Adaptive responses and invasion: The role of plasticity and evolution in snail shell morphology

Erica Jean Kistner

Only a select few non-native species achieve high abundance and inhabit broad ranges outside of their native habitat. Success of invasive species can be attributed to two contrasting mechanisms: phenotypic plasticity and adaptive evolution. Two studies involving the New Zealand mud snail Potamopyrgus antipodarum examined the role that plasticity and evolution play in a successful invader. In the first study, variation in shell morphology was compared between P. antipodarum with sympatric populations of Pyrgulopsis robusta, a native snail, along the Snake River, Idaho, USA. The Analysis of Variance (ANOVA) found the effect of site on shell morphology to be significant, indicating morphological variation across the sample sites. The Canonical Variate Analysis (CVA) revealed parallel adaptive responses in the shell shape of both species consistent with water flow variation across the sample sites, but whether these responses are evolved or plastic remains unclear. In a common garden experiment, responses in shell shape morphology of three geographically distinct invasive populations of the New Zealand mud snail Potamopyrgus antipodarum were compared to determine the presence of plastic and evolved responses. CVA and traditional length measurements revealed significant differences between F1 and maternal lineages, suggesting a plastic response. However, offspring maintained among-populations differences in both shell shape and life history traits. Furthermore, broad sense heritability estimates for shell traits were high, indicating a genetic component. The generational reduction in shell size was attributed to a reduction in water flow in the common garden environment, indicating an adaptive shift in shell shape. A significant population by generation interaction suggests that plasticity and evolution are not mutually exclusive explanations for the differences in shell shape among populations. Variation in shell morphology of P. antipodarum suggests both phenotypic plasticity and adaptive evolution play an important role in invasion success. Invasive P. antipodarum matched the adaptive morph of its native counterpart and was able to alter its shell morph within a generation when grown in a common environment, suggesting that invasive populations exhibit adaptive responses to new environmental parameters. The results suggest that plasticity initiates phenotypic change, followed by genetic changes in the direction of the plastic response.

Adaptive responses and invasion: The role of plasticity and evolution in snail shell morphology

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