Sorry to hear this. We had an arrow crab for a bristle worm problem and he got big fast and ate our shrimp and a couple of small fish. Hauled his hungry butt right back to the lfs!!!The shell is in pieces, so I'm strongly leaning towards either mantis shrimp of bobbit worm. I'll look with some red light tonight and see if I can find anything. There was a brief day or so where a hole in the rock had what looked like small white tentacles hanging out. I thought it was an anemone but now am questioning that.