If you here the snapping when the crab approaches the hole in the rock then you are dealing with a pistol shrimp. Pistol shrimp have pinchers and will make that snapping noise as a warning mechanism to potential intruders. A Mantis shrimp would only make that noise if it was hitting something. They don't give any warning.
My Guess: PISTOL SHRIMP. Almost 100% Positive!
