Antipredatory secosterols from the octocoralPseudopterogorgia americana

The chemical defense of the Caribbean octocoralPseudopterogorgia americanawas investigated using feeding assays performedin aquariaandin situwith a natural assemblage of predatory fishes. Using a bioassay-guided scheme, crude extracts, fractions and pure compounds were incorporated into palatable foods and tested. Only the secosterol fraction deterred fish feeding. Two secosterols were identified from the bioactive fraction by spectroscopic methods and comparison with literature data: 9,11-secogorgosterol (Compound 1) and 9,11-secodinosterol (Compound 2), both of which have been previously described fromPseudopterogorgiaspp. Our results suggest that symbiont-produced gorgosterol and dinosterol are oxidized by the coral host to C-ring secosterols, which then provide a chemical defense to both coral and alga.