DEFENSIVE PROPERTIES OF SECONDARY METABOLITES FROM THE CARIBBEAN GORGONIAN CORAL ERYTHROPODIUM-CARIBAEORUM

Field experiments support previous laboratory assays indicating that crude lipid-soluble extracts of the Caribbean encrusting gorgonian octocoral Erythropodium caribaeorum (Duchassaing & Michelotti) deter predation by reef fishes. An extract of E. caribaeorum incorporated into carrageenan food strips at the same volumetric concentration as it occurred in gorgonian tissues deterred feeding of a natural assemblage of fishes on the same reef from which the gorgonian had been collected. Field assays using carrageenan strips containing naturally occurring concentrations of several purified secondary metabolites isolated from the crude extract revealed that the feeding deterrent effects were present in a fraction containing chlorinated diterpenoid (C20) compounds (erythrolides), but not in a fraction containing sesquiterpene (C-15) hydrocarbons (of which erythrodiene, a previously unreported marine natural product, was a major component). Three compounds were purified from the erythrolide fraction: erythrolide A, erythrolide B and erythrolide D. Only the last 2 of these compounds, when assayed independently, inhibited feeding of reef fishes at the natural concentrations found in E. caribaeorum. While the erythrolides appear to defend E. caribaeorum from reef predators, the function of sesquiterpene hydrocarbons, which occur in high concentrations in the lipid-soluble extracts of many octocorals, remains unclear.