A nut cracker is used for cracking nuts. What is the gadget used for cracking shells of crabs called?
A rock is often used for cracking crab shells.
However, I suppose that you are actually referring to a 'Crab Cracker', or 'Seafood Cracker'.
Lobster and Crab Cracker - Cooking.com
Zyliss Seafood Cracker - Zyliss Kitchen Tools