This mass resembling worn coral was about 10 cms across. On the surface were crescent shaped structures which were the open ends of calcareous tubes constructed by tubeworms. They have clustered together and probably grown on a snail, completely enveloping it and forming a hard mass, as suggested by Audrey Falconer(Marine Research). The mass was partially covered with sand but red algae could be seen growing on the mass.
The tubes are built by annelid fanworms from the family Serpulidae. The worms have branchial crowns in two lobes, one of them has a stalked operculum (lid). The branchial croown form the gills and also helps to capture food.
The worm lives within the tube and retracts into the tube when in danger or when the tide is out, pulling the operculum down tight to shut the opening of the tube. A dense mass of tubes can form a microhabitat for other marine creatures. My thanks to Audrey Falconer ( Marine Research) for identifying this mass.
Spotted in a rock pool in an intertidal zone of a rocky shoreline ( Cape Conran, Victoria). These tubeworms are seen from Southern Queensland all along the southern coast to Western Australia.