Variation in chemical defenses against herbivory in southwestern Atlantic Stypopodium zonale (Phaeophyta)

Abstract

Many seaweeds produce secondary metabolites that deter herbivory, but intraspecific variation in both the identity and the concentration of chemical defenses has been documented rarely. In order to evaluate the occurrence of chemical variation in the brown seaweed Stypopodium zonale along the Brazilian coast, we investigated the defensive properties of crude extracts and major secondary metabolites found in specimens from two distant locations (Forno inlet, Búzios, southeastern Brazil and Fernando de Noronha archipelago, northeastern Brazil) against herbivory by the crab Pachygrapsus transversus and the sea urchin Lytechinus variegatus. At natural concentrations, the extracts from S. zonale from Búzios and Fernando de Noronha significantly deterred feeding by P. transversus and L. variegatus, although the former was more effective as a defense than the latter. Corroborating these results, the major metabolites atomaric acid, found in individuals from Búzios, and stypoldione, from Fernando de Noronha specimens, also inhibited herbivory, but atomaric acid was more effective as a defense against L. variegatus and P. transversus than stypoldione. The variation between S. zonale from two distant and different geographic locations in Brazil suggests that defensivechemicals from this seaweed are not qualitatively or quantitatively absolute or invariant characteristics of the species, and may represent an ecological specialization to successfully prevent herbivory.