Selective predation by parrotfishes on the reef coral Porites astreoides

Direct predation by parrotfishes (Scaridae) may be an important stressor and mortality agent for reef-building corals, yet the patterns and consequences of such predation have received little attention. In a Belizean back reef habitat, we found that parrotfish predation caused mortality of the common reef-building coral Porites astreoides with > 1.3 %, of P. astreoides colonies showing partial or total colony mortality. Predation was not uniformly distributed, as completely intact colonies occurred adjacent to heavily grazed colonies exhibiting > 50 % tissue lost to parrotfish predation. We examined whether differential predation on P. astreoides colonies might reflect differences in either coral nutritional quality or in coral-associated macroborers. Nutritional quality of coral tissue, measured as C:N ratios, did not differ significantly between grazed and intact P, astreoides colonies. However, significantly higher densities of coral-associated macroborers (which included barnacles, polychaetes, and vermetid molluscs) were found in grazed portions compared to ungrazed portions of the same P. astreoides colony. Thus, one explanation for selective grazing on P. astreoides colonies is that parrotfishes may be targeting coral areas with higher densities of macroboring organisms. Comparison of C:N ratios among possible dietary components suggests that macroborer consumption could potentially provide nutritional benefits to mainly herbivorous parrotfishes. These results corroborate a growing body of evidence to indicate that parrotfish predation is an important source of coral mortality, and provide a novel explanation for selective coral predation by parrotfishes.