Relationships between coral quality and parrotfish corallivory: A long-term predator-exclusion experiment on the Belizean barrier reef

A detailed understanding of the dual role that parrotfish play as key herbivores and potentially important corallivores is essential to the study of coral reef health and trophodynamics. Some Caribbean parrotfish regularly consume live coral, and show both interspecific and intraspecific feeding selectivity. Parrotfish preferentially consume Montastraea spp. corals, which are dominant Caribbean reef builders, but potential causes of parrotfish selectivity among colonies remain unknown. Here we describe an 18-month manipulative experiment designed to determine what characteristics of coral tissue quality might be a cause for, or an effect of, parrotfish corallivory. We manipulated coral exposure to parrotfish grazing via corallivore exclusion cages in Belize, comparing initially grazed versus intact (non-grazed) Montastraea spp. colonies. We investigated several nutrition-related characteristics (zooxanthellae density, C:N ratio, %C, and %N) as well as defensive characteristics (nematocyst density and skeletal hardness) to determine if any of these variables accurately predicted parrotfish grazing. We found substantial differences in coral nutritional quality (C:N) associated with parrotfish grazing, although these differences appeared to be a consequence rather than a cause of parrotfish selectivity. In addition, other coral characteristics including zooxanthellae density and nematocyst density also changed slightly as a result of parrotfish grazing, but neither appeared to influence the likelihood of grazing. Increased nematocyst densities may represent an inducible coral defense, although this is unlikely to be effective against parrotfish. These results indicate that parrotfish grazing has major physiological consequences for Montastraea spp. corals, but causes of preferential grazing by parrotfishes on certain colonies require further investigation.