One of the serpulid species. Small feather duster species reaching a max of 0.5 cm in diameter when open. 
We believe possibly apomatus sp. a temperate feather duster found around the Mediterranean. They can grow as individual or form small groups that make great hosts to many species of invertebrates such as pods, micro brittle stars, small crabs and other small animals. They are filter feeders using their feather like appendages called radioles to pull in food and for use as gills. This species has a hard calcified tube with no segments and does not contain an operculum (trap door snails use for protection). This tube can grow up to 12 cm long depending on its environment. They have separate sexes releasing gametes into the water typically during the warmer summer months.