Production of fine sediments in reef environments is a poorly understood but probably significant process. On the Great Barrier Reef, Cliona viridis is locally abundant. The sponge has been found from Lizard Island in the north, down to the southern part of the Central Region, and it is likely even more widespread. Although on individual reefs the sponge may occur almost anywhere, it is most abundant on lagoon path reefs ("bommies" in Australian), where it overgrows the substrate and bores to a constant depth of about 1.3cm. Sponge infestation reaches a maximum at a depth of 4 m covering almost 8% of the vertical sides of bommies. It is frequently the dominant benthic organism. Serial underwater photography demonstrates that colonies can expand laterally at a rate of about 1 cm yr⁻¹ removing approximately 40% of the substrate. This species is a major producer of fine sediment on the Great Barrier Reef. / Thesis / Master of Science (MS)