Marine and terrestrial herbivores display convergent chemical ecology despite 400 million years of independent evolution

Significance We report, for the first time to our knowledge, compounds that specialist marine herbivores use to find their prey. The seaweed Halimeda incrassata produces metabolites that deter feeding by generalist herbivores. However, a specialist sea slug, Elysia tuca , follows these defensive compounds and not only attacks the seaweed but does so preferentially while the seaweed is reproducing. Elysia sequester Halimeda’s chemical defenses (to deter predators) and chloroplasts (becoming photosynthetic). Elysia feeding reduces Halimeda growth by ∼50%, but the alga drops branches occupied by Elysia , possibly to avoid fungal infection associated with herbivory and to rid itself of Elysia . These interactions parallel many involving terrestrial insects and plants, even though marine and terrestrial herbivores have evolved independently for 400 million years.