Feeding-deterrent effect of phlorotannins from the brown alga Ecklonia stolonifera against the abalone Haliotis discus hannai

For the purpose of clarifying the reason why the laminariaceous brown alga Ecklonia stolonifera are not grazed by marine herbivorous animals, such as abalones and sea urchins, feeding-deterrent effect of the methanol extract from this alga was tested on the abalone Haliotis discus hannai by the bioassay with cellulose plate method. Concurrently, another two laminariaceous algae Eisenia bicyclis and Ecklonipopsis radicosa were examined. A large amount of the water-soluble fraction was obtained from the methanol extract of Eck. stolonifera showed potent feeding-deterrent activity when compared with those from the other two species. The active compounds that were isolated from the water-soluble fraction by a combination of column (Sephadex LH-20) and thin-layer (silica gel) chromatography, were characterized as phloroglucinol oligomers and polymers, phlorotannins, on the basis of spectroscopic analysis. The feeding-deterrent activity of phlorotannins depends upon the degree of polymerization of phloroglucinol (the polarity of the polyphenol). We concluded that the algal phlorotannins play an important role in the chemical defense against marine herbivores.