Hierarchical structure and multiscale mechanical properties of the gooseneck barnacle (Capitulum Mitella)

Stalked barnacles represent an evolutionarily earlier lineage than acorn barnacles. They rely not only on rigid shell plates but also a unique flexible yet protective stalk to withstand powerful wave-induced mechanical forces in intertidal environments. However, their mechanical properties remain poorly understood. In this study, we investigated the gooseneck barnacle (Capitulum mitella) from a materials science perspective. Multiscale characterization using projection X-ray microscopy (PXM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and field-emission electron probe microanalysis (FE-EPMA) revealed the hierarchical structure and elemental composition of the stalk and its external protective scales. Nanoindentation results showed that the mineralized core within these scales was approximately one order of magnitude harder than the surrounding non-mineralized region under both dry and hydrated conditions, exhibiting a sharp mechanical transition at their interface. Creep test further quantified viscoelastic properties of both regions, elucidating their distinct energy dissipation pathways for buffering localized impacts. At the macroscale, uniaxial tensile testing of the stalk demonstrated pronounced mechanical anisotropy. Compared with longitudinal loading (0°), transverse loading (90°) increased ultimate stress, elastic modulus, and toughness by 49%, 75%, and 188%, respectively, accompanied by distinct crack propagation trajectories and digital image correlation (DIC) strain distributions. Furthermore, varying the strain rate across two orders of magnitude (10^-3 to 10^-1 s^-1) yielded no statistically significant changes in tensile properties, demonstrating a stable mechanical defense. Overall, this work elucidates the hierarchical structure and mechanical adaptation mechanisms of C. mitella, offering design principles for bio-inspired protective armor applications. STATEMENT OF SIGNIFICANCE: Arthropods typically possess rigid exoskeletons, but gooseneck barnacles exhibit a unique, flexible stalk armored with mineralized scales for structural support and protection. To date, only a study has reported the tensile properties of the stalk, relied on chemically treated or dehydrated tissues and only investigated the chitinous cuticle. Considering the barnacle's ecological adaptation to intertidal zones, its sensitivity to hydration and strain rates, alongside the scales' protective role, remains unexplored. This work bridges this gap by systematically investigating the scale's heterogeneous microstructure via nanoindentation and elucidating the synergistic effects of moisture and strain rate on tensile properties of stalk. These comprehensive datasets will significantly advance future bio-inspired armor designs.