BIOEROSION IN ACROPORA ACROSS THE CONTINENTAL-SHELF OF THE GREAT-BARRIER-REEF

The degree of internal bioerosion was examined in the dead basal porti ons of live branches of the scleractinian coral Acropora formosa colle cted from six reefs across the continental shelf in the central region of the Great Barrier Reef, Australia. The bioeroders included the spo nges Cliona spp. and Cliothosa spp., the boring bivalve Lithophaga sp. , and sipunculid and polychaete worms. Total internal bioerosion exhib ited higher means and variances inshore and at the mid-shelf than the outer shelf specimens, which were characterized by low means and low v ariances. Bioerosion by Cliothosa and all sponges combined declined sl ightly across the shelf. Bivalves accounted for a small proportion of the internal bioerosion in A. formosa. The bioerosion pattern exhibite d by worms (polychaetes and sipunculids) was similar in pattern to tha t of the sponges. Ah groups exhibited lowest levels of bioerosion at t he outer shelf. Highest variance in the data was observed at the intra -branch/intra-colony and the intercolony levels. Inter-site variance w as 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 b ioerosion in Acropora formosa was 2-3 times higher than in Porites lob ata in this region. The low level of bioerosion at the outer shelf ver sus the inner- and mid-shelf areas may be partially due to lower level s of productivity and lower concentrations of terrestrially derived or ganic matter. Other potential factors may include higher fish grazing/ predation activity on the outer shelf.