Male fiddler crabs are famous for their mismatched front claws — one great big and threatening, one eensy-weensy. (I used to use this as a metaphor for the split in my SAT scores.) But what's really interesting about this lopsided look is that it seems to serve multiple purposes. The big claw can be used to attract lady crabs — wave it around and it becomes the crabby equivalent of, "Yo! Adrian!" But the big claw can also be used as a practical weapon, where two male crabs go at each other like fancy fencers with one arm behind their backs.
And the extent to which the big claw is for looks or for violence seems to vary a lot depending on the species of fiddler crab, writes scientist John Christy. Some have a lightweight claw that's better for waving at the girls, but weaksauce in a fight. Others have a heavy, dangerous claw that's difficult to use for long-distance flirting. Christy and his team are in the process of trying to figure out what selection forces leave some crabs optimized for love and others for the battlefield. In the meantime, though, they made this awesome crab fight video, set to a stirring, John Williams-esque soundtrack.