Chemical ecology and the search for marine antifoulants

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Abstract
The gorgonian octocoralLeptogorgia virgulata (Phylum Coelenterata, Class Anthozoa) is rarely overgrown by fouling organisms and is avoided by most predators. Laboratory experiments suggest that secondary metabolites and calcium carbonate spicules interact synergistically to provideL. virgulata with an effective defense against predatory fish. In spite of these defenses,L. virgulata is consumed by the symbiotic, trophically specialized gastropodNeosimnia uniplicata, which closely mimics the appearance of the gorgonian.Neosimnia uniplicata is readily eaten by fish and appears not to acquire an effective antipredator defense from its gorgonian host. Extracts ofLeptogorgia virgulata andNeosimnia uniplicata strongly inhibited the settlement of the barnacleBalanus atnphitrite. Bioassay-directed purification of the more potent antifouling agents fromL. virgulata led to the isolation of two previously described, diterpenoid hydrocarbons, known as pukalide and epoxypukalide. A third inhibitor of barnacle settlement, whose structure is presently unknown, was obtained from bothL. virgulata andN. uniplicata. When assayed for ability to inhibit barnacle settlement, these three compounds possessed EC50 values ranging from 19 to 55 ng/ml. These secondary metabolites may prevent the overgrowth ofL. virgulata by fouling organisms in nature. The allelochemicals ofL. virgulata, N. uniplicata, and other marine organisms may provide nonpolluting alternatives to existing, commercial antifoulants based on derivatives of tri-n-butyltin.