Effects of nitrogen concentrations in turtlegrass Thalassia testudinum on consumption by the bucktooth parrotfish Sparisoma radians

Abstract

Terrestrial herbivores often preferentially consume plants that have higher nitrogen concentrations. In these circumstances, low nitrogen concentrations in exposed tissues would be an effective defense against herbivory. In marine environments, the relationship between nitrogen con- centration in plants and herbivore preferences has only recently been explored, and results are inconsistent. We examined preferences of the bucktooth parrotfish Sparisoma radians for the turtle- grass Thalassia testudinum with high or low nitrogen concentrations in both field and laboratory experiments. These species were chosen for study because (1) turtlegrass, the dominant seagrass in the Caribbean, is the primary food resource of the bucktooth parrotfish and (2) S. radians is known to be undeterred by the chemical defenses (i.e. phenolic compounds) and physical defenses (i.e. tough tissues) of T. testudinum. In field choice experiments, we found that S. radians consumed a greater proportion of T. testudinum shoots with high nitrogen concentrations than shoots with low nitrogen concentrations (68 vs 5%). Similarly, in laboratory experiments, S. radians consumed more T. tes- tudinum with high nitrogen than with low nitrogen concentrations (25 vs 5%). To determine whether S. radians used the color or shape of shoots as a basis for its preferences, we repeated laboratory experiments using an agar mixture with powdered T. testudinum leaves of either high or low nitro- gen concentration. S. radians again showed a significant preference for the high nitrogen food source. We, therefore, conclude that mechanisms for choice of high nitrogen food were most likely related to gustation and/or olfaction of chemical cues (i.e. nitrogen or phenols).