The shell structure of the Pinna nobilis species constitutes a model for others formed by bivalves of the Ostreida order. The outer part is built of monocrystalline columns whose axes remain parallel to the calcite c-axis. The present work reveals a new microstructure induced by mantle damage in the early stage of growth. The calcite c-axes, oriented perpendicularly to the strongly rough outer surface, deviate significantly from the shell thickness direction. The inclination angle is maintained up to the nacre layer. The transfer is made by the monocrystalline prisms which initially run along the c-axis and then deflect taking the thickness direction. They form coherent systems with low-energy twin boundaries. The uncovered twin relationships significantly improve the mechanical properties, as demonstrated using the nano-indentation and impact tests. Moreover, compression tests were performed, which confirms that the untypical structure exhibits a unique combination of high fracture toughness and strength.