Opposing genetic patterns of range shifting temperate and tropical gastropods in an area undergoing tropicalisation

Aim: the poleward range expansion of tropical species, and range contraction of temperate species (known as tropicalisation) has mainly been studied from an ecological perspective, with little research on its genetic consequences. Here, we used distributional and genetic data to document the consequences of tropicalisation in rocky shore gastropods and assess more broadly the future implications of tropicalisation on phylogeographic patterns.Location: nineteen sampling sites along >3000 km of the eastern Pacific rocky intertidal zone, from the tip of the Baja California Peninsula to southern California.TaxonTemperate gastropods: Lottia conus, L. strigatella, Fissurella volcano and Tegula gallina.Tropical gastropods: Fissurella rubropicta, Nerita funiculata and N. scabricosta.Methods: we determine historical and modern distributions of tropical and temperate species by combining historical records with current field surveys. Using a section of the cytochrome oxidase subunit I gene, we utilised comparative phylogeography, analysis of molecular variance, FST pairwise comparison, mismatch distributions of haplotype differences and neutrality tests to detect genetic signatures of tropicalisation and to better understand its consequences.Results: we identified range contractions in two temperate species and range expansion in all three tropical species. We detected genetic signatures of range expansion in the tropical species through unimodal distributions of pairwise haplotype differences and strongly negative values for the Fu and Li D and F* statistics. We found population subdivision and phylogeographic breaks in three temperate species, although the geographic location of the breaks differed among species.Main conclusions: genetic signatures and field surveys indicate recent range expansions in tropical species, supporting tropicalisation along the studied coastline. Conversely, we found phylogeographic breaks in temperate species, suggesting that tropicalisation may cause genetic erosion of evolutionary distinct lineages with range-contraction. The different locations of the phylogeographic breaks among temperate species suggests that some barriers are species specific