Revisiting the relationship between sea surface temperature and coral species richness in Japan: nestedness and regional variation beyond temperature

In Japan, coral species richness has long been known to decline with decreasing winter sea surface temperature (SST). However, recent taxonomic revisions, together with the accumulation of occurrence records from temperate regions, have enabled re-evaluation of this relationship using an updated nationwide database. Although high-latitude coral communities in Japan have long been suggested to form subsets of lower-latitude communities, quantitative tests of this nested structure along a detailed thermal gradient remain limited. Here, we analyzed reef-building coral occurrence records from 1059 sites across Japan collected between 1929 and 2020, examining relationships between winter SST, species richness, and community structure across 30 regions, as well as regional factors associated with departures from SST-based expectations. A total of 482 coral species were recorded. Regional species richness was strongly positively related to winter SST. Occurrence site count was also significantly associated with species richness, but the SST effect remained significant after accounting for differences in site count. The SST–richness relationship was not fully described by a simple linear gradient: Exploratory analyses indicated a pronounced steplike shift around 20 °C. Coral communities also exhibited a clear nested structure along the winter SST gradient, with those in colder SST bands tending to form subsets of those in warmer bands. This nested pattern persisted after accounting for differences in occurrence site count among SST bands. However, winter SST alone did not fully explain regional variation in species richness. Wakayama Prefecture had substantially higher species richness than predicted from winter SST, with 134 species recorded compared with 62.6 species predicted by the SST-only model and 93.0 species predicted after accounting for occurrence site count. Analyses focused on the Kushimoto area suggested that Kuroshio-Current-driven larval transport and local retention, together with coastal geological and geomorphological conditions, may contribute to this unusually high diversity. These findings indicate that winter SST provides the broad-scale framework for coral diversity and community structure in Japan, while regional oceanographic, geological, and geomorphological conditions further modify this pattern. They provide a useful framework for developing conservation strategies that consider future range shifts and connectivity among coral communities under ongoing ocean warming.