The Enemy Release Hypothesis (ERH) predicts that successful recruitment of an introduced species in the recipient region results from escaping co-evolved enemies. The success to cope with previously non-encountered grazer species represents one critical aspect for introduced seaweeds in this context. In laboratory experiments, the ability of the introduced red alga Mastocarpus stellatus to induce anti-herbivore defences against two grazer species at two seasons (spring and fall) was compared to that of the indigenous red alga Chondrus crispus. Grazers were selected by living either allopatrically (Littorina littorea) and sympatrically (Idotea granulosa) with Mastocarpus on Iceland, i.e. the donor region of the Mastocarpus population on Helgoland. Algae were exposed to each grazer species separately or left ungrazed during the induction phase and subsequently cultivated without herbivores (reduction phase). Bioassays at the end of both phases using fresh algae and reconstituted food assessed changes in algal palatability. Preference of control over grazed algae indicates an induction of anti-herbivory defences. Direct comparisons revealed that Littorina prefers Chondrus over Mastocarpus, while Idotea shows no preference between both algae. Irrespective of season, Idotea but not Littorina induced defences in fresh and reconstituted Mastocarpus feed originating from Helgoland and Iceland. In contrast, both grazer species induced anti-herbivory defences in fresh and reconstituted feed of the indigenous seaweed Chondrus in spring, but not in autumn. The differential anti-herbivory responses of the introduced Mastocarpus to allopathic and sympatric grazers represent a trait optimization, minimizing the impact of each type of herbivore, relative to their impact on the indigenous algae.