Dominica Critter ID

At 25 feet, sand, turtle grass.

This thing was flat, like a pancake, and attached to a shell that had been taken over by a hermit crab. It seemed to have a center opening or mouth (which I don't have a good picture of but can get one if needed) and some reddish bumps around the outer edge. Upon further "inspection", these pink, thread like, tentacles started extending from the bumps..

Any ideas?

Thanks, Bud

You're right, it's an anemone. Some species have these string-like defensive organs called acontia. When the anemone is touched the acontia are released through the mouth or through pores on the body wall. They are sticky & adhere to whatever contacts them and they carry nematocysts - stinging cells - to drive off the intruder.