Substrate Effects on the Bioeroding Demosponge Cliona orientalis. 2. Substrate Colonisation and Tissue Growth

Sponge bioerosion is a result of tissue expansion of endolithic sponges in cal-cium carbonate substrates. The efficiency of erosion by the sponges can be affected bysubstrate features, which are thus also likely to influence the way in which the spongewill grow. A field experiment was conducted, in which sponge tissue was grafted tobiogenic blocks cut from the coralsGoniopora tenuidens, massive Porites sp., Astreopora listeri, Favites halicora, Favia pallida, Goniastrea retiformis and Cyphastrea serailia, and the clamTridacna squamosa, to investigate colonisation capabilities andgrowth patterns of Cliona orientalis Thiele, 1900 after 9 months of the experiment. C. orientalisis not substrate-specific. It invaded>90 % of the different substrateblocks and penetrated them to varying depths, but usually only down to slightly morethan 1 cm. Lateral penetration clearly exceeded depth penetration. Enlargement of surface area versus restricted depth penetration benefits the symbiotic zooxanthellae located in the sponge's surface. Structural irregularities and barriers such as coraldissepiments temporarily deflected the direction of tissue growth and created character-istic tissue patch patterns in different substrates. Tissue growth may be more pronounced in substrates of higher density and lower pore volume, but evidence was only slight. Protection against predation is better in denser materials, which may stimulatethe sponge's tissue growth especially in shallower substrate depth. In more porous substrates, favoured by grazers and corallivores, relatively more tissue was located in deeper layers.