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

Coral reef ecosystems harbor high biodiversity, yet corals are increasingly threatened by elevated water temperatures due to climate change. Interestingly, several coral species are expanding their distributions northward into temperate areas of Japan. These recently colonized marginal populations may encounter harsh environments and potentially lower genetic diversity or unique adaptive strategies. In this study, we tested these hypotheses by assessing the genetic diversity and connectivity of the corals and their symbiotic Symbiodiniaceae. We focused on two dominant species, Acropora cf. glauca and Acropora solitaryensis, which are expanding their distribution northward. We utilized the MIG-seq method to estimate their genomic structure. Additionally, we examined the ITS2 region sequences for Symbiodiniaceae. Our results indicated that the genetic diversity of the two coral species was consistent across all examined areas, suggesting they are not particularly vulnerable to environmental changes in terms of genetic diversity in their northernmost habitats. However, the genetic compositions of Symbiodiniaceae in the northernmost populations, Tateyama and Tsushima, were significantly different from those in other temperate areas. This suggests that corals may adapt to cold-water environments through different Symbiodiniaceae compositions. This study presents the first field-based evidence that corals dramatically change their symbiotic partners at the forefront of the range expansion due to climate change. Our findings highlight that special adaptive strategies involving sudden changes in symbiotic partners are crucial for corals at the range margins. In addition, the result challenges the prevailing theory that the limit of northward migration is solely determined by the cold tolerance of host corals, but the availability of suitable symbiotic algal species that can adapt to low sea temperatures may also determine the shift in coral distribution. Consequently, caution is warranted in predicting future coral distribution and resilience under climate change.