Palatability of an introduced seaweed does not differ between native and non-native populations

Generalist consumers potentially limit introduced prey populations when they are willing and able to consume those prey, but because generalist consumers vary in feeding preference, they are not equally effective in resisting invasions. Previous work suggested that during the introduction of the red seaweed Gracilaria vermiculophylla to North American and European shorelines, populations evolved enhanced levels of constitutive and activated prostaglandin-based defenses against Littorina snails and the isopod Idotea balthica. However, when we offered tissue from a total of 700 G. vermiculophylla thalli from 14 native Japanese and 25 non-native sites to a North American population of the generalist amphipod Ampithoe valida, native and non-native populations were consumed at statistically indistinguishable rates. In contrast to previous results using Idotea and Littorina, we found no evidence that mechanical wounding of G. vermiculophylla altered palatability of thalli to A. valida. These results indicate either the strength of anti-herbivore defense varies across the geographic range in the seaweed’s native range in the Northwest Pacific, that the effectiveness of G. vermiculophylla to deter herbivores varies across generalist herbivore species, or both. In either case, we question the conclusion that rapid phenotypic shift in anti-herbivory traits to generalist herbivores occurred in this species. Future studies should focus on whether shifts in anti-herbivore defenses are adaptive by experimentally assessing species-specific herbivore impacts in native and non-native ranges.