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

作为“Isoyake”现象的致病因子之一，我们推测珊瑚红藻产生的一些化感物质在维持Isoyake中起着重要作用。为了证明这一点，我们研究了抑制海带藻类生长、吸引海胆幼虫、诱导海胆幼虫定居和变态以及刺激海胆摄食的珊瑚藻代谢物。我们的结果表明，大量的裸球海胆持续栖息在Isoyake地区的珊瑚坪上，因为珊瑚红球藻，Lithophyllum yessoense，产生化感物质，如糖甘油脂，吸引海胆的寄生幼虫，诱导它们的定居和变态，并刺激幼海胆和成年海胆的进食。因此，海胆聚集在大量平静的珊瑚滩和Isoyake显然是保持破坏性的青少年macralgae放牧他们。另一方面，在低海水温度下，食草动物，如海胆，往往会减少对大型藻类，包括海带群落的放牧压力。温性珊瑚红藻在低水温时，其生长和次生代谢会减慢。次生代谢活性的降低可能导致抑制海带孢子发育和生长的化感物质的产生减少。因此，海带游动孢子在珊瑚红藻上可以正常发育，导致北海道西南海岸Isoyake地区珊瑚坪的海带床恢复。

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.