Geographic variation in seaweed induced responses to herbivory

Abstract

Recent work on European (Swedish and English) populations of Ascophyllum nodosum has detected inducible defenses in response to snail grazing that include a decrease in algal palatability and an increase in phlorotannin concentration, a class of potential chemical defenses. However, tests of induced responses in other A. nodosum populations are lacking and we thus have a limited understanding of how widespread such responses are in fucoid algae. We exposed a North American (Massachusetts) A. nodosum population to Littorina obtusata, the same snail species used in previous studies, and tested for the presence of inducible responses during 2 experiments. In contrast to European populations, the North American population of A. nodosum did not develop any such responses. Exposure to either direct grazing or just waterborne cues associated with grazing for 26 d failed to suppress the palatability of artificial foods made from seaweed tissues exposed to these treatments. In addition, exposure to grazer cues did not increase seaweed phlorotannin levels compared to controls. The lack of induced responses in the North American population cannot be attributed to tissue type or potential seasonal variation in inducibility because each of our 2 independent experiments tested for responses in 2 tissue types (apical tips and basal shoots) in both the winter and spring. Furthermore, consistent with European work, the North American population lacked obvious grazing damage at the time of collection, suggesting that the differences we observed were unrelated to recent history with grazers. Thus, our work and that of others indicates that A. nodosum displays considerable inter-population variability in its response to a single herbivore species. © Inter-Research 2007.