Seaweed susceptibility to herbivory: chemical and morphological correlates

The susceptibility of 82 species of tropical seaweeds to grazing by herbivorous fishes was assessed on 8 different coral reefs in the Florida Keys, USA. Most species were simultaneously assayed for the presence or absence of unusual secondary metabolites and recorded as having either calcified or uncalcified thalli. Both production of secondary metabolites and of a calcified thallus were associated with low susceptibility to herbivory. However, the relative importance of calcification versus chemical deterrents cannot be assessed for the calcified species since almost all these also contained secondary metabolites. Eighty-five % of calcified species, but only 39 % of uncalcified species, produced secondary metabolites. Secondary metabolites were produced by 71 % of the species least susceptible to herbivory (75 % eaten). Calcified thalli were produced by 50 % of the lowest preference species but by only 9 % of the highest preference species. Thus, several common reef seaweeds appear to resist herbivory by relying primarily on chemical deterrents (genera Dictyota, Dilophus, Stypopodium, Lobophora, Avrainvillea, and some Caulerpa species) but many appear to combine both chemical and morphological defenses (genera Penicillus, Halimeda, Rhipocephalus, Udotea, Amphiroa, and Galaxaura). We suggest that multiple defenses will be common among seaweeds on coral reefs since herbivore diversity is high and it is unlikely that any single defense will be effective against the many types of herbivores that encounter these plants. This may account, in part, for the diversity of secondary metabolites produced by some tropical seaweeds.