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Bioerosion of coral reefs generates the fine and white coral sand characteristic of tropical islands. The coral is converted to sand by internal bioeroders such as algae, fungi, bacteria (microborers) and sponges (Clionaidae), bivalves (including "Lithophaga"), sipunculans, polychaetes, acrothoracican barnacles and phoronids, generating extremely fine sediment with diameters of 10 to 100 micrometres. External bioeroders include sea urchins (such as "Diadema") and chitons. These forces in concert produce a great deal of erosion. Sea urchin erosion of calcium carbonate has been reported in some reefs at annual rates exceeding 20 kg/m2.