Query: When water temperatures become too high, coral becomes stressed and expels the algae, which leave the coral a bleached white color.

Relevant passages:
Coral bleaching <=SEP=> Coral bleaching occurs when coral polyps expel algae that lives inside their tissues . Normally , coral polyps live in an endosymbiotic relationship with the algae and that relationship is crucial for the coral and hence for the health of the whole reef . Bleached corals continue to live . But as the algae provide the coral with 90 % of its energy , after expelling the algae the coral begins to starve . Above-average sea water temperatures caused by global warming have been identified as a leading cause for coral bleaching worldwide . Between 2014 and 2016 , the longest global bleaching events ever were recorded . According to the United Nations Environment Programme , these bleaching events killed coral on an unprecedented scale . In 2016 , bleaching hit 90 percent of coral on the Great Barrier Reef and killed 29 percent of the reef 's coral . In 2017 , the bleaching further expanded to areas of the park that were previously spared , such as the central one .   __ TOC __

Coral reef <=SEP=> Coral reefs are diverse underwater ecosystems held together by calcium carbonate structures secreted by corals . Coral reefs are built by colonies of tiny animals found in marine waters that contain few nutrients . Most coral reefs are built from stony corals , which in turn consist of polyps that cluster in groups . The polyps belong to a group of animals known as Cnidaria , which also includes sea anemones and jellyfish . Unlike sea anemones , corals secrete hard carbonate exoskeletons which support and protect the coral polyps . Most reefs grow best in warm , shallow , clear , sunny and agitated waters .   Often called `` rainforests of the sea '' , shallow coral reefs form some of the most diverse ecosystems on Earth . They occupy less than 0.1 % of the world 's ocean surface , about half the area of France , yet they provide a home for at least 25 % of all marine species , including fish , mollusks , worms , crustaceans , echinoderms , sponges , tunicates and other cnidarians . Paradoxically , coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients . They are most commonly found at shallow depths in tropical waters , but deep water and cold water corals also exist on smaller scales in other areas .   Coral reefs deliver ecosystem services to tourism , fisheries and shoreline protection . The annual global economic value of coral reefs is estimated between US$ 29.8-375 billion . However , coral reefs are fragile ecosystems , partly because they are very sensitive to water temperature . They are under threat from climate change , oceanic acidification , blast fishing , cyanide fishing for aquarium fish , sunscreen use , overuse of reef resources , and harmful land-use practices , including urban and agricultural runoff and water pollution , which can harm reefs by encouraging excess algal growth .

Coral <=SEP=> Corals are marine invertebrates in the class Anthozoa of phylum Cnidaria . They typically live in compact colonies of many identical individual polyps . The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton .   A coral `` group '' is a colony of myriad genetically identical polyps . Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length . A set of tentacles surround a central mouth opening . An exoskeleton is excreted near the base . Over many generations , the colony thus creates a large skeleton that is characteristic of the species . Individual heads grow by asexual reproduction of polyps . Corals also breed sexually by spawning : polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon .   Although some corals can catch small fish and plankton using stinging cells on their tentacles , most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates in the genus Symbiodinium that live within their tissues . These are commonly known as zooxanthellae and the corals that contain them are zooxanthellate corals . Such corals require sunlight and grow in clear , shallow water , typically at depths shallower than 60 m. Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters , such as the enormous Great Barrier Reef off the coast of Queensland , Australia .   Other corals do not rely on zooxanthellae and can live in much deeper water , with the cold-water genus Lophelia surviving as deep as 3000 m . Some have been found on the Darwin Mounds , north-west of Cape Wrath , Scotland . Corals have also been found as far north as off the coast of Washington State and the Aleutian Islands .