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Regulation of anti‐herbivore defence by Fucus vesiculosus in response to various cues

1 We examined whether the marine macroalga Fucus vesiculosus induces defences against herbivory and, if so, which factors trigger this induction. In addition, we assessed whether induced defences are reduced after consumption stops. 2 Induced effects were measured as changes in palatability rather than changes in the chemistry of the algae. We also tested for reductions in growth rate to determine whether induced defence incurs metabolic costs. 3 We tested whether direct grazing, feeding on neighbouring plants, clipping and presence of a non‐grazing herbivore could trigger induction. The isopod Idotea baltica and the gastropod Littorina littorea were used as herbivores. 4 Both direct feeding of I. baltica and feeding on neighbouring plants induced chemical defence in F. vesiculosus, whereas the snail L. littorea only induced defence by direct grazing. Simulated herbivory (clipping), or the presence of herbivores without grazing, did not lead to defence induction. All induced defences were reversed within 2 weeks of consumption ending. 5 Thus, F. vesiculosus differentiates between physical damage and natural herbivory. Furthermore, feeding by I. baltica on F. vesiculosus releases signals that trigger neighbouring Fucus individuals to induce defence. 6 We found no evidence that metabolic costs incurred as a consequence of induced defence were sufficient to lead to growth reduction. 7 This algal species demonstrates defence plasticity (i.e. induction and reduction of anti‐herbivore defences ‘on demand’), with the response depending on both grazer identity and grazing pattern matter.