107 / 2025-03-31 22:14:44

Biomineralization of Annelida：Study on the microstructure of calcareous tubes in modern serpulids

biomineralization, serpulid, tube ultrastructure, organic matrix-mediated.

Theme 5: Life-Mineral Interaction and Evolutions > Session 5c: Microbe-mineral Interactions in Ocean Environments

Serpulid belongs to the phylum Annelida and is one of the few groups capable of building skeletal structures by secreting calcium carbonate minerals.  The degree of controls over tube biomineralization in annelids is relatively weaker compared to typical biomineralizers such as mollusks and brachiopods, making them more susceptible to variations in marine environmental conditions. This study examines the microstructure of the calcareous tube in “Hydroides elegans” using Scanning Electron Microscopy at the micro-nano scale to understand biomineralization. The tube wall has three layers: an innermost organic sheath, an intermediate laminated fiber, and an outermost prismatic microstructure. While the lamello-fibrillar structure, made of needle-like aragonite crystals with high organic content, is common in cephalopods, it differs in serpulids in terms of microstructure, composition, and organic content. In serpulids, calcite minerals form prismatic bundles woven into fibers, bound by organic matrices, with the outer prismatic structure growing radially outward. The outermost layer of the tube wall is usually composed of prismatic layers, which grow outward in a radial manner, providing a hard shell of the calcareous tube.Unlike the previously assumed “granular secreting” mode of carbonate deposition, this study demonstrates that the biomineralization of the laminated fiber structure in serpulids is controlled by an "organic matrix-mediated" process. The formation of the lime tube is a process of layer-by-layer deposition to thicken the tube wall. The collar gland secretes organic macromolecules, adsorbs calcium ions and induces calcium carbonate to nucleate in a specific area, and mediates the growth rate, direction and crystal morphology of the crystal. Insects secrete organic sheets, which can separate the mineral layer and determine the thickness and arrangement of the mineral layer, so as to realize the layered regulation of the mineralized layer. This layer-by-layer deposition allows Serpula to control the thickness, shape and strength of the tube wall. To sum up, the presence of numerous extraneous mineral particles within the calcareous tube indicates that this biologically controlled process is relatively weak.