One of your arms mutates into a massive, crab-like claw.
Benefit
You gain a natural weapon which deals 2d6 slashing and bludgeoning damage (assuming you are Medium size). You gain a +4 mutation bonus on grapple checks to initiate or maintain a grapple or to pin an opponent (but not to resist or escape a grapple).
Special
The claw can be used to hold items, but it is incapable of fine manipulation. The claw cannot attack with held weapons or perform somatic spell components, nor can it wear gloves or other magic items unless they are specifically designed for it (which may increase the crafting cost).