CALCIUM-CARBONATE AND SECONDARY METABOLITES IN TROPICAL SEAWEEDS - VARIABLE EFFECTS ON HERBIVOROUS FISHES

On coral reefs where herbivorous fishes are both numerous and diverse, many seaweeds combine mineral and chemical defenses. We hypothesized that combined defenses affect a greater number of herbivore species than either single defense, and in this study we experimentally tested this hypothesis. We first incorporated calcium carbonate (aragonite form) alone at a range of concentrations into an artificial diet composed of freeze-dried samples of the noncalcified, green alga Enteramorpha clathrata, carrageenan, cornstarch, and water. In field experiments where diets were made available to a natural fish community, aragonite had variable effects: in some trials aragonite deterred feeding, in others it had no significant effect, and in one trial it even stimulated feeding. Parrotfishes (Scaridae) and the surgeonfish Ctenochaetus striatus (Acanthuridae) showed a significant preference for food containing aragonite in field observations, while the surgeonfish Naso vlamingii and three damselfishes (Pomacentridae) were observed to feed preferentially on food without added aragonite. In the laboratory, the surgeonfishes Acanthurus nigrofuscus, A. triostegus, A. xanthopterus, and Nasa unicornis and the rabbitfish Siganus spinus (Siganidae) significantly avoided food with added aragonite. However, the parrotfishes Scarus sordidus and S. schlegeli consumed significantly more food containing aragonite than plain food. Ctenochaetus striatus avoided aragonite only at the highest aragonite concentration and showed no preference at lower aragonite concentrations. Artificial diets containing natural concentrations of aragonite (80% dry mass), diterpenoid secondary metabolites from Halimeda macroloba (1% dry mass), and a combination of both defenses (80% aragonite and 1% aragonite-free dry mass of diterpenes) were tested in the laboratory against four fish species. Scarus sordidus and Ctenochaetus striatus avoided secondary metabolites but not aragonite, and Naso lituratus and Siganus spinus were deterred by aragonite but not by secondary metabolites. All fishes were deterred by one defense or the other, and all fishes but Scarus sordidus avoided food containing the combined defenses. Combinations of aragonite and secondary metabolites increased the number of fsh species that were effectively deterred from grazing relative to either single defense.