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.

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