Ecological studies on sea urchins-marine forests interactions in Kuroshio waters

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To clarify the effect of grazing of the sea urchins Hemicentrotous pulcherrimus and Anthocidaris crassispina on growth of the brown large alga Eisenia bicyclis forest and seasonal sea urchins - marine forest interactions, study was undertaken in a permanent site at Hinomisaki, Wakayama Prefecture, western Japan. Densities of both the species of sea urchins markedly decreased from May to July due to mass mortality cased by a large amount of freshwater inflow. Mass mortality enabled to colonize juvenile E.bicyclis and the adult beds enlarged to upper subtidals at the depth of 1 m regardless of being under warmer oceanographic condition. While, in articulated corallines beds at the depths of>1 m, juvenile E.bicyclis disappeared. Maturation size of H.pulcherrimus and A.crassispina was>18 mm diameter. Adult H.pulcherrimus in articulated corallines migrated to the kelp bed from autumn to winter when gonads were in maturation to spawningTo clarify the differences in the growth and gonad size of H.pulcherrimus and A.crassispina among algal sere, the study was conducted in July at a fucoid bed and a kelp bed in Gobo and two algal turfs in Mihama, Wakayama Prefecture. The growth of H.pulcherrimus in the fucoid bed and the kelp bed, where the large perennial Myagropsis myagroides and Ecklonia cava at the climax stage dominated, respectively, are higher than that in the algal turfs dominated by the small perennial articulated corallines at the seral stage. While, the most rapid growth of A.crassispina was observed in the fucoid bed. The gonad indices (gonad wet weight x 100/body wet weight) of both species of the sea urchins were highest in the fucoid bed. These results suggest that growth and gonad production of H.pulcherrimus and A.crassispina are affected by algal sere as reported in H.pulcherrimus and Strongylocentrotus nudus in northern Japan, although they differed among dominant species of algae at the climax stage.