Chemical defense of the Caribbean sponge Agelas clathrodes (Schmidt)

Marine sponges have been a rich source of natural products, but only in recent years have studies been undertaken to evaluate the ecological functions of these compounds. Previously, we found that crude organic extracts of the tissues of 6 species of the genus Agelas from the Caribbean were all strongly unpalatable to a predatory reef fish in aquarium assays. In this study, we used bioassay-directed isolation techniques to identify the deterrent metabolites in one of these species, Agelas clathrodes (Schmidt). Crude organic extracts of A. clathrodes at natural concentrations deterred feeding of the reef fish Thalassoma bifasciatum (Bloch) in aquarium assays, and of a natural suite of reef fishes in field assays. Separation of the crude extract by column chromatography yielded a series of fractions, of which only the polar fractions were deterrent in both aquarium and field assays. Two previously described compounds, oroidin (1) and 4,5-dibromopyrrol-2-carboxylic acid (2), were identified by high resolution mass spectrometry and NMR spectrometry as the deterrent metabolites in the active fractions. Purified samples of both compounds deterred feeding in aquarium assays, both separately and in combination, at concentrations found in the sponge tissue. In addition, both compounds were identified by analytical thin-layer chromatography as constituents of the crude extracts of A. conifera (Schmidt), A. dispar Duchassaing and Michelotti, A. inaequalis Pulitzer-Finali, A. sceptrum (Lamarck), and A. wiedenmeyeri Alcolado. These results suggest that sponges of the genus Agelas share a common chemical defense against fish predators.