Armed on the back: Hidden biomineralized scales in the ventral girdle of chiton Acanthopleura loochooana.

Flexible protective armors are found in large animals such as fish skins, snake skins, and pangolin scales. For small-sized invertebrates, such armors are paid less attention and overlooked. Chitons, a type of marine mollusk, possess mineralized armors covering the whole dorsal body. The dorsal scales in the girdle tissue are well known, in this study, we reported hidden mineralized scales in the ventral side of chiton Acanthopleura loochooana girdles for the first time. The ventral surface is covered with scales with ca. 40 μm in length, forming continuous but overlapped scales. Additionally, scales are formed from aragonitic spicule-like and square-like scales, embedded in the cuticle layer. Nanoindentation testing results showed that the hardness and elastic modulus of ventral scales were ∼20 % higher compared to those in the dorsal scales, exhibiting good hardness and wear resistance. The combination of the ventral scales and cuticle, along with the regular arrangement of ventral scales, may allow chitons to simultaneously address complex and variable attachment interfaces while also providing wear-resistant protection. This study provides insights for designing protective structures that balance flexibility and durability. STATEMENT OF SIGNIFICANCE: Biomineralization is universal in nature and provides protection and support for animals. However, mineralization of dermal skin is not commonly seen. Herein, for the first time, we reported hidden minerals covering the whole ventral side of skin in a small marine animal, chitons. Calcium carbonate minerals are arranged regularly and manifest different morphology in different regions. Additionally, these minerals are embedded in a continuous cuticle layer covering the whole animal. The material also indicates a higher wear-resistant property. This study extends our understanding of the diverse functionality of biominerals and provides a prototype for designing wear-resistant materials.