S-3-3

CHEMICAL DEFENSES IN CORAL REEF SEAWEEDS

Valerie J. Paul
University of Guam Marine Lab, UOG Station, Mangilao, Guam 96923

Seaweeds in many coral reef habitats experience intense grazing pressureby generalist herbivores; therefore, it is not surprising that tropicalseaweeds produce a diverse array of chemical defenses and also combinechemical and structural defenses to reduce herbivory. Calcified green seaweedssuch as Neomeris annulata and Halimeda spp. have higher concentrationsof secondary metabolites in the tips and higher concentrations of CaCO3in the bases of the thalli. Parrotfishes are not deterred by highlevels of calcium carbonate in their foods, but avoid secondary metabolitesof N. annulata and Halimeda spp. Rabbitfishes and some surgeonfishesare strongly deterred by CaCO3 in their diets. Noncalcifiedseaweeds can also occupy a variety of reef habitats, and the types andoncentrations of compounds they produce can vary among collection sites. This chemical variation may influence the distribution of these seaweedsin different reef habitats. Chemical variation in seaweeds can bevery pronounced for the filamentous cyanobacteria, which can form largemats in many coral reef habitats. Most generalist reef herbivoresavoid these chemically rich seaweeds. However, one specialist herbivore,the sea hare Stylocheilus longicauda, feeds on Lyngbya majuscula,a common cyanobacterium in reef habitats. The concentration and typesof secondary metabolites vary among collections of L. majuscula,and this variation can influence feeding by both generalist and specialistherbivores.