I always thought a piston shrimp was a sand dweller???? The click sound liked it comes from the top of my tank in one of the huge ass rocks. But like i said its a single click nothing more. It also amazes me how crabs can hide so well u can see a large crab then the crunch themselves into som odd shapped hole. I Know a few time i took a Metal rod and jammed it in a hole to kill a crab