Elucidation of "Isoyake" phenomenon: Study on allelochemicals of coralline red algae

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As one of the causative agents of "Isoyake" phenomenon, we hypothesized that some allelochemicals produced by coralline red algae play an important role in maintaining Isoyake. In order to prove this, we investigated coralline algal metabolites which suppress growth of Laminaria algae, attract sea urchin larvae, induce larval settlement and metamorphosis in sea urchin, and stimulate feeding of sea urchins.Our results show that numerous sea urchins of Strongylocentrotus nudus continually inhabit coralline flats in Isoyake areas because coralline red alga, Lithophyllum yessoense, produces allelochemicals such as glycoglycerolipids that attract planktonic larvae of sea urchins, induce their settlement and metamorphosis, and stimulate feeding in juvenile and adult sea urchins. Therefore, sea urchins aggregate in large numbers at calm coralline flats and Isoyake is apparently maintained by destructive grazing of juvenile macralgae by them. On the other hand, at low seawater temperature herbivores, such as sea urchins, tend to reduce the grazing pressure toward macroalgae, including the kelp community. At low seawater temperature, temperate coralline red algae may slow down their growth and secondary metabolism. The reduced activity of secondary metabolism may lead to low production of allelochemicals that suppress the development and growth of Laminaria sporelings. Hence, Laminaria zoospores settling on coralline red algae may develop normally, leading to the recovery of kelp beds in coralline flats of Isoyake areas in the southwest coast of Hokkaido.