Effect of stony corals extracts on feeding by Acanthaster planci

The preference of the starfish Acanthasterplanci (Linnaeus, 1758) for 3 species of corals which survive A. planci outbreaks (Diploastrea heliopora, Porites rus, and Coscinaraea colurnna), and the effects of organic and aqueous extracts of these corals on A. planci feeding, were investigated experimentally in the laboratory. We hypothesized that extracts of these corals deter feeding by A. planci, but instead we found no evidence that chemical defenses were produced by these stony corals against A. planci. In whole-coral experiments, D. heliopora and P. rus were consumed significantly less than Acropora aspera, a preferred coral species. Organic extracts from D. heliopora stimulated feeding at low concentrations, but not at higher natural concentrations compared with solvent controls. Organic extracts of P. rus and C. colurnna did not deter or stimulate A. planci feeding at low and high natural concentrations compared with controls. Aqueous extracts from all 3 nonpreferred corals tended to stimulate feeding at high concentrations relative to controls, but only aqueous extracts of P. rus at high concentrations significantly enhanced A. planci feeding. When aqueous extracts of A. aspera were compared with the aqueous extracts of the 3 non-preferred corals, A. aspera extracts were significantly preferred over D. heliopora and C. colurnna, but not over P. rus. Overall, aqueous extracts of all coral species tended to stimulate feeding by A. planci and most organic extracts had no effect on feeding. None of the extracts deterred A. plancifeeding relative to controls as would be expected for extracts of chemically defended species. Some other defense appears to keep non-preferred corals from being consumed by A. planci, and we discuss the possibility that chemical crypsis may be one mechanism.