The Giant Clam is an animal that tried to eat Sandy. SpongeBob rescued Sandy, but SpongeBob then got stuck in the clam's mouth. Sandy tosses the Giant Clam in the air to free SpongeBob. This happens in the pilot episode, Help Wanted.
In No Free Rides, SpongeBob sees a sign called "Giant Clams" and huge clams start eating SpongeBob while he is holding onto his boat, being driven by Mrs. Puff.