Giant crawling clams are bivalves that are common in certain parts of the Pacific Ocean. They are capable of moving across the seabed, and are considered very dangerous. Larger clams can clamp down on a unlucky diver, especially extremities like legs, and they are "as good as dead." The clams seem to be able to sense movement, as they snapped shut and attempted to snag Troy as he swam by them. The only way to make them let go is the shoot a projectile like a harpoon into their constrictor muscles.
Troy Tempest nearly got snagged by several while swimming by them. The Ghost of the Sea got his leg caught in one, but Troy managed to free him by shooting the clam's constrictor muscles with his harpoon.