A dolabellane diterpene from the brown alga Dictyota pfaffii as chemical defense against herbivores

Laboratory and field experiments assessed the defensive properties of the crude extract and major secondary metabolite found in the Brazilian brown alga Dictyota pfaffii. Natural concentrations of the crude organic extract of D. pfaffii significantly inhibited feeding by the sea urchin Lytechinus variegatus and generalist herbivore fishes in the field. In contrast, the crude extract did not inhibit feeding by the crab Pachygrapsus transversus. Chemical defensive action against the sea urchin and the generalist fishes was due to the diterpenoid 10,18-diacetoxy 8-hydroxy-2,6-dolabelladiene found as the major natural product in D. pfaffii. However, this compound did not inhibit feeding by P. transversus as readily as the crude extract. This is the first report showing that Dictyota species produce dolabellane diterpenes as chemical defense against herbivory. In addition, these results widen the action spectrum of secondary metabolites found in species of this brown algal genus. Since dolabellanes constitute the major skeleton type found in Dictyota, we suggest that these diterpenes may play an important ecological function as defenses to the Dictyota species worldwide.