Crab in sign language looks like the claws of a crab opening and closing. Take both hands and touch your thumbs to your fingers to form a claw, then open and close your claws. An alternative ASL sign for crab is to take just the pointer or index fingers and touch them with the thumbs multiple times.
Crab can be taught from pictures or online videos (unless you are lucky enough to have access to live crabs either in an aquarium or a natural setting).
If you are lucky enough to live by the beach, teach this sign during afternoon walks when you see crabs by the shore.