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Marine Natural Products Influencing Larval Settlement and Metamorphosi of Benthic Invertebrates



Most benthic invertebrates produce planktonic larvae, which swim or float for a while and embark on search for location to settle and metamorphose. Many factors influence this process, among which chemical cues derived from conspecific adults or prey organisms are believed to be important; larvae start to settle in response to chemical cues. Although various compounds have been reported to induce larval settlement and
metamorphosis for many species, those which real chemical cues have been known are quite few. On the other hands, settlement of larvae of other species endangers the survivorship of benthic organisms which in turn possess chemical defense against larval settlement. An array of marine natural products have been obtained as inhibitors of settlement of the barnacle Balanus amphtirite which is one of most popular fouling organisms. Since the Fusetani Biofouling Project started in October 1991, we have been trying to discover from marine organisms compounds which induce larval settlement and metamorphosis of the ascidian Halocynthia roretzi, which resulted in the isolation of more than 40 active metabolites including simple benzenoids, alkaloids, amino acid derivatives, and polyacetylenes. These compounds were found to mimic a real cue, lumichrome. We have also attempted to isolate antibarnacle metabolites from sessile marine organisms, mainly sponges and nudibranchs. More than 60 active substances including terpenoids, steroids, amino acid derivatives, alkaloids, and polyacetylenes have been obtained. Sesqui- and diterpenoids containing isocyano and related functionalities showed potent activity, but their toxicity was low, thus suggesting their promising properties for nontoxic antifoulants.
