Tiny plant-like organisms called zooxanthellae live in the tissues of many animals, including some corals, anemones, and jellyfish, sponges, flatworms, mollusks and foraminifera. These microscopic algae capture sunlight and convert it into energy, just like plants, to provide essential nutrients to the corals. In exchange, they have a place to live inside the animal's body. But when the zooxanthellae are under stress, such as high temperatures, they will die or leave their host—a process known as bleaching.
slideshow
Zooxanthellae and Coral Bleaching
Close-up of a Coral Polyp
Credit: © osf.co.uk. All rights reserved.The brownish-green specks seen in this coral polyp are the zooxanthellae that most shallow, warm-water corals depend on for much of their food. Like plants, zooxanthellae capture energy from the sun and turn it into food, some of which the coral eats in exchange for protection.
Where Do They Live?
Credit: Smithsonian InstitutionAs you can see in this diagram, the zooxanthellae live within the tissue of their host coral. The coral polyp itself lives in a cup it built from calcium carbonate; decades of piled up calcium carbonate cups create the reef.
Coral Colony
Credit: Photo Collection of Dr. James P. McVey, NOAA Sea Grant ProgramFlower-like clusters of polyps make up this coral colony. Their pink color comes from the zooxanthellae living inside. More about coral reef ecosystems can be found in our Coral Reefs featured story.
Bleached Corals, Pacific Ocean
Credit: Wolcott HenryWhen the reef is under stress from high temperatures, pollution, or other threats, the zooxanthellae abandon their coral hosts in a process called "bleaching." The coral animals can survive for a short time without their main food source by catching particles from the water with their tentacles, but they are more susceptible to disease and other disturbances. When they die, just the white skeleton is left behind as if it had been bleached.
Bleached Corals
Credit: Emma Hickerson/NOAAThese white corals in the Gulf of Mexico’s Flower Garden Bank National Marine Sanctuary are bleached due to an increase in water temperatures, which causes corals to lose the microscopic algae that provide them with food. Bleaching spells trouble for coral reefs.
Diseased Coral
Credit: Amanda FeuersteinA photo taken at a reef near Bocas del Toro, Panama. The reef suffered a mass bleaching event in the summer of 2010, when water temperatures were unusually high. Bleached corals have lowered defenses against disease, so often will suffer further damage and death as disease moves in after a bleaching event. In this photo, healthy brown coral gives way to the frontlines of disease.
Resilient Coral
Credit: Amanda FeuersteinIt can be hard for coral to recover for a bleaching event. The best bet is the some healthy tissue still remains deep in the skeleton and, if conditions improve, this coral can grow and recover, spreading to the rest of the skeleton. The corals pictured here are still in recovery after a mass bleaching in Panama in the summer of 2010. You can see some bleaching on the tops, but the sides are looking good.