2018 Volume 11 Issue 1+2 Pages 11-23
The zooxanthellate scleractinian coral Alveopora japonica is endemic to the northwestern Pacific, ranging from Taiwan to Korea and Japan. This coral has recently expanded into seaweed beds in the temperate waters of these regions, as the seawater temperature increased. The largest known bed of A. japonica (1940 m2) was found in 2012 at Yashiro Island, in the Seto Inland Sea in the temperate region of Japan, where the minimum seawater temperature is the lowest among the regions inhabited by this coral. We surveyed the coverage of seaweeds and A. japonica in this coral bed during the seaweed-abundant and seaweed-sparse seasons in 2014. Throughout the year, the coverage of Ecklonia kurome and sargassaceous plants was low in places where the coverage of A. japonica was high. The maximum seawater temperature was within the optimal range for the growth of these seaweeds, indicating that the increasing temperature did not adversely affect seaweed growth. Evidently, a local environmental stress rather than the increase in seawater temperature caused the decline of the canopy-forming seaweeds that, in turn, allowed the coral bed to expand. Our research provides baseline data to compare with and analyze future changes at the edge of A. japonica distribution.