Optical asymmetric transmission design with mechanical robustness in the shell of Polinices albumen

Asymmetric transmission (AT) is crucial for optical devices like isolators and circulators. A key, yet unmet, challenge is integrating this functionality into sturdy, bulk materials that can withstand harsh operating conditions. Here, we report a previously overlooked AT phenomenon in the gastropod Polinices albumen shell, which exhibits a distinctive two-layered crossed-lamellar structure. We found that the inner layer's optical transmittance substantially exceeds that of the outer layer. Intriguingly, within each layer, alternating bright and dark lamellae reverse their optical contrast upon a 180° rotation of the incident light, revealing pronounced direction-dependent transmission. This AT effect might stem from the shell's multilevel architecture, the composition and the precisely controlled crystallographic orientation of aragonite crystals (CaCO₃). Notably, this structure concurrently delivers high hardness and fracture resistance, highlighting a natural design paradigm that successfully reconciles optical and mechanical functionalities. The evolutionary change in the shell's optical properties-where the juvenile shell, represented only by the apex, exhibits the transparency characteristic of the inner layer, while the adult shell, represented by the outer surface, shows high reflectivity-reflects its natural developmental transition from a prey-like state to a predator-like one. As a result, the observed AT emerges as a product of this developmental process. Our work elucidates a previously unrecognized biological strategy for AT and provides a blueprint for fabricating bio-inspired optical materials (e.g., infrared windows, interference coatings) for demanding applications in harsh environments. STATEMENT OF SIGNIFICANCE: This work reports the discovery of inherent asymmetric optical transmission in a mollusk shell, Polinices albumen, achieved through a hierarchical architecture of interlocking aragonite fibers with controlled crystal orientation. Unlike synthetic materials that struggle to combine directional optical function with mechanical durability, this natural system exhibits both high-contrast asymmetric transmission and high hardness and fracture resistance. The findings provide new insights into nature's design strategies for multifunctional structural materials and offer a bioinspired blueprint for developing advanced optical-mechanical integrated systems, such as robust optical sensors, infrared windows, or protective coatings capable of performing in harsh environments.