Unique Mg-calcite skeletal ultrastructure in the tube of the serpulid polychaete Ditrupa

The tube wall in Ditrupa is composed of two layers of Mg-calcite. The outer, hyaline layer has a unique type of ultrastructure, unknown in other invertebrate skeletons, here termed regularly ridged prismatic structure (RRP). This structure is characterized by ridged lateral surfaces of the prisms, which interlock the prisms. RRP structure exclusively characterises the genus Ditrupa and had already evolved in the Eocene D. strangulata and Miocene D. gracillima. The ridges of RRP structure could have a shell strengthening function in that the prism surfaces tightly interlock, which could have given a mechanically stronger structure by binding individual crystallites. RRP structure could have evolved as an adaptation to the unattached life style, with the ability to move around, which could have placed an increased demand on the tube's mechanical properties. This complicated tube structure cannot be explained by the usual serpulid mineralization system in which calcareous granules from calcium-secreting glands are transported to the tube aperture and solidify in contact with seawater. In Ditrupa, mineralization must be directly cell controlled.