Snail snacks: introduced seaweeds facilitate performance of an introduced marine snail

Facilitative interactions among introduced species can accelerate invasion and community change, yet positive interactions are often overlooked compared to competition and predation. In marine systems, positive facilitation is more commonly demonstrated indirectly through habitat modification, with fewer studies examining direct performance impacts. While introduced herbivores in non-marine systems consume and even prefer introduced food sources, this pattern is less studied in marine environments. In this study, we tested grazing preferences and growth responses of the introduced periwinkle Littorina littorea and the native Littorina obtusata, when fed native and introduced seaweeds from southern New England. L. littorea grazed broadly across native and introduced species but exhibited 2–3 × higher growth on introduced seaweeds relative to natives. In contrast, L. obtusata showed significant preference for native seaweeds and demonstrated enhanced growth only on the native fucoids Ascophyllum nodosum and Fucus vesiculosus. Specialization on fucoids may increase vulnerability of this native snail as these seaweeds decline under warming and face increasing competition from introduced species. Conversely, enhanced growth of L. littorea on introduced seaweeds supports growth-based unidirectional facilitation, where introduced primary producers enhance the performance of an introduced generalist grazer. Together, these results suggest that growth advantages for L. littorea by introduced seaweeds could scale to population-level effects and shape rocky intertidal communities.