While chemical defenses can determine plant persistence in terrestrial ecosystems and some marine macroalgae, their role in determining seagrass persistence in areas of intense grazing is unknown. As a first step toward determining if concentrations of feeding deterrents in seagrasses increase following herbivore attacks, we conducted 4 experiments using a common macrograzer (sea urchin Lytechinus variegatus) and 2 phylogenetically divergent seagrass species (Thalassia testudinum and Halodule wrightii). Macrograzer impacts on production of phenolic acids and condensed tannins varied somewhat idiosyncratically with season, urchin density, and distance from urchin damage. In general, phenolic concentrations were higher in both turtlegrass and shoalgrass in summer than in fall. Grazing led to increased condensed tannin concentrations in T. testudinum but had few effects on turtlegrass phenolic acid concentrations. Turtlegrass p-hydroxybenzoic acid concentrations increased locally in the grazed area and in areas >2 cm above grazing. Similarly, condensed tannins in grazed H. wrightii leaves increased with urchin density in summer but decreased in fall while having few predictable effects on phenolic acid concentrations. Shoalgrass gallic acid concentrations increased in the lowest portions of grazed leaves. In choice feeding experiments in which urchins were offered agar food made with nighand low-phenolic seagrass tissue, high phenolic concentrations did not reduce urchin feeding, suggesting that the increases in phenolic concentrations we saw in turtlegrass and shoalgrass did not protect these plants against future sea urchin herbivory but rather some other factor, such as mesograzer feeding or pathogenic infection.
Steele, L. & Valentine, J. F. (2012(. Idiosyncratic responses of seagrass phenolic production following sea urchin grazing. Marine Ecology Progress Series, 466: 81-92.