Substrate effects on the bioeroding demosponge Cliona orientalis. 1. Bioerosion rates

Bioeroding sponges are highly specialised to live in and to erode various natural and manmade calcareous substrates. They encounter very different substrate fea-tures. Previous field observations suggest that damage caused by sponge bioerosionmay vary with substrate density and architecture. This study aims to experimentally in-vestigate influences of structurally different calcareous substrates on bioerosion activities of Cliona orientalis Thiele, 1900, an important eroder of inshore Great BarrierReef calcium carbonate. Blocks were made of the corals Goniopora tenuidens, massive Porites sp., Astreopora listeri, Favites halicora, Favia pallida, Goniastrea retiformis and Cyphastrea serailia, and of the clam Tridacna squamosa. They were grafted with C. orientalis tissue and re-examined after 9 months. Block weight loss, increase of porevolume and differences in breaking stability were measured as indicators of sponge ero-sion.Erosion caused byC. orientalisdiffered between substrates and was significantly morepronounced in denser materials with lower pore volume and in coral blocks with morestructural barriers. Coral substrates with imperforate thecae and thicker dissepimentwalls were more strongly eroded than those with perforate thecae and thinner dissepi-ments. At similar growth rates, more material has to be removed in denser material withmore barriers compared to more porous substrates. Existing pores will be occupied, re-sulting in lower erosion rates. Erosion capabilities of the sponge could best be detectedby the blocks' loss in dry weight, but the sponges also significantly reduced blockbreaking stability. Change in pore volume was not found to be a reliable parameter toinvestigate sponge erosion.