Arch and beam: deployment of microstructural fabrics in relation to loads exerted on the shells of bivalved molluscs in different functional regimes

Abstract

Two apparently contradictory models have been proposed, relating the disposition of microstructural fabrics of the bivalve shell to stresses they experience. These models are here shown to apply to the function of the shell in different circumstances. In its day-to-day operation, the shell acts as a pair of beams, loaded under opposing stresses exerted by the adductor muscles and the ligament. The resulting strain is built into the shell as new layers are added to its growing interior surface (Wainwright 1969). The distribution of stresses induced by attempts to crush the shell, or by violent adduction intended to prevent it from being opened, is different. Here, the shell acts as a dome, a ‘shell’ in the architect's sense. Compressional stress develops in the outer layer and tension in the inner layer. The distribution of shell microstructures in many bivalves is biomechanically consistent with the need to resist these latter stresses. The shell is prestressed in the right direction to resist this deformation. However, the built-in strain is an exaptation in relation to this function. Any added resistance to crushing it provides is fortuitously advantageous, since it becomes prestressed as an unavoidable consequence of shell growth and articulation.

Keywords:

• bivalve shell
• biomechanics
• microstructure
• dual functions
• exaptation
• Mollusca

Acknowledgements

I am deeply grateful to Wolf-Ernst Reif for lively and productive discussion of issues discussed in this paper, and so much else besides. I thank Robert J. Ruiz for his assistance in preparation of the figures; Beto Frota for permission to reproduce Figure 2(A); and Geerat J. Vermeij and another reviewer for their constructive suggestions for improvement of the manuscript. Sonderforschungsbereich 230, Universität Stuttgart and Universität Tübingen, and Franklin & Marshall College provided financial support for this research.