Tridacna squamosa, Lamarck, 1819 (Bivalvia Cardiida
Cardiidae, known as the fluted giant clam) is one of the largest-sized
bivalve shells, which is equipped with a strong and tough bioceramic
shell to effectively protect itself from the attack of predators.
To better understand the mechanical defense mechanism, the relationship
between the microstructure, composition, and mechanical properties
of the Tridacna squamosa shell was investigated.
We find that the Tridacna squamosa shell is composed
of aragonite CaCO3 and a small portion of organic matter,
which are well-arranged, assembling a multiscale, inhomogeneous, and
anisotropic structure. Three levels of microstructure units are identified,
including the smallest aragonite rods, medium sheets, and block-like
lamellae. Such multiscale structures are the main contributor to creating
abundant fracture surfaces much larger than the case for single mineral
components, leading to multiple toughening mechanisms observed in
Vickers indentation experiments, such as pulled-out of mineral platelet
and crack deflection. The material inhomogeneity in the cross-sectional
direction indicates that the material is stronger at the inner layer
than that at the outer layer, which also facilitates an effective
defense against the predator attack. This study may provide insights
into the design of biomaterials with the desired mechanical properties.