Bioerosion in Acropora across the continental shelf of the Great Barrier Reef

Abstract. The degree of internal bioerosion was examined in the dead basal portions of live branches of the scleractinian coral Acropora formosa collected from six reefs across the continental shelf in the central region of the Great Barrier Reef, Australia. The bioeroders included the sponges Cliona spp. and Cliothosa spp., the boring bivalve Lithophaga sp., and sipunculid and polychaete worms. Total internal bioerosion exhibited higher means and variances inshore and at the mid-shelf than the outer shelf specimens, which were characterized by low means and low variances. Bioerosion by Cliothosa and all sponges combined declined slightly across the shelf. Bivalves accounted for a small proportion of the internal bioerosion in A.formosa. The bioerosion pattern exhibited by worms (polychaetes and sipunculids) was similar in pattern to that of the sponges. All groups exhibited lowest levels ofbioerosion at the outer shelf. Highest variance in the data was observed at the intra-branch/intra-colony and the intercolony levels. Inter-site variance was high in worms and bivalves. Boring sponges generally dominated the bioeroder community. The relative abundance of Cliona declined on the outer shelf while the relative abundance of worms increased. Percent bioerosion in Acroporaformosa was 2-3 times higher than in Porites lobata in this region. The low level of bioerosion at the outer shelf versus the inner-and mid-shelf areas may be partially due to lower levels of productivity and lower concentrations of terrestrially derived organic matter. Other potential factors may include higher fish grazing/predation activity on the outer shelf