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. Robin Dunham and his sternman Roger Eaton found this four-clawed lobster in a trap while fishing gear off Stonington, ME in July. There are several characteristics that make the lobster in the photos extremely unusual. The most obvious abnormality is that there are three claws on one side. The presence of the double seizer, though, is not nearly as rare as the condition of having more than one claw attached at the main body segment.