I don't think those are FD's. They are a type of worm. If you look closely after feeding or, stirring up sandbed they will let the slime go out to catch food. You can actually watch them slowly pull the slime back in to eat. I had a big one in my tank that I glued shut. It would disturb my p. xenia and surrounding corals with the slime. I can't remember the name of them though. |