Corals can grow only in shallow water, where light can penetrate, because thezooxanthellae on which they dependneed light. Calcareons algae also requiresunlight. Particular types of coral andalgae have different depth limits—somecan live deeper than others—but reefsrarely develop in water deeper thanabout 50 m (165 ft). Because of this,coral reefsare found only on the conti-nental shelves, around islands, or on topof seamounts. Many types of coral live indeep water and do not need light, butthese corals do not contain zooxanthellaeor build reefs. Corals also prefer clear waters, since water clouded with sedi-ment or plankton does not allow light topenetrate very well.