The Patterns, Causes, and Consequences of Parrotfish Corallivory in Belize

Parrotfish are important members of coral reef communities because they consume macroalgae that would otherwise outcompete reef-building corals for space. However, some Caribbean parrotfish also feed directly on live corals, and thus may negatively impact coral fitness and survival. Here, I explore the patterns, causes, and consequences of parrotfish corallivory along the Belizean barrier reef. I demonstrate that parrotfish display both inter- and intraspecific grazing selectivity, preferring corals in the genus Montastraea regardless of reef habitat, and certain colonies within species. Grazed colonies are repeatedly grazed, suggesting that consequences of corallivory are likely amplified for these colonies. I show that parrotfish corallivory can negatively impact coral fitness and survival directly by causing mortality, reduced growth and reproductive potential, and delayed recovery after bleaching events. The most dramatic change detected in coral tissue following parrotfish grazing was decreased coral nutritional quality (decreased %C and %N, increased C:N). Chronic corallivory also exacerbates the influence of environmental stressors and can alter coral-zooxanthellae symbioses. Interestingly, parrotfish seem to selectively target Montastraea annularis colonies high in lipid-rich gonads and Porites astreoides corals with high macroborer densities. Colonies avoided by parrotfish may have effective defenses, but I found no evidence of intraspecific differences in Montastraea spp. skeletal hardness or nematocyst densities, and evidence for coral chemical defense is thus far inconclusive. Nonetheless, Montastraea spp. corals mount a short-term inducible response to grazing via increased nematocyst densities that may be an effective deterrent against other corallivores. Taken together, my results provide strong evidence that parrotfish corallivory plays an important ecological role in coral reef trophodynamics. Although live coral cover is declining throughout the Caribbean, I found that the abundance of some corallivorous parrotfishes has increased significantly over the last two decades, suggesting that the role of parrotfish may become increasingly important. There are over 160 known corallivores worldwide, thus the results presented here serve as a basis to understand the potential role of all corallivores and the implications for coral fitness. My results yield novel insight into the causes and patterns of corallivory, and reveal the previously underestimated negative consequences of parrotfish predation on coral reefs.