Allele shift under laboratory selection and local adaptation to low salinity in Baltic Sea mytilid mussels

Baltic blue mussels colonise and dominate benthic habitats with much lower salinity than any other marine mytilid population globally. Surprisingly, all Baltic populations are hybrids of Mytilus edulis x M. trossulus genotypes with the former dominating hybrid genotypes in the western (high salinity) and the latter in the eastern part of the Baltic (low salinity). Here, we tested if low salinity selects for M. trossulus dominated hybrid genotypes and whether populations along the salinity gradient are locally adapted to their specific salinity regimes. Using laboratory larval rearing trials, we can show that Baltic M. trossulus hybrids have higher fitness when exposed to salinities <10 psu whereas Baltic M. edulis hybrids have higher fitness at a salinity of 16 psu. In addition, we can demonstrate that populations from the centre of the hybrid cline can be selected towards Baltic M. trossulus hybrids at low salinities, with allele shifts significantly beyond genetic drift expectations. We conclude that salinity driven selection can shape mussel populations and hence allows for local adaptation to extremely low environmental salinity. Future climate change driven desalination therefore has the potential to shift the Baltic Sea hybrid gradient to the west, with important implications for ecology and aquaculture.