Abstract Sea shells are composed of calcium carbonate crystals interleaved with layers of viscoelastic proteins, having dense, tailored structures that yield excellent mechanical properties. Shells such as conch ( Strombus gigas), giant clam ( Tridacna gigas), and red abalone ( Haliotis rufescens) have hierarchical architectures that differ depending on growth requirements and shell formation of the particular mollusk. Mechanical tests have been carried out on these shells for a comparison of strength with respect to the microstructural architecture and sample orientation. The mechanical response is found to vary significantly from specimen to specimen and requires the application of Weibull statistics in order to be quantitatively evaluated. The complex micro-laminate structure of these biocomposite materials is characterized and related to their mechanical properties. The red abalone has the highest compressive (233–540 MPa) and flexure strengths of the three shells. The giant clam has the lowest strength (87–123 MPa) and the conch has an intermediate value (166–218 MPa) in compression. The high compressive strength observed in the abalone is attributed to an optimization of microstructural architecture in the form of 2-D laminates, enhancing the fracture toughness of this shell material and enabling higher stresses to develop before fracture.