Identifying environmental drivers of shell shape variation in the freshwater gastropod Campeloma decisum (Say, 1817)

Campeloma decisum (family Viviparidae), Pointed Campeloma, is a common and widely distributed freshwater gastropod found in varied habitats across the eastern United States and southeastern Canada. Substantial conchological variation is known to be present in the species, but this variation and what it corresponds to has not been quantified. Our study sought to use geometric morphometric analyses to test which environmental factors may be driving shell shape variation in C. decisum. Campeloma decisum specimens (n = 458) and environmental data were collected from 22 sites in six lakes in central and northern Michigan. Specimens were photographed with apertures orthogonal to the camera lens and 26 landmarks were digitized onto each shell image to estimate shell and aperture shape. A Procrustes superimposition was performed to scale and rotate the shells to focus analyses solely on shape. A linear discriminant analysis of principal components (PCA-LDA) was run to quantify shape variation among the sites and lakes sampled. Correlations were tested for among environmental variables and PCA-LDA axes that defined shell shape. The PCA-LDA assigned 46.6% of snails to their site of origin and 86.9% to their lake of origin. Shell shapes of C. decisum were most strongly correlated with Secchi depth, ammonia concentration, pH, alkalinity, phosphate concentration, magnesium concentration, and percent sand substrate. Higher ammonia and phosphate concentrations often correlated with more elongate shapes (i.e., taller spire). The pH at sites and lakes appeared to be a strong driver of spire decollation with more acidic conditions likely breaking down calcium carbonate in shells. In contrast to a previous study on the often sympatric freshwater gastropod Elimia livescens (Menke, 1830) (family Pleuroceridae), our study did not find lake fetch (driving wave energy) to be major a driver of shell shape in C. decisum. However, to better test this, we recommend collecting additional C. decisum specimens from lakes with longer fetch to expand the dataset and further effects of lake morphometry on shell shape.