ALBERT

Description: Grazing-induced plant defences that reduce palatability to herbivores are widespread in terrestrial plants and seaweeds, but they have not yet been reported in seagrasses. We investigated the ability of two seagrass species to induce defences in response to direct grazing by three associated mesograzers. Specifically, we conducted feeding-assayed induction experiments to examine how mesograzer-specific grazing impact affects seagrass induction of defences within the context of the optimal defence theory. We found that the amphipod Gammarus insensibilis and the isopod Idotea chelipes exerted a low-intensity grazing on older blades of the seagrass Cymodocea nodosa, which reflects a weak grazing impact that may explain the lack of inducible defences. The isopod Synischia hectica exerted the strongest grazing impact on C. nodosa via high-intensity feeding on young blades with a higher fitness value. This isopod grazing induced defences in C. nodosa as indicated by a consistently lower consumption of blades previously grazed for 5, 12 and 16 days. The lower consumption was maintained when offered tissues with no plant structure (agarreconstituted food), but showing a reduced size of the previous grazing effect. This indicates that structural traits act in combination with chemical traits to reduce seagrass palatability to the isopod. Increase in total phenolics but not in C:N ratio and total nitrogen of grazed C. nodosa suggests chemical defences rather than a modified nutritional quality as primarily induced chemical traits. We detected no induction of defences in Zostera noltei, which showed the ability to replace moderate losses of young biomass to mesograzers via compensatory growth. Our study provides the first experimental evidence of induction of defences against meso-herbivory that reduce further consumption in seagrasses. It also emphasizes the relevance of grazer identity in determining the level of grazing impact triggering resistance and compensatory responses of different seagrass species.

Description: Plants optimise their resistance to herbivores by regulating deterrent responses on demand. Induction of anti-herbivory defences can occur directly in a grazed plant or from emission of cues to the environment, which modifies the interaction of adjacent plants with their consumers. This study confirmed the induction of anti-herbivory responses by water-borne cues between adjoining con-specific seaweeds and firstly examined whether such responses also exist in seaweeds among adjacently positioned hetero-specifics (= eavesdropping). Furthermore, differential abilities and geographic variation in eavesdropping by a non-indigenous seaweed as well as native seaweeds were assessed. Twelve-day induction experiments using the nonindigenous brown seaweed Sargassum muticum were conducted in the laboratory in Portugal and Germany with one local con-familiar (Portugal: Cystoseira humilis, Germany: Halidrys siliquosa) and one local hetero-familiar native species (Portugal: Fucus spiralis, Germany: F. vesiculosus). All seaweeds were grazed by a local isopod species (Portugal: Stenosoma nadejda, Germany: Idotea baltica) and were positioned upstream of con- and hetero-specific seaweeds. Grazing-induced modification in seaweed traits were tested in three-day two-choice feeding assays between cueexposed and cue-free (= control) pieces of both fresh and reconstituted seaweed pieces. Both Fucus species reduced their palatability when positioned downstream of isopod-grazed con-specifics. In contrast, the palatability of non-indigenous S. muticum remained constant in the presence of upstream grazed con-specifics and native hetero-specifics. Yet, both con-familiar, but none of the hetero-familiar native species reduced palatability when located downstream of grazed S. muticum. These patterns of grazer-deterrent responses mediated by water-borne cues were observed on both European shores and were identical between assays using fresh and reconstituted seaweeds. Thus similar to terrestrial plants, seaweeds may eavesdrop to optimise chemical resistance to consumers, though this ability appeared species-specific. Furthermore, this study suggests that native species may asymmetrically benefit from the arrival of a non-indigenous species as only natives were eavesdropping.