Large Worm Snail

Found singly or in large groups, burrowed into dead and live corals with just a small tube poking out, over coral and rocky reefs.
They feed on detritus and plankton.

These worm shells are unusual as they bury into live and dead corals and have a black trapdoor called an Operculum
They produce a mucous net (very much like a spider web) to snare their food of plankton and detritus.
They produce the net and then suck it back in at intervals, redigest it to get the food and then cast it back out again.