We've had an 8 inch bristle worm (fireworm), which was actually the larger of two halves. About six months earlier, we had speared it in an attempt to remove it from the tank, and it split into two and both parts disappeared into the rock. A tank breakdown finally located the two healthy worms.
It could stretch to over a foot in length to reach food, and we are sure it was the source of mysterious fish disappearances (small gobies).
My wife still gets shivers remembering this beast ...
