Environmental drivers of shell shape in a freshwater gastropod from small and large lakes

Elimia livescens (Menke 1830) is a freshwater gastropod common in the Laurentian Great Lakes region. Geometric morphometric analysis was conducted to determine the variation in shell shape among sites along the shoreline of Lake Michigan (4 sites) and 3 inland lakes on Beaver and Manitou Islands (8 sites) in Michigan, USA. Canonical Variates Analysis (CVA) showed significant variation in shell shape among the sites at inland lakes and those on the Lake Michigan shoreline. Based on shell shape, 82.5% of individual snails could be correctly assigned to their lake of origin, and 62.8% could be correctly assigned to their site of origin. CVA axes were tested for correlations with the environmental variables. Shell shape in E. livescens was correlated most strongly with fetch length and specific conductance. Higher fetch lengths produce higher-energy wave action and may select for individuals with a larger aperture, more globose shape, and lower spire (better for anchorage). Lower fetch lengths produce lower-energy wave action and may select for snails with a smaller aperture, more elongate shape, and higher spire (better for navigating above soft sediment). Environmental processes like those documented in classic studies on marine snails appear to affect shell shape in freshwater snails in similar ways. Furthermore, our study highlights that fetch plays a key role in structuring benthic communities and may play a parallel role in structuring populations. Our study suggests that shell shape variability is a critical factor in regional adaptation and success in this widely distributed gastropod.