The boring sponge Cliona caribbaea is ubiquitous on the shallow terraces around Grand Cayman Island, British West Indies, occurring as brown sheets covering and growing over both live corals and hard substrate. Throughout the zone of the radial grooves, on the southwest corner of Grand Cayman, C. caribbaea covered 5% of the total bottom area between -1 and -10 m. Values up to 10% occurred commonly. Lateral extension rates were 4 cm/yr. The sponge removes an average of 20% (up to 45%) of the substrate, which has an average specific gravity of 1.7. Conservative estimation of typical rates of substrate removal yields 8 kg/m ² /yr, a value of about double the typical rates of reef calcification. The silt produced by the sponge is rapidly transported downslope.