Zooxanthellae and Coral Bleaching
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.
Close-up of a Coral Polyp
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.
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Where Do They Live?As 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.
Flower-like clusters of pink polyps make up this coral colony.
Photo Collection of Dr. James P. McVey, NOAA Sea Grant Program
Bleached Corals, Pacific OceanWhen 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.
It 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.