Feeding preferences and host associations of specialist marine herbivores align with quantitative variation in seaweed secondary metabolites

Consequences of congeneric quantitative variation in secondary metabolites of seaweeds on diet specificity and host association in specialist marine herbivores have received little attention. We investigated quantitative variation in caulerpenyne and oxytoxin 1 in 7 species of green seaweeds from the genus Caulerpa, along with the feeding preferences and host associations of 4 co-occurring sacoglossan molluscs. C. taxifolia and C. sertularioides contained high concentrations of metabolites and were preferred least by all herbivores. Algae with intermediate metabolite concentrations (C. racemosa, C, serrulata, and C. cupressoides) were preferred by Elysia tomentosa and Lobiger viridis. Oxynoe viridis and Stiliger smaragdinus had strong preferences for different low concentration Caulerpa species (C. racemosa var. laetevirens and C. lentillifera), suggesting not all feeding preferences are based exclusively on the major metabolites. In situ host associations of L. viridis and S. smaragdinus mirrored their feeding preferences, but this was not the case for E. tomentosa. Furthermore, those algal species with the highest and lowest metabolite concentrations had the lowest overall densities of sacoglossans. The results imply that the direct influence of quantitative variation in Caulerpa chemistry may only be limited to host associations in some sacoglossans. However, feeding pressure from multiple herbivore species with unique preferences could still contribute to variation in chemical defence amongst congeneric algae.