Phenotypic Plasticity and the Endless Forms of Freshwater Gastropod Shells

Abstract

Freshwater gastropod shells display a striking amount of variation. Shell characters are the foundation of most freshwater gastropod taxonomy and the basis for identifying most species. However, intraspecific shell variation is common, and the mechanisms that give rise to this variation are often unclear. One source of shell variation is phenotypic plasticity, in which one genotype gives rise to multiple phenotypes as a response to environmental cues. This phenomenon is often invoked as an explanation for intraspecific shell variation in gastropods, but its existence has not been confirmed experimentally or otherwise in many gastropod lineages. I review the evidence for phenotypic plasticity in freshwater gastropods, and I discuss areas of research needed for a better understanding of intraspecific shell variation. Phenotypic plasticity is well documented in the superorder Hygrophila, but evidence in other freshwater gastropod groups is limited or nonexistent because of the scarcity of common garden experiments for those groups. Despite statements to the contrary, studies that show correlations of shell traits with environmental factors, population genetic analyses, and phylogenetic inference fail to provide evidence of phenotypic plasticity. Researchers must be careful not to postulate about phenotypic plasticity without evidence. I argue that phenotypic plasticity should not be the default hypothesis for explaining intraspecific shell variation in freshwater gastropods and that more common garden experiments are needed to test its existence. Genomic studies of mantle gene expression and transgenerational epigenetic studies also will increase our understanding of gastropod shell variation.