Genetic and morphological variation is associated with differences in cold-water tolerance and geographic expansion among invasive snails (Melanoides tuberculata) in central Texas

Melanoides tuberculata (Thiaridae) is an old-world freshwater snail that is and now circumtropical. After being introduced in the 1960s via the aquarium trade, populations of M. tuberculata are thriving in spring systems of Texas (USA). Field surveys and experimental investigations of temperature tolerance suggest M. tuberculata is stenothermal, and thus range expansions outside of aquatic habitats with water temperatures between 18{degrees} and 32{degrees}C should be unlikely. However, in 2012 snails were detected in natural aquatic habitats with seasonal temperatures below the experimentally determined lethal thermal minimum. To test whether genetic and phenotypic variation might be associated with cold-water tolerance and range expansion, we sequenced the 16S ribosomal rRNA gene and measured qualitative conch morphology of 170 snails collected at 26 sites in three central Texas rivers. We also conducted phylogenetic analyses of M. tuberculata collected globally and in Texas to determine potential source populations and estimate the number of invasion events. Our results show snails detected in variable temperature habitats are genetically divergent and morphologically distinct from snails collected in habitats with stable temperatures. These data are consistent with at least three introduction events into Texas of M. tuberculata sensu lato that are characterized by distinct habitat preferences, physiological tolerances, and/or adaptive behaviors.