We found that the sea star A. planci was attracted to AA and LA isolated from viscera of the sea urchin T. pileolus. Since AA and many prostanoids ŽWeinheimer and Spraggins, 1969., metabolites of AA, have been isolated from corals ŽLight, 1973., it can be expected that AA in corals functions as one of the chemoattractants for A. planci. Moore and Huxley Ž1976. reported that betaine exhibited feeding–inducing activity based on random testing of several kinds of the known compounds such as amino acids. Our studies confirmed the presence of AA and betaine in corals. Ormond et al. Ž1973. reported that corals fed on by A. planci were more attractive than intact corals. Although the release of betaine might be crucial for the attractive activity for the sea star, we consider that AA plays an important role in the chemoattraction of the sea star
A. planci. Aggregative predation by A. planci is very often observed on reef flats. Our research
indicates that damaged corals may release AA and attract more A. planci. On the other hand, it is known that phospholipase A2 ŽPLA2 . contained in the body of A. planci ŽShiomi et al., 1985. produces AA from glycerophopholipid. Therefore, AA released by PLA activity in A. planci may, in turn, attract further A. planci and so on ŽFig. 6.. The 2 pheromonal function of these unsaturated fatty acids in seawater is quite interesting. Identification of the receptors on the sea star for AA and LA will require further study. In the present paper, we found the feeding attractants for the sea star A. planci and confirmed the efficiency of the attractants in the aquarium experiments. Although further studies were extensively needed for practical use of our attractants in the coral reefs, we were able to indicate a possibility of new methods to protect corals against A. planci.