1Marine Biological Research Institute of Japan, Co. Ltd., Suita, Osaka 564-0051, 2Kansai International Airport, Co. Ltd., Izumisano, Osaka 549-0001, 3Kansai Airport Research Institute, Chuo, Osaka 540-0012, 4Graduate School of Agriculture, Kyoto University, Sakyou, Kyoto 606-8502, Japan
Seaweed biomass, and the distribution and feeding activities of the sea urchins Hemicentrotus pulcherrimus and Anthocidaris crassispina were examined on a seawall composed of stones in Osaka Bay for three years. Perennial small algae, Gigartinaceae (Chondrus spp. and Chondracanthus spp.) was observed at a depth of 2 m throughout the year and annual species of algae, such as Sargassum spp. and Undaria pinnatifida, grew thickly during winter to early summer. Total biomass of these seaweeds was 500 to 600 g-dry weight/m2 from March to July. The biomass decreased during summer and fall, but was maintained at above 200 g-dry weight/m2. The densities of sea urchins were high: H. pulcherrimus and A. crassispina were 20-40 ind./m2 and 4-7 ind./m2, respectively. Gut content indices (percentage of gut content weight in body weight) were 1-8%, 3-7%, respectively, and the urchins fed sufficiently. In the gut contents of the urchins, high frequencies of Gigartinaceae and Sargassum spp. were found with ratios similar to those of the seaweed community. The biomass of the seaweed community was maintained at a stable level despite high grazing pressure by the sea urchins.