Seagrass deterrence to mesograzer herbivory: evidence from mesocosm experiments and feeding preference trials

Two laboratory experiments documented the effects of mesograzers (i.e. the gastropod Crepidula ustulatulina and the isopod Paracerceis caudata) on phenolic acid and condensed tannin production in 2 regionally abundant seagrasses - Thalassia testudinum (turtlegrass) and Halodule wrightii (shoalgrass). Subsequent paired choice experiments tested the hypothesis that phenolic acids and condensed tannins produced by these seagrasses deter mesograzer feeding. At the scale of the shoot, grazing by gastropods and isopods led to similar to 40 to 50% decreases in concentrations of some phenolic acids and similar to 20% decreases in condensed tannins in turtlegrass leaves. At a more refined spatial scale, concentrations of 2 of these compounds increased by 25 to 85% in areas near tissues damaged by C. ustulatulina and P. caudata in turtlegrass. In contrast, isopod feeding increased the concentrations of some shoalgrass phenolic acids by similar to 30 to 50%, while gastropod grazing led to similar to 25 to 50% higher concentrations of condensed tannins in shoalgrass leaves, suggesting that grazer identity and seagrass species play important roles in seagrass deterrent production. Amphipods (Batea catharinensis) consistently preferred agar food made from seagrass leaves with low phenolic concentrations in choice feeding experiments, indicating that phenolics can act as feeding deterrents to these mesograzers.