Nacre, a natural composite consisting of biogenic aragonite and protein, possesses superior strength and toughness compared to its brittle aragonite components. In this work, we first show that dry nacreous sections exhibit complete brittle fracture along the tablet interfaces at the proportional limit under pure shear of torsion. We quantitatively separate the initial tablet sliding primarily resisted by nanoscale aragonite pillars from the following sliding resisted by various microscale toughening mechanisms. In addition, we use the pure shear of torsion to demonstrate how hydrated nacre resists the initial tablet sliding by tuning its nanoscale toughening mechanisms. In hydrated nacre, hydrogen bonds between water molecules and organic matrices provide temporal paths for stress redistributions, through which the shear resistance is gradually transferred from mineral bridges to contacted nanoasperities. In the subsequent sliding, dynamical interactions between nacreous tablets enable substantial plasticity before the catastrophic failure of hydrated nacre. Microscale growth layers between nacreous tables possess distinctive aragonite structures, including columns, spherulites and organic matrices. High temporal resolution experiments were performed to elucidate the tensile and shear behavior of growth layers under dry and hydrated conditions. Hydrated growth layers exhibited lower strengths and larger failure strain than hydrated nacre under both shear and tensile loadings. However, they successfully confined or deflected cracks within themselves when failure happened.