Later in life the crusher claw becomes sexually dimorphic. Reproductive males develop a disproportionately large crusher claw that females find irresistibly attractive. Occasionally, a lobster with more than two claws is landed. Repetition of body parts occurs in segmented animals such as lobsters and insects. Each body segment has a particular set of regulatory genes that tells the animal which appendages to grow on that part of the body.
Abnormalities arise when the regulatory genes give the wrong signal.
On her right side, this female lobster has a normal seizer claw. On the left, she has a smaller, normally functioning crusher claw. That in itself is not particularly unusual. Lobsters often lose and regenerate claws. It can take many molts for the smaller claw to catch up in proportionate size.