Climate change drives corals to switch algal symbiotic partners at the forefront of northward migration

Coral reef ecosystem harbors high biodiversity, although corals are highly threatened by elevated water temperature due to climate change. Several species of corals, on the other hand, are expanding northward and expanding their distributions in the northern areas in Japan. The recently colonized marginal populations of species distribution created by northward migration, however, would be a harsh environment for the coralsspecies and might have lower genetic diversity and/or have specific adaptive strategies. In this study, to test these hypotheses genetic diversity of coral itself and Symbiodiniaceae have been assessed. Genomic diversity of two dominant temperate zone species (Acropora cf. glauca and Acropora solitaryensis) were estimated using the MIG-seq method. We used the MIG-seq method and ITS2 regions sequences for Symbiodiniaceae. As a result, genetic diversity of two coral species was similar in all the areas examined in this study, suggesting that they are not particularly vulnerable to environmental changes in the northernmost areas in terms of genetic diversity. On the other hand, the genetic compositions of Symbiodiniaceae in the northernmost two populations, Tateyama and Tsushima, were strikingly different from those in other temperate areas, suggesting that corals may have selectively adapted to the cold-water environment by having unique Symbiodiniaceae composition. This study demonstrated that corals dramatically change their symbiotic partners in the front-lines of northward migration. These findings imply special adaptation strategies by dramatically changing symbiotic partners can be found in corals at the marginal populations and thus caution is required when predicting future coral distribution and resilience associated with climate change.