Compromise between mechanical and chemical protection mechanisms in the Mytilus edulis shell

The shell of Mytilus edulis is a multilayered system for protecting this bivalve. In contrast to well-developed research on the nacre materials, the protective function of the complete M. edulis shell has not been widely studied. In particular, the question of why nacre is situated on the inner side of the shell rather than on the outer side remains unclear. Herein, the acid resistance of different shell layers was compared using etching tests and the mechanical protection performance of the shell was tested using three-point bending. Two bending loads, including static and dynamic, were applied on the shell samples from outside in (i.e. out-in bending) and from inside out (i.e. in-out bending). respectively. Our etching results show that the external prismatic calcite endows M. edulis with stronger acid resistance than if nacre was on the outside. In contrast, the static out- in and in-out bending tests reveal that a better mechanical protection of the shell against slow mechanical attacks is achieved if the nacre is on the outside. However, the shell has the same mechanical properties against dynamic mechanical attacks regardless of nacre location. Briefly, the nacre should be on the outside of the shell for better mechanical protection while the outside location of the prismatic layer offers a stronger resistance against etching. The inside natural location of nacre is a compromise between mechanical and chemical protection mechanisms against a complex survival environment. This strongly contributes to our understanding of biological design principles and further development of shell-inspired protective materials.