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

Shell is a multilayered system for protecting the bivalve Mytilus edulis. In contrast to well-developed research on the nacreous component, the protective function of the M. edulis shell as a whole system is not fully understood. Especially, the question of why nacre is situated at the inner side of the shell rather than on the outer 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., the out-in bending) and from inside out (i.e., the in-out bending), respectively. Our etching results show that the outside existence of prismatic calcite endows M. edulis with stronger acid resistance than if when the nacre would be outside. On the contrary, the out-in and in-out bending tests reveal that a better mechanical protection of the shell against slow mechanical attacks is achieved if when the nacre was outside. 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 a better mechanical protection while the outside location of the prismatic layer offers a stronger resistance against etching. The inside natural location of nacre is the compromise between mechanical and chemical protection mechanisms against complex survival environment. This strongly contributes to our understanding of biological design principles and further development of shell-inspired protective materials.