In the rocky sublittoral zones of Japanese waters, macroalgal communities are seen to flourish where it provides a luxuriant environment for the marine inhabitants. However, in recent years, these macroalgal communities are declining and being replaced by crustose coralline red algae. This phenomenon is called "Isoyake" and has been seen to spread particularly along the southwest coast of Hokkaido. In "Isoyake" areas dense population of sea urchins and coralline flats similar to the sea desert are observed. Rise in seawater temperature, excessive grazing by sea urchins, sloughing of epidermal tissue of crustose algae and decrease in organic and inorganic nutrients have been suggested as the primary factors involved in causing and maintaining this "Isoyake" phenomenon. In addition to these factors, we supposed that secondary metabolites of coralline red algae also play important roles in causing and maintaining Isoyake phenomenon. Therefore, our objectives are to clarify following two problems on the basis of allelochemicals. 1) Why do Laminaria communities recruit in "Isoyake areas" in the year when seawater temperatures are low? 2) Why do sea urchins gather numerously in "Isoyake areas"? In order to prove these questions, we investigated suppression of Laminaria's growth, attraction of sea urchin larvae, inducement of larval settlement and metamorphosis of sea urchins, and the feeding stimulant of sea urchins. As bialgal cultures of an articulated coralline red alga Corallina pilulifera and Laminaria sporelings were performed under various temperature, it became obvious that both the Laminaria spp.'s gametophytes maturation and normal sporophyte formation were suppressed at higher temperatures. Present findings also indicate that crustose coralline red alga Lithophyllum yessoense and C. pilulifera not only induced larval settlement and metamorphosis of the sea urchins, Strongylocentrotus intermedius and S. nudus, but also attracted swimming larvae of these sea urchins. The benthic diatoms isolated from the surfaces of L. yessoense also displayed similar activities. Bioassay-guided separation of C. pilulifera extract afforded several glyceroglycolipids which induce larval settlement and metamorphosis of sea urchins. In addition, these glycolipids attracted the swimming larvae and some of them were shown to be feeding stimulants for adult sea urchins. Similar glycolipids were also found to be present in L. yessoense. Consequently, it is suggested that glycolipids play important roles in maintaining Isoyake phenomenon.
