Structural and functional characteristics of the exoskeletal architecture of the cuttlebone

Cuttlefish, a unique group of marine mollusks, possess a lightweight, highly porous (∼93%) internal shell called the cuttlebone, which functions as a buoyancy chamber while withstanding high water pressures. This study quantifies the physical, chemical, and mechanical properties across different anatomical zones of the cuttlebone to reveal their role in optimizing structural performance. The internal morphology of the cuttlebone exhibits a hierarchical architecture, with varying dimensions of wavy pillars and lamellae, and porosity increasing from 88% in the posterior to 93% in the anterior region. Elemental and spectroscopic analyses confirmed the presence of calcium carbonate in the aragonite form, while thermogravimetric analysis revealed zone-dependent organic-inorganic ratios, establishing a compositional gradient that governs interfacial toughening across the zones. Uniaxial compression tests on dried samples revealed regional variations in strength (0.79-1.28 MPa), toughness (0.23-0.61 MPa), and stiffness (0.83-4.39 MPa), with consistently high strength-to-weight ratios (>1 MPa g^-1). Sea-water-hydrated samples showed increases in compressive strength of 83%-117% across all zones, demonstrating that water activates the organic matrix as an active toughening constituent rather than degrading structural integrity. Image analysis of compression showed strain localization via pillar buckling and sequential chamber collapse. In puncture testing, hydrated samples exhibited lower puncture resistance than dried ones, revealing a loading-mode-dependent asymmetry in the hydration response. The dorsal shield demonstrated significant puncture resistance and stiffness by effectively arresting crack propagation through its mineralized lamellae. These observations highlight the cuttlebone's unique microarchitecture across different zones, which effectively balances buoyancy, mechanical strength, flexibility, and energy dissipation. These findings provide valuable insights for the bioinspired design of lightweight structures and materials capable of absorbing higher compressive loads without immediate collapse.