Water Turbidity and Plant Density Influence Shell Shape in Painted Turtles (Chrysemys picta)

An organism's body shape is influenced by its ecology and environment through selective pressure and phenotypic plasticity. The shape of turtle shells is impacted by locomotor efficiency, and shell shape has been shown to differ between populations occupying different water flow regimes. However, it is not known whether other aspects of the environment, such as water turbidity or plant density, similarly impact shape by influencing locomotor efficiency. We used geometric morphometric methods to analyze the shell shapes of Painted Turtles (Chrysemys picta) from several populations in Wisconsin. Water turbidity was found to be correlated with different shell shapes among the populations. In more turbid water, turtle shells were narrower than shells in clear water, and there was an interactive effect between water turbidity and plant density such that shells were anteriorly taller in high plant densities with high water turbidity than in low water turbidity. Shell shape also differed between the sexes and varied with adult size. These correlations suggest that aspects of the aquatic environment in addition to water flow may impact turtle shell shape by influencing locomotor efficiency and that this variation is detectable even within a species and region.