Coral reefs at 34°N, Japan: Exploring the end of environmental gradients

We describe the architecture and development of the highest latitude coral reefs currently known on Earth, located in Japan at 34°N. The reefs are distributed within turbid inner bays and undergo winter sea-surface temperatures (SSTs) that fall to 13 °C, well below the generally accepted lower limit (18 °C in winter) of tropical coral reef formation. Despite low SSTs and high turbidity, coring indicates reefs ranging to 555 cm in thickness since ca. 4.3 k.y. ago. The reefs exhibit high adaptability to this extremely marginal environment. Variability of the Tsushima Warm Current and the Asian monsoon could have affected the evolution of these reefs. The corals are dominantly faviids, in contrast to Acropora and Porites that tend to dominate low-latitude, tropical-subtropical reefs. Defining the end of the distributional range, the reefs serve as baselines for understanding coral reef distribution along SST and turbidity gradients.