Whether crabs or humans, we like balance. There are, as always, exceptions.
Some crabs have one large claw and one small one.
Far more valuable to the soft-bodied hermit crab is the ability to wriggle into other creatures' shells. Without shelter, the hermit crab is a tempting snack for predators. So its abdomen twists sideways, the better to fit into the spiral shells of snails. So much for symmetry. Two of the hermit crab's rear legs also have dwindled to nearly nothing, usually on the right side, further dashing our symmetrical hopes.