Microstructures, crystallography and growth patterns of serpulid tubeworms (Class Polychaeta)

Serpulid polychaetes are global marine worms that secrete tubes of calcium carbonate, in which they live. Despite extensive previous research on their microstructures, there are no crystallographic data and their biomineralization process remains unclear. Herein, we review the microstructures of seven different serpulid species and study their chemical composition, mineralogy and crystallography, using X-ray diffraction, Raman and Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam, electron backscatter diffraction, and thermogravimetric analysis. Generally, serpulid tubes have a high amount of organic matter (~ 7.5 wt. %), consisting of chitin and proteins, and the calcite is always present as medium to high magnesium calcite. We identified three main microstructures: granular-prismatic and lamello-fibrillar calcite, and fibrous aragonite. They all display an axial texture, which is stronger in the lamello-fibrillar calcite, with the c-axis aligned with the elongation axis of the crystals. Our findings demonstrate that only some instances of the granular-prismatic and the lamello-fibrillar calcite are biogenic (primary) microstructures. Conversely, other instances of the granular-prismatic calcite and the fibrous aragonite are a consequence of a recrystallization process (i.e. secondary). Substitution may occur on either primary or secondary calcitic microstructures (replaced by aragonite). Secondary microstructures retain remnants of the previously substituted microstructures, such as vestigial crystals or major growth increments. The high-Mg nature of the calcite favors the recrystallization processes. We hypothesize that the plywood arrangement of the lamello-fibrillar is obtained by the ordering of a chitin fibrillar precursor into a cholesteric liquid crystal phase. Subsequently, calcite would grow by oriented nucleation onto the organic fibrils.