Chemically mediated resistance to mesoherbivory in finely branched macroalgae along the western Antarctic Peninsula

Along the western Antarctic Peninsula, benthic macrophytic biomass rivals some of the denser algal communities in the world. However, there is a noticeable lack of filamentous epiphytes compared with other algal-dominated communities. One possible explanation is that epiphytic species and emerging filaments from profuse endophytes are controlled by grazing pressures from an abundant mesograzer community. Amphipod gut contents from the study area revealed the presence of algal filaments that could support this hypothesis. However, this gut-derived algal material may be from ingestion of any of the several sub-tidal, finely branched rhodophyte species, which are often inhabited by numerous amphipod mesograzers. Palatability of several of these finely branched macrophytes (Halopteris obovata, the only phaeophyte in this study, Cystoclonium obtusangulum, Pantoneura plocamioides, Picconiella plumosa, Plocamium cartilagineum, as well as the known edible control alga Palmaria decipiens) were tested against two of the most abundant amphipod mesograzers, Gondogeneia antarctica and Prostebbingia gracilis, in a series of fresh-thallus feeding assays. Several artificial food bioassays, utilizing artificial foods mixed with algal extracts, were also conducted to test for any chemical grazing deterrents produced by the macroalgae. Results indicate that all the finely branched algae tested were unpalatable to both grazers, most likely due to chemical defences in the red algae. These results suggest that filamentous material found in amphipod guts is not finely branched rhodophytes despite observation of mesograzers associating with these algae. Possibly, mesograzers of the western Antarctic Peninsula use these chemically defended algae as a refuge from predation and, in turn, graze on palatable epiphytes and emergent filaments from endophytes, growing on their host macrophytes.