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The role of natural products in keeping the surfaces of the sea stars Linckia laevigata, Fromia indica, Cryptasterina pentagona and Archaster typicus free of fouling organisms was investigated. Conditioned seawater of these sea stars did not have any effects on the settlement of the ecologically relevant diatoms Amphora sp. and Nitzschia closterium and the bryozoan Bugula neritina. However, dichloromethane, methanol and aqueous extracts of whole sea stars at 100, 10, 1 and 0.1 µg cm–2 had concentration-dependent effects on the settlement of these fouling species and the polychaete Hydroides elegans. Based on bioassay-guided fractionation and analysis with proton nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry (GC-MS), the most bioactive fractions contained several fatty acids and sterols. To determine whether the compounds responsible for the observed antifouling effects were present on the surface of all 4 sea star species, surface-associated compounds were tested against Amphora sp., N. closterium, B. neritina and H. elegans. These compounds reduced the settlement of at least 2 of the 4 fouling species for each sea star species. Using surface extractions and GC-MS analysis, the most abundant surface-associated fatty acids and sterols of each sea star species were identified and quantified. Hexadecanoic acid, cholesterol, lathosterol and sitosterol were tested at surface-associated concentrations, ranging from 1000 to 1 ng cm–2, in settlement assays. Whereas the settlement of B. neritina was not affected by any of the compounds, hexadecanoic acid, cholesterol and lathosterol significantly reduced the settlement of N. closterium at surface-associated concentrations

Chemical antifouling defences of sea stars: effects of the natural products hexadecanoic acid, cholesterol, lathosterol and sitosterol