Coral Reefs

Some cnidarians help build reefs. Polyps produce calcium carbonate skeletons which can eventually form massive reefs. Reef-building corals generally have zooxanthellae which form a symbiotic relationship with the coral.Coral polyps are comprised of a long tube with grasping tentacles on the end. The zooxanthellae are photosynthetic and help feed the coral. In return the coral lives close to the surface so that the zooxanthellae can get the optimal amount of sunlight. Coral reefs are only found on continental shelves, surrounding islands, and on top of sea mounts. Coral reefs also require a water temperature of 20 Celsius or over to survive. Also coral reefs do not do well in fresh water.