DANIEL, T.L.; GLINOGA, G.P.; WARD, P.D.; Univ. Washington, Seattle; Univ. Washington, Seattle; Univ. Washington,Seattle: Biomechanics and origins of shell coiling

Mechanisms driving the origin and early evolution of coiling in molluscan shells remain enigmatic. With the notion that shell form follows from an evolutionary arms race between mollucan prey and their predators, we asked whether coiling could reduce the magnitude of peak stress in shells exposed to point forces derived from potential predators. To address this issue we used computer aided design methods to generate a wide range of shell forms, from simple straight cones and cylinders to highly coiled and tapered surfaces. We used finite element methods (MSC, Marc-Mentat Program) to examine how shell shape and size influences the magnitude and distribution of stresses within the shell. With paired point forces applied to opposite sides of the shell aperture, we examined how the strains, Cauchy stresses, and displacements varied with size, shape and extent of coiling. We find that, for cylindrical shells, modest coiling reduces the magnitude of the peak stresses in the shell. Moreover, ever greater coiling leads to ever lower stresses. In conical shells, there is a slight increase in the stress with subtle (5 degree) coiling. Ever greater coiling (greater than 10 degrees) leads to decreased stresses in conical shells. Changes in shell size do not alter the general pattern of reduced stress with increased coiling. These results suggest that peak stress reduction in response to predatory forces is one possible mechanism driving the evolution of coiling.