Microstructural architecture of the argonauts' shell-like eggcase reveals the formation logic of a convergent extended phenotype

Argonautid octopods of the genus Argonauta possess a shell-like biomineralized external structure called an eggcase. A classical behavioral observation suggested that this structure is produced by the first dorsal arms rather than by the mantle tissue as seen in typical molluscan shells. In this study, we performed detailed microstructural analyses using scanning electron microscopy to investigate the characteristics of normal, undamaged eggcases and regions that had undergone post-damage repair. Our analysis revealed that the eggcase comprises five layers: an outermost organic membrane, an outer spherulitic-fibrous prismatic layer, a middle organic layer, an inner spherulitic-fibrous prismatic layer, and an innermost organic membrane. Both prismatic layers exhibit bidirectional growth from the middle organic layer, a unique feature not observed in typical molluscan shells. We propose a four-stage formation sequence to explain the observed microstructural architecture: nucleation on the organic scaffold, bidirectional crystal growth from the organic mid-layer, perpendicular growth relative to the eggcase surface, and final encapsulation by an organic membrane. Interestingly, the growth pattern and crystal shapes in the prismatic layers resemble of the internalized shells of cuttlefish, the calcified skeleton of stony corals, and avian eggshells, suggesting functional convergence. We also identified two repair mechanisms: reattachment of broken fragments and regeneration via new secretions. These findings call into question the previous assumptions about the role of the first dorsal arms in calcification. The eggcase also exemplifies the formation of a functionally and structurally complex extended phenotype through convergence.