Purchased a Derasa clam a few weeks ago. Been doing fine since I first put him in the tank. Pretty much stayed where I put him. This morning I noticed he was totally dead. Nothing left but the shell. This happen in less than 24 hours. Nothing in my tank had changed and Neither the fish or the rest of my corals look stressed. I have heard this is not unusually but certain caught me off guard. Any theory on what happened.