Understanding the Evolution of Shell Shape in Snails

The shape of snail shells is a complex and important factor contributing to their fitness, and needs to be adapted to different selection pressures in different environments. However, the link between form and function, and the evolutionary processes that create the wide diversity seen in the shell shape of gastropods, is still not fully understood. In order to connect the environmental adaptations of snail shells to phenotypic plasticity and genetics it is crucial to have quantification methods which relate to biologically relevant components. I therefore developed a high-throughput snail shell quantification method building on the classic ideas of Raup, which is able to describe the variability in shape and growth within two ecotypes of the intertidal snail species Littorina saxatilis. This method was also used to gain new insights into the genetic architecture involved in different aspects of shell shape in L. saxatilis, including detecting and describing allometric growth. As the quantification method directly describes how to construct computer generated shell models, I investigated whether and how shape affects the hydrodynamic properties of the shells. I performed flume experiments with both real shells and 3D printed shell models in water flow as well as computational fluid dynamics simulations. The results from the fluid dynamics analysis gave some new insight into how shape affects the optimal orientation against a steady current, but also suggests that foot size has a larger influence on attachment ability, and that the hydrodynamic efficiency of the shell is likely secondary to having a shape which can accommodate a large enough foot. In conclusion, a better quantification of shape and growth can lead to an improved understanding of how and why snail shells have the shape that they do, and how the shape relates to evolutionary processes such as local adaptation and speciation.