gadget for cracking crab shell

Montana

sorry, that was a joke....there's a member here named Montana that worked at a lobster plant, and she was a claw cracker.

Actually, that's what I think you would call a nut cracker in that situation, a claw cracker.

Cracker is the main term. You can say nut cracker or crab cracker or lobster cracker etc. It's basically the same device, but you can make it more specific by mentioning the item it will be cracking.

In Florida the gadget is called a "crab hammer".