Relative impact of parrotfish grazing scars on massive Porites corals at Lizard Island, Great Barrier Reef

Abstract

Scraping and excavating parrotfishes are well known for their marked differences in jaw morphology and feeding behaviour. Despite the importance of parrotfishes to reef systems and the well-known differences within the group, few studies have compared the functional importance of scraping versus excavating parrotfishes in shaping coral reef benthic communities. We compared the abundance, depth and dynamics of scraping and excavating parrotfish grazing scars on massive Porites spp. colonies along a reef gradient at Lizard Island, northern Great Barrier Reef. Scraping grazing scars were more abundant at most study sites than excavating scars. Excavating grazing scars were relatively rare but exposed more coral skeleton than scraping scars. Approximately 70% of excavating scars had some degree of filamentous algal growth in the scar compared with just 5% of scraping scars. Scraping grazing scars on massive Porites spp. completely disappeared after 2 mo whereas excavating grazing scars remained almost unchanged over this period. Groups of excavat- ing scars were more tightly clustered, exposed more coral skeleton and presented higher algal cover than grouped scraping scars. The deep, long-lasting excavating scars probably provide more suitable sites for the settlement of benthic algae and other invasive taxa on coral colonies. In contrast, the abundant and frequent grazing scars of scraping parrotfish may represent a more constant drain on energy supplies for coral colonies. These results highlight the differences between parrotfishes with distinct feeding modes and indicate that they differently impact not only algal communities but also coral colonies.