My theory? They mimic the look of the the buzzillion bristle worms (see inset) found in the same mud flats the crab inhabits. On top of the carapace, they aren't going to be highly visible to other crabs, so they probably don't serve any reproductive purpose. Anyway, it's all about the size of that claw when it comes to fiddler crab romance. They only other purpose those little patterns might serve would be camouflage and possibly defense. Bristle worms are nasty little little critters. Their bristles can break off and get stuck in human skin and possibly the tongues and mouth bits of crab eaters like birds and fish, causing pain and inflammation. It wouldn't be the only example of an animal that borrowed the look of another poisonous or unpleasant tasting creature as a defense.
Tom Bradley © 2015