Chemical defense in the seaweed Ochtodes secundiramea (Montagne) Howe (Rhodophyta): effects of its rnonoter- penoid components upon diverse coral-reef herbivores

Many species of tropical red algae are known to produce halogenated secondary metabolites that have been hypothesized to function in chemical defense. The tropical fleshy red alga Ochtodes secundiramea (Montagne) Howe is a typical example known to produce the halogenated monoterpenoids ochtodene and chondrocole A. In this study, the susceptibility of 0. secundiramea to grazing by herbivorous fishes was examined by offering an array of 11 red, green, and brown seaweeds to herbivores in field assays in the Grenadine Islands of the Caribbean. 0. secundiramea was among the least susceptible to grazers, with no transplants being lost to herbivory during our assay. To examine the possible rBles of the monoterpenoids in reducing herbivory, the metabolites ochtodene and chondrocole C were isolated and purified by high-performance liquid chromatography (HPLC). These monoterpenoids were tested in field assays for their feeding deterrent effects toward herbivorous fishes in the Grenadine Islands and on Guam in the tropical western Pacific. In both sets of experiments, ochtodene was an effective feeding deterrent and chondrocole C was not. These compounds, and a nonpolar mixture (more than three compounds) of halogenated monoterpenes from Ochtodes, were also tested for their feeding deterrent effects toward herbivorous amphipods collected in the Grenadine Islands. Only the unresolved monoterpene mixture was an effective feeding deterrent toward the amphipods. Herbivory on coral reefs is intense and the diversity of herbivore types is great. It appears that the variety of secondary metabolites synthesized by seaweeds such as 0. secundiramea may be effective in deterring different reef herbivores.