Wave-exposure rapidly induced changes in morphological seaweed traits that modify trophic interactions with grazers

Seaweeds are known to change their phenotype in response to wave-exposure. Shifts in shape or texture due to phenotypic plasticity may affect subsequent interactions between seaweeds and other species. In two transplantation experiments, we tested the ability of the brown seaweed Fucus vesiculosus to acclimate to different levels of wave-exposure and whether this acclimation affected its consumption by different grazer species. In different years, a transplantation of F. vesiculosus between exposed and sheltered sites provoked a rapid shift of apical thallus toughness, measured as puncture strength, towards values found in conspecifics that remained throughout the experiment at the respective recipient site (= replants). The time needed to significantly change thallus toughness was 9 times shorter in F. vesiculosus that were transplanted from the sheltered to the exposed site (8 days) than in the opposite direction (i.e. 72 days). Prior to transplantation, both the North Sea isopod Idotea baltica and the amphipod Echinogammarus marinus consumed significantly more sheltered-site than exposed-site apical pieces of F. vesiculosus. This pattern was confirmed with F. vesiculosus and I. baltica from the Baltic Sea population. In contrast, no discrimination was apparent when the North Sea grazers could chose between reconstituted food made of either sheltered-site or exposed-site F. vesiculosus. At the end of the transplantation experiment, grazers consumed equal amounts of transplanted and replanted F. vesiculosus. Results indicate that (i) wave exposure altered morphological rather than non-morphological properties (ii), a toughening of tissue with an increase of exposure is faster than the relaxation of toughness with decreasing exposure and (iii) trait plasticity modified the suitability of F. vesiculosus as food for mandible-bearing grazer species. Thus, grazing impact was reduced when wave exposure increased; possibly offering seaweeds an opportunity to trade-off costs that may be associated with a live on high-energy shores