Substrate Effects on the Bioeroding Demosponge Cliona orientalis. 1. Bioerosion Rates

Abstract

Abstract. Bioeroding sponges are highly specialised to live in and to erode various natural and man‐made calcareous substrates. They encounter very different substrate features. Previous field observations suggest that damage caused by sponge bioerosion may vary with substrate density and architecture. This study aims to experimentally investigate influences of structurally different calcareous substrates on bioerosion activities of Cliona orientalisThiele, 1900, an important eroder of inshore Great Barrier Reef 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 pore volume and differences in breaking stability were measured as indicators of sponge erosion. Erosion caused by C. orientalis differed between substrates and was significantly more pronounced in denser materials with lower pore volume and in coral blocks with more structural barriers. Coral substrates with imperforate thecae and thicker dissepiment walls were more strongly eroded than those with perforate thecae and thinner dissepiments. At similar growth rates, more material has to be removed in denser material with more barriers compared to more porous substrates. Existing pores will be occupied, resulting in lower erosion rates. Erosion capabilities of the sponge could best be detected by the blocks' loss in dry weight, but the sponges also significantly reduced block breaking stability. Change in pore volume was not found to be a reliable parameter to investigate sponge erosion.