The Hierarchical Organization of the Layered Fibrous Shell of Chamelea gallina Guides Fracture Pathways

Molluscan shells combine mineralized layers with distinct microstructures that can influence crack propagation. Here, we provide a through-thickness, multiscale characterization of the all-aragonitic shell of the striped venus clam Chamelea gallina and relate its hierarchical textural organization to observed fracture-surface trajectories. Optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and spatially resolved Fourier-transform infrared spectroscopy and X-ray diffraction resolve an outer region comprising (i) convergent and divergent fibrous sublayers separated by a preferred fracture plane, (ii) a porous transition layer with inverse-tulip microstructures, and (iii) a structurally distinct homogeneous inner layer whose crossed-lamellar organization becomes visible after etching. The key advance is the resolution of this outer architecture from nanogranule alignment to curved fibrous layers and a porous transition region, together with the identification of recurring changes in fracture-surface direction at its interfaces. FTIR and XRD reveal through-thickness variations in vibrational-band ratios and crystallographic parameters. As the fracture analysis is based on post-fracture morphology, crack steering and damage localization are discussed as plausible structure-related mechanisms supported by the observed features. The architecture suggests transferable design principles, including graded orientation, interface-guided deflection, and localized porosity, for mechanically robust bioinspired composites and provides a structural framework for future studies of shell evolution and function.