Reactive oxygen species as a marine grazing defense: H2O2 and wounded Ascoseira mirabilis both inhibit feeding by an amphipod grazer

• We investigated the role of ROS in marine anti-grazing defense.
• One micrometer H2O2 decreased feeding by the amphipod Gondogeneia antarctica.
• Wounded Ascoseira mirabilis releases ROS and inhibited G. antarctica feeding.
• Data are consistent with a role for ROS as an anti-herbivore defense in nature.

Marine macroalgae can release reactive oxygen species (ROS) upon wounding and grazing. Here we address the potential role of ROS in herbivore defense. We performed feeding assays in the presence of varying concentrations of hydrogen peroxide (H2O2), a common type of ROS. H2O2 inhibited feeding by a marine amphipod grazer, Gondogeneia antarctica, over broad levels of concentration, and its potency was strongly dependent on its rate of decay in natural seawater. Because it is possible that the inhibitory levels of H2O2 are encountered in the vicinity of a sympatric macroalgal wound, we suggest that H2O2 has the potential to act as a direct anti-grazing defense in marine ecosystems. Since some sympatric macroalgae release a burst of non-H2O2 ROS, we also performed experiments to evaluate the role of these naturally-produced ROS on sympatric grazers. The presence of wounded Ascoseira mirabilis (which releases a burst of non-H2O2 ROS after wounding) during a feeding assay inhibited feeding of G. antarctica compared to the presence of intact A. mirabilis. These data are consistent with a role for ROS as a direct anti-herbivore defense in nature. However, the data are also consistent with hypotheses that involve other putative activated anti-grazing defenses.