The zooxanthellae live in various organisms such as corals, giant clams, anemones etc. They are microscopic, single-celled photosynthetic algae, which form a symbiotic relationship with corals, jellyfish, anemone and various other organisms. The algae is able to photosynthesise and produce energy, which the coral needs to build its calcium carbonate structure. When there is too much zooxanthellae growing on the coral, the coral will secrete chemicals to expel the zooxanthellae from the structure. However, when there are changes in the temperature, the corals might expel too much zooxanthellae. Without these zooxanthellae, the corals are unable to survive.