Water-borne defence induction of a rockweed in the wild

Abstract

Laboratory-scale studies suggest that water-borne chemical cues released in grazing of marine macroalgae act as info-chemicals allowing the yet-undamaged conspecifics to increase their resistance before the actual herbivore attack. The relevance of this finding in natural populations has thus far remained unexplored. We investigated spread of water-borne herbivore resistance in a field experiment using the littoral foundation species Fucus vesiculosus. We exposed algal genotypes for water-borne cues in the distances of 0.5 and 2 m from the grazed cue-emitting genotypes. We measured herbivore resistance with preference experiments and recorded changes in the phlorotannin content. We recorded changes in biomass and length of the genotypes to test whether water-borne resistance correlates with the grazing damage on the cue-emitter or incurs costs for the genotypes in terms of reduced growth. In two of our four experimental blocks, induced resistance via water-borne cues took place and spread to undamaged neighbours that were 2 m distance apart. This provides evidence of water-borne resistance induction in the wild as well as of small-scale spatial variation in the spread of induction. The undamaged neighbours induced resistance to a lesser extent than the directly grazed cue-emitters did, but resistance was consistently expressed among all 26 algal genotypes and independent of the amount of grazing damage on the cue-emitter. Herbivore resistance correlated negatively with the growth in the grazed cue-emitting genotypes, while water-borne resistance was independent of the variation in growth. Our results show, for the first time for marine macroalgae in a natural setting, that information about the increased risk of herbivory can be carried by water-borne cues and is used to prime or induce defences prior to an actual attack. The results we present furthermore indicate that water-borne resistance differs quantitatively and qualitatively from the resistance triggered by actively feeding herbivores. Ability to induce resistance prior the herbivore arrival may greatly influence the ability of littoral macroalgae to withstand the generally intense herbivory of marine benthic environments. A plain language summary is available for this article.