Microstructure and formation of the calcareous operculum in Pyrgopolon ctenactis and Spirobranchus giganteus (Annelida, Serpulidae)

The calcareous operculum of Pyrgopolon ctenactis is composed of spherulitic prismatic structures. The opercular cup consists of regular spherulitic prismatic crystals; the talon has two layers, an inner with an irregular spherulitic prismatic structure (150 μm thick) and an outer with a regular spherulitic prismatic structure (110 μm thick). The outer regular structure has thick (1 μm) organic interprismatic sheets unique in biomineralization of this group, but similar to that of Bivalvia. We infer that control over biomineralization is stronger during the formation of the outer regular layer, with its thick organic interprismatic sheets, than during the formation of the inner irregular spherulitic prismatic structure, without such sheets. In Spirobranchus giganteus, opercular formation differs from that of P. ctenactis. S. giganteus has numerous pores in its opercular plate, and calcification starts with the formation of an outer irregularly oriented prismatic structure followed by an oriented prismatic structure without interprismatic sheets.