Variation in multiple traits of vegetative and reproductive seagrass tissues influences plant-herbivore interactions.

12 páginas, 3 figuras, 1 tabla.Plant–herbivore interactions have strong ecological and evolutionary consequences, but have been traditionally overlooked in marine higher plants. Despite recent advances in seagrass ecology that highlight the importance of herbivory, the mechanisms that regulate the feeding behaviour of seagrass consumers remain largely unknown. Herbivores have been shown to reduce the sexual reproductive success of seagrasses through direct consumption of inXorescences and seeds, but we know little about intraspeciWc variation in susceptibility to grazing of diVerent seagrass tissues. We contrasted the relative palatability of reproductive and vegetative tissues of the temperate seagrass Posidonia oceanica in the Weld, and we assessed the feeding preferences among these tissues of the main consumers of the plant, the Wsh Sarpa salpa and the urchin Paracentrotus lividus. Moreover, we identiWed the plant traits that explained the observed feeding behaviour. We provide strong evidence for herbivore selectivity among seagrass tissues. In the Weld, 70–90% of inXorescences were damaged by herbivores compared to 3– 60% of leaves of similar age. In feeding assays, the urchin P. lividus showed over a twofold preference for reproductive tissue at various stages of development. By contrast, we detected no feeding activity on either leaves or inXorescences from the Wsh S. salpa, which is known to migrate to deeper waters soon after Xowering starts and during the period of fruit maturation. Despite being the preferred food of urchins, inXorescences were chemically defended, had higher levels of phenolics and lower nutrient and caloriWc content than leaves. We experimentally demonstrated that leaf structural defences are the primary factor in determining urchin feeding preferences. Removal of plant structure results in a drastic shift in urchin selectivity towards the most nutritious and less chemically defended leaf tissue, indicating that multiple mechanisms of defence to herbivory may coexist in seagrasses.This work was supported by grant REN-2002-04020-C02 from the Spanish Ministry of Science and Education.Peer reviewe