Pleurocerid snails generally exhibit shell variation with the environment and predation; shells are thicker, more conic, and harder to crush upstream than downstream. Little is known, however, about whether the density of shell material varies in a similar fashion and how it correlates with other shell characteristics and the environment. Using eight populations of Duck River Lithasia geniculata, we measured shell material density as a function of X-ray radiopacity and shell thickness and correlated it with river mile and crushing strength. Populations differed in their density, which was positively correlated with river mile whether adjusted for shell thickness or not. Regression indicated that shell density showed a positive correlation with thickness and a negative correlation with crushing strength. Our results in L. geniculata are the first to show variation in shell material density in pleurocerids, and our data suggest adaptive trade-offs in response to hydrology and predation pressures.
Pleurocerid snail diversity has historically been defined by morphological shell features, and these features are believed to be phenotypic responses to environmental factors. More recently, authors have shown the importance of genetics in shell variability, even to the exclusion of any environmental effect. Work by Dillon on Elimia proxima in the 1980s aimed to address this genetic versus environmental correlation of shell features, without reaching a solid conclusion. Using Dillon’s original data and specimens, geometric morphometric analysis was used in a phylogenetic context to show that E. proxima shell variation at the population level is correlated with genetics and not with the environment. This data is the latest example to find that shell variation as the result of environmental factors is too simplistic an explanation in pleurocerids.
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