Large-scale patterns in internal bioerosion of Porites: cross continental shelf trends on the Great Barrier Reef

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Abstract
Internal bioerosion of the coral Porites lobata Dana (Coelenterata, Scleractinia) was examined on 5 reefs across the central region of the Great Barrier Reef, Australia: Orpheus Island, Britomart Reef, Rib Reef, Davies Reef, and Myrmidon Reef. Ninety corals were sliced into discs (nt = 270 replicates) and X-rayed to identify internal bioeroders and quantify the degree of bioerosion caused by each bioeroding group. Total internal bioerosion decreased significantly with distance offshore, across the continental shelf. Abundances of bivalves and sponges decreased the most significantly. Sponges were significantly lower on the outer shelf. Abundance of sipunculans and polychaetes showed only a nominal decrease across the shelf. Boring bivalves dominated the inshore area while the relative abundance of sponges increased with distance from shore. The relative abundance of worms (polychaetes and sipunculans) remained generally constant across the shelf. The explanatory hypothesis is offered that there may be a link between internal bioerosion and the already documented decreasing productivity and increased abundance of grazing fish with distance from shore. Three predictions are also made: (1) that observed changes in internal bioeroders may influence sediment composition across the continental shelf, with inshore sediments being more poorly sorted, and possessing more fines produced by internal bioeroders; (2) that offshore lagoonal sediments may have a greater contribution derived from grazing fish; and (3) that the observed higher levels of bioerosion inshore may increase susceptibility of corals there to dislodgement and fragmentation imposed by physical disturbance.