OUCH!: stinging epifaunal anemones reduce parrotfish grazing pressure in a Caribbean seagrass ecosystem.

Mutualisms in which one organism defends another from consumers are powerful determinants of community structure in many ecosystems. Here, we investigated how the presence of a stinging epifaunal anemone, Bunodeopsis antilliensis, influenced fish grazing on its host, the seagrass Thalassia testudinum, in a Caribbean seagrass meadow. Experimental removal of anemones yielded a more than 60% increase in parrotfish herbivory on blades of T. testudinum and proportionate reductions in seagrass height and biomass over a five-day period. A survey of a seagrass meadow where anemones were common (1.2 ± 2.1 anemones per blade) showed that individual seagrass blades with attached anemones were generally taller, minimally epiphytized, and had fewer parrotfish bite marks. The same survey showed that anemone density at the plot level was negatively associated with parrotfish herbivory. A seagrass preference assay revealed that anemones preferentially colonized healthy, non-epiphytized leaves over senescent or heavily epiphytized ones, suggesting that host condition influences anemone settlement preference. Taken together, our results demonstrate that B. antilliensis provides a protective benefit to T. testudinum, potentially reducing herbivory pressure and promoting seagrass persistence. Our study provides experimental evidence consistent with a plant-animal grazing-protection mutualism in a seagrass ecosystem.