Seaweed community structures in the temperate zooxanthellate scleractinian coral Alveopora japonica bed in the western Seto Inland Sea, Japan

有藻性イシサンゴ類のニホンアワサンゴは，台湾から韓国，日本に分布し，近年，海藻藻場での群生が確認されているが，生育地の海藻群落構造はほとんど研究されていない。山口県屋代島は，このサンゴの分布域では最も水温が低いにもかかわらず，2012年に大規模な群生地（1940m2）が確認され，2013年に瀬戸内海国立公園の海域公園に指定された。2014年に，海藻の繁茂期と少ない時期に海藻群落構造の調査を行った。年間を通して，サンゴ被度の高い場所では大型褐藻類の被度が低かったが，群生地の水温は，これらの種の生育適温の範囲内であった。水温上昇以外の局所的な要因により，大型褐藻被度が低下し，群生地が拡大したと考えられる。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.This research was mainly supported by the Supporting Grant 2014 from the Fujiwara Natural History Foundation, Japan to AK