The effects of nitrogen content in turtlegrass, Thalassia testudinum, on food selection by the bucktooth parrotfish, Sparisoma radians (Florida)

There are conflicting results in the literature concerning the role of plant nitrogen content in determining marine herbivore food preferences. To clarify whether nitrogen content of plants affects herbivore choice, field and laboratory experiments were conducted using turtlegrass (Thalassia testudinum), the dominant seagrass in the Caribbean, and the bucktooth parrotfish (Sparisoma radians), a common browser and inhabitant of T. testudinum. In field experiments, S. radians consumed 60% more, and in laboratory experiments 20% more, T. testudinum shoots containing high nitrogen. To eliminate visual cues, preference tests using an agar mixture with powdered T. testudinum containing high or low nitrogen were conducted, and again S. radians showed significant preference for the high nitrogen food source. Thus, olfaction and gustation are also likely mechanisms underlying preference. Selective feeding on high nitrogen plants could influence the outcome of competitive interactions among seagrasses, leading to broad-scale changes in the composition of seagrass meadows.