresh water crustatean easily recognizable by the over sized claw. However, it's only the males that sport this big claw. Females have both their claws tiny. Anyway, the fiddler crab uses its big claw in a display of strength, arm wrestling their rivals. If the big claw were to be lost, the smaller claw becomes bigger and a new small claw sprouts, or so I've read.

Taken at the Virginia Aquarium.