ALBERT

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.