Seaweed susceptibility to herbivory: chemical and morphological correlates

Abstract

The susceptibility of 82 speaes of tropical seaweeds to grazing byherbivorous fishes was assessed on 8 different coral reefs in theFlorida Keys, USA. Most species were simultaneously assayed for thepresence or absence of unusual secondary metabolites and recordedas having either calcified01 uncalcified thalli. Both productionof secondary metabolites and of a calcified thallus were associatedwith low susceptibility to herbivory. However, the relative importanceof calcification versus chemical deterrents cannot be assessed forthe calcified species since almost all these also contained secondarymetabolites. Eighty-five YO of calcified speaes, but only 39 O/Oof uncalcified species, produced secondary metabolites. Secondarymetabolites were produced by 71 Yo of the species least susceptibleto herbivory (<25 percent eaten) but by only 20 percent of the speaes most susceptibleto herbivory (>?5 Yo eaten). Calcified thalli were produced by 50YO of the lowest preference species but by only 9 Yo of the highestpreference speaes. Thus. several common reef seaweeds appear to resistherbivory by relying primarily on chemical deterrents (genera Mctyota,Dilophus, Stypopodium, Lobophora, Avrainvillea, and some Caderpespecies) but m y ap pear to combine both chemical and morphologicaldefenses (genem PenicUus, Halimede, Rhipocepbalus, Udotea, Amphima,and Galaxaura). We suggest that common among seaweeds on coral reefssince herbivore diversity is high and it is. unlikely that any singledefense will be effective against the many types of herbivores thatencounter these plants. This may account, in part, for the diversityof secondary metabolites produced by some tropical seaweeds.