Ribs of Pinna nobilis shell induce unexpected microstructural changes that provide unique mechanical properties

The authors thank Mr Łukasz Niedzielski from Keyence International for the possibility of performing microstructural analysis using a VHX- 7000 digital microscope. Thanks to Damian Sapijaszka who collected the P. nobilis specimen for scientific research. The work was supported by the Polish National Agency for Academic Exchange (grant PPI/APM/ 2018/1/00049/U/001) and the National Science Center (grant UMO- 2018/29/B/ST8/02200). MS was supported by the European Union from the resources of the European Social Fund (Project No.WNDPOWR. 03.02.00-00-I043/16). AGC was funded by project CGL2017- 85118-P of the Spanish Ministerio de Ciencia e Innovaci´on. Jos´e Rafael García March (Universidad Cat´olica de Valencia, Spain) provided the images of Pinna nobilis of Fig. A1.The reinforcement function of shell ribs depends not only on their vaulted morphology but also on their microstructure. They are part of the outer layer which, in the case of the Pinna nobilis bivalve, is built from almost monocrystalline calcitic prisms, always oriented perpendicular to the growth surfaces. Originally, prisms and their c-axes follow the radii of rib curvature, becoming oblique to the shell thickness direction. Later, prisms bend to reach the nacre layer perpendicularly, but their c-axes retain the initial orientation. Calcite grains form nonrandom boundaries. Most often, three twin disorientations arise, with two of them observed for the first time. Nano-indentation and impact tests demonstrate that the oblique orientation of c-axes significantly improves the hardness and fracture toughness of prisms. Moreover, compression tests reveal that the rib area achieves a unique strength of 700 MPa. The detection of the specific microstructure formed to toughen the shell is novel.Polish National Agency for Academic Exchange (grant PPI/APM/ 2018/1/00049/U/001)National Science Center (grant UMO- 2018/29/B/ST8/02200)European Social Fund (Project No.WNDPOWR. 03.02.00-00-I043/16)CGL2017- 85118-P of the Spanish Ministerio de Ciencia e Innovaci´o