Experimental evidence of chemical deterrence against multiple herbivores in the seagrass Posidonia oceanica

There is increasing evidence of the importance of herbivory in seagrass communities, but the factors that regulate seagrass–herbivore interactions remain largely unknown. Many terrestrial plants and marine algae chemically deter herbivores using secondary metabolites, yet their incidence and role on seagrasses have received little attention. Posidonia oceanica, the dominant seagrass in the Mediterranean Sea, suffers a high level of herbivory and produces phenolics, a type of compounds known to deter feeding in algae and terrestrial plants. We have experimentally quantified the chemical deterrence of P. oceanica against 3 types of sympatric consumers by comparing feeding on palatable agar-based foods containing aqueous and organic extracts of P. oceanica at natural concentrations with that on appropriate controls. The effect of plant chemistry was measured on the feeding of the sea urchins Spharechinus granularis, Paracentrotus lividus and Arbacia lixula, the mesograzer gastropod Cerithium vulgatum, and the general fish assemblage inhabiting P. oceanica meadows. Since these 5 experiments were run independently, we used meta-analysis to test the overall hypothesis that P. oceanica is chemically defended against multiple consumers and to compare the magnitude of their inhibition. Our results clearly showed that the seagrass extracts significantly deterred feeding, although inhibition varied between consumers. S. granularis was the most deterred organism, while the mesograzer C. vulgatum was the only species unaffected by P. oceanica extracts. These results represent the first experimental evidence for seagrass chemical deterrence against a range of consumers, and emphasise the potential importance of chemically mediated interactions between seagrass and herbivores.