FUNCTIONAL MORPHOLOGY OF GASTROPOD SHELL ORNAMENTATION AND AN EXPERIMENTAL TEST OF THE ANTI-PREDATOR DEFENSE HYPOTHESIS

Many mollusk shells exhibit prominent external ornaments which are hypothesized to function as a defense against shell-crushing predators. Ornamentation may provide protection by inhibiting attack and/or by enhancing shell mechanical properties, but only a handful of studies have experimentally tested the structure-performance linkages that permit such protective mechanisms. As the mechanical consequences of ornamentation have rarely been quantified, we used compression tests of 3D-printed gastropod shell models to investigate how ornamental spines affect a shell’s resistance to crushing forces. The relationship between morphology and mechanical strength in real shells is complicated by factors such as shell microstructure and taphonomic history. 3D-printed models provide greater experimental control by using a standardized constructional material, which allows the role of spine morphology to be isolated. We used a microCT scan of a shell of the modern strombid gastropod Strombus pugilis to generate a digital 3D model. Two theoretical morphotypes representing alternate degrees of ornamentation were also created by digitally lengthening and removing the spines on the original model, respectively. The models were printed using a resin 3D printer, then subjected to compression testing between two flat steel plates to simulate the forces applied by a vertebrate shell-crushing predator. Results suggest that the presence of spines modifies the overall resistance of shells to crushing by increasing both the maximum force and the amount of work required to break a shell. These findings provide direct structure-performance linkages in favor of the hypothesis that shell-crushing predation is a selective pressure driving the evolution of mollusk shell ornamentation.