Effects of secondary metabolites and CaCO3 on feeding by surgeonfishes and parrotfishes: Within-plant comparisons

Tropical seaweeds and 'plant-like' animals often contain both secondary metabolites and high concentrations of minerals. Typically, secondary metabolites are most concentrated in the tip of the organism and minerals in the base. We used within-individual variation in defenses in the alga Neomeris annulata to test the hypothesis that surgeonfishes are deterred from feeding by calcium carbonate but not by chemical defenses, whereas parrotfishes are deterred from feeding by chemical defenses but not by calcium carbonate. Our results supported this hypothesis. The parrotfish Scarus sordidus preferred to feed on an artificial food containing CaCO3 compared to control food, but preferred control food over food containing a sesquiterpene-rich organic extract of N. annulata. Two species of parrotfishes fed primarily on the metabolite-poor, CaCO3-rich basal regions of N. annulata, whereas 2 species of surgeonfishes fed primarily on the metabolite-rich, CaCO3-poor tips. Variation in grazing location had important consequences for N. annulata. Individuals grazed on the tips grew in length more slowly than individuals grazed around the base. Individuals grazed on the lips were temporarily rich in CaCO3; individuals grazed around the base were temporarily rich in sesquiterpenes. These patterns were probably not a result of induction of defenses, but rather a consequence of selective removal of CaCO3-poor and sesquiterpene-poor material respectively.