Shell shape and tissue withdrawal depth in 14 species of temperate intertidal snail

Predation is an important selective agent in the evolution of gastropod shell form. The adaptive value of shell elongation, however, has received relatively little attention in the context of predation. Arguments for its functional significance include the hypothesis that shell elongation facilitates soft tissue withdrawal inside the shell, thus elongation may improve fitness when predation by shell entry is intense. Here we ask whether species with more elongate shells also retract more deeply, a long-held assertion that until now lacked empirical support. To do this, we measured shell elongation (shell height divided by breadth) and soft-tissue withdrawal depth (angular retraction) in 14 species of temperate marine snail, covering a wide range of shell forms. Results show shell elongation to be a good predictor of angular retraction across genera and distantly related species. However, the elongation-retraction relationship was not apparent in comparisons among closely related species (specifically, species of Nucella and Littorina), nor were there such relationships among individuals within species (except, perhaps, within a few species having high shell-shape diversity). Competing arguments for the functional significance of shell elongation are discussed, as is the potential role of environment in affecting shell elongation and withdrawal depth independently.