Biogenic Design of the Flexible, Resilient, and Hard Mineral Protector in Door Snails

Organism design incorporates diverse materials with varying properties, such as hard skeletons of biogenic minerals and soft organic skins. However, achieving a balance of flexibility, resilience, and hardness remains a challenge even for organisms. Door snails have a calcareous door (clausilium) that covers the aperture. The clausilium combines hardness for defense and flexibility for opening and closing. Here, this work focuses on the biogenic design of a clausilium stalk as a unique architecture balancing several properties. This study investigates the stalk, a twisted ribbon with high flexibility and resilience, which is identified as a synapomorphic structure in 22 Clausiliidae species across seven subfamilies and 17 tribes. Internal observations reveal a double-layered structure: a hard, dense envelope with aragonite rods arranged in the b axis and a flexible, low-density core with randomly packed aragonite nanoparticles and organic matter. The anisotropic hierarchical design seen in nature is surely useful in the development of artificial materials that combine flexibility, resilience, and hardness.