Understanding the invasion success of Sargassum muticum: herbivore preferences for native and invasive Sargassum spp

Several ecological models explain the success of introduced seaweeds by herbivore–prey interactions. The ‘enemy release hypothesis’ states that invaders benefit from a lack of natural enemies compared to the recipient community. The ‘novel weapons hypothesis,’ however, highlights the importance of chemical defense compounds of introduced species that are more effective in invaded regions than native counterparts. In order to explain the tremendous invasion success of the brown alga Sargassum muticum, we compared the palatability and nutritional value of S. muticum individuals from their native and invaded habitat (Japan and North Sea, respectively) with noninvasive congeneric species from Japan (S. fusiforme,S. horneri), and a native competitor from the North Sea (Fucus vesiculosus). Different feeding assays using artificial food with either freeze-dried algae or algal extracts and three dominant North Sea mesograzers were performed to detect feeding preferences. All herbivores preferred the local brown alga F. vesiculosus, followed by North Sea S. muticum, while Sargassum spp. from Japan were the least preferred. Since nutritional value did not correlate with feeding preference and algal extracts had the same effect as algal powder, we could demonstrate a deterrent activity of algal secondary metabolites. The preference of herbivores for the sympatric S. muticum population compared to the allopatric Japanese population could indicate a resource allocation from chemical defense to reproduction and growth. Due to the low palatability of Sargassum spp. from Japan, it might be reasonable to include additional Sargassum species in North Sea monitoring to prevent their establishment in European waters.