Biomineralising polychaete

Serpulid tubes are composed of aragonite, calcite or mixture of both polymorphs.

In addition to the tubes, some serpulid species secrete calcareous opercula.

There are thin organic sheets in serpulid tube mineral structures.

These sheets have evolved as an adaptation to strengthen the mechanical properties of the tubes.

[1][2][3][4][5] Fossil and present-day cirratulid bioconstructors have been confirmed to be the only known occurrence of double-phased biomineralization processes in the animal kingdom.

Serpula israelitica , longitudinal section of the tube, calcitic lamello-fibrillar structure
Ditrupa arietina ( Serpulidae ), outer tube layer, calcitic regularly ridged prismatic structure, showing interlocked prisms
Glomerula piloseta ( Sabellidae ), longitudinal section of the tube, aragonitic spherulitic prismatic structure