Selection of increased resistance against epiphytism during the invasion history of Gracilaria vermiculophylla

Fouling is a stressor that might determine the fate of seaweeds, but reports of algal adaptation to epibiosis are scarce. Previous comparisons have shown resistance to epibionts can be higher in non-native than in resident seaweed species, but we do not know whether it is an intrinsic trait of the non-natives or it has been acquired during the invasion process. We here compared native and non-native populations of the same algal species to elucidate this question. Resistance against two groups of epiphytes was assessed in living thalli and in artificial substrata coated with surface extracts, both gained from four Asian (native) and four European (non-native) populations of the red alga Gracilaria vermiculophylla. Two diatom species and two filamentous macroalgae were used as micro- and macro-epiphytes, and one of each type was collected in Asia, while the other came from Europe. Laboratory assays were done in both distributional ranges of G. vermiculophylla and in different seasons. We used a fully crossed design with the factors (i) ‘Origin of Gracilaria’, (ii) ‘Origin of epiphytes’, (iii) ‘Season’ and (iv) ‘Solvent used for extraction’. Both groups of epiphytes, regardless of their origin, attached less to living thalli and to surface extracts from non-native G. vermiculophylla. Fewer diatoms attached to hexane-based extracts, while fewer Ceramium filaments settled on extracts gained with dichloromethane. Our results show for the first time that non-native individuals of a seaweed are better defended against epiphytes than native conspecifics. Furthermore, we found evidence that at least a part of the defence is based on extractable secondary metabolites. We suggest that an enhanced defence against epiphytes after introduction is one reason for G. vermiculophylla’s invasion success. Our observation may also apply to other basibiont–epibiont interactions and could be a key feature of seaweed bioinvasions