This is an interesting one, because there are two very different points of view. The large claw is for communication with other crabs, to challenge other males and to attract females. The small claw is for feeding, to bring food to its mouth.

Most of the sites with general information on Fiddler Crabs say that if the male crab loses its large claw, its small, feeding claw then becomes the large claw, and the crab regrows the lost claw as a small, feeding claw.

A search through scientific papers on handedness in Fiddler Crabs showed a very different picture, with quite a few studies showing that as juveniles, both claws are the same size, and that losing one claw will cause the other to become the large claw. These studies also found that once the crabs became adults, their handedness was fixed, so a lost large claw would grow back as a large claw.