Chemical defenses of the tropical green seaweed Neomeris annulata Dickie: effects of multiple compounds on feeding by herbivores

Abstract

The calcareous green alga Neomeris annulata Dickie is widely distributed in shallow inshore waters of many reef habitats in the Caribbean and Pacific. We examined the effects of three brominated sesquiterpenes found in Neomeris annulata on Guam against three reef herbivores that are not strongly deterred by CaCO 3 in their diets: the parrotfishes Scarus sordidus Forsskål, S. schlegeli (Bleeker) and the sea urchin Diadema savignyi Michelin. The effects of each metabolite were examined separately to compare their effectiveness toward the different herbivores. The major brominated sesquiterpene (compound 1; ca. 1% dry mass in thallus tips) deterred feeding by all three herbivores at or below natural concentrations, as did the other two minor compounds. The terpene most effective at deterring feeding by all three herbivores was a closely related minor compound (compound 2: ca. 0.5% dry mass in thallus tips). We then tested for possible interactions among the three metabolites and synergistic effects by seeing whether natural combinations of the three metabolites enhanced their effectiveness as chemical defenses. The natural mixture of metabolites was never more effective than either compound 1 or compound 2 alone, suggesting no enhanced or synergistic effect of multiple metabolites. In fact, at higher concentrations approaching natural concentrations found in the thallus tips, compound 2 was more effective at deterring feeding by S. schlegeli by itself than in the naturally occurring mixture of terpenes.