Loud click + dissapearing inverts + empty shells = mantis?

[vic622]

Over the past 6-8 months, I've had a run of disappearing inverts, the odd small fish and heard a single, loud click every once in a while. 75gal tank with ~90lbs of rock.

There are plenty of empty shells and even some shell debris - it's hard to say how much is from the mystery culprit because I've added some to the substrate, too. Having said, that, though I have seen some that I'm sure aren't supposed to be there...

In the past two or three weeks I've also noticed some of the substrate has also been moved around in about a 3-4" fan shape from under a pile of live rock.

Mantis or Pistol?

How to get rid of it?

Has anyone had any luck with the X-terminator mantis trap?