Insights into the composition,morphology, and formation of the calcareous shell of the serpulid Hydroides dianthus

To date, the calcareous tubes of serpulid marine worms have not been studied extensively in a biomineralization context. The structure and composition of the tube shell and adhesive cement of the marine tubeworm Hydroides dianthus were studied using a variety of characterization techniques, including powder XRD, FTIR, SEM, EDX, and AFM. The tube and cement were determined to be inorganic–organic composite materials, consisting of inorganic aragonite (CaCO₃) and Mg-calcite ((Ca_0.8Mg_0.2)CO₃) crystals, and both soluble and insoluble organic matrices (SOM and IOM). SEM imaging revealed a variety of crystal morphologies. AFM nanoindentation of the inorganic components yielded Young’s moduli of ～20 GPa in the wet state, and ～50 GPa in the dry state. Amino acid analysis of the SOM indicated substantial amounts of acidic and non-polar neutral amino acids. Part of the insoluble organic tube lining was identified as being composed of collagen-containing fibres aligned in a criss-crossed structure. The SOM and organic tube lining were found to contain carboxylated and sulphated polysaccharides. In an artificial seawater solution, the SOM and the organic tube lining mediated CaCO₃ mineralization in vitro.