Zooxanthellae are microscopic dinoflagellates (Symbiodinium spp.) that live inside the tissues of larger animals, mostly corals. Their photosynthesis provides a large amount of nutrition for the corals, which supplement it with capturing zooplankton from the water with their tentacles. When the corals are stressed, usually by excessive temperatures, the zooxanthellae are expelled (or leave) resulting in "bleaching" of the coral. If bleaching persists over a long time, the coral does not get enough nutrition and will die. Bleached corals have become more common as temperatures have warmed due to global climate change. It is possible that some strains of Symbiodinium are more temperature-tolerant, and may replace the less tolerant strains, enabling more corals to survive climate change. Zooxanthellae are also found in other animals including some species of jellyfish and in giant clams (Tridacna spp.).
Judith S. Weis (Lead Author);Marion McClary (Topic Editor) "Zooxanthellae". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth January 31, 2007; Last revised Date January 31, 2007; Retrieved May 23, 2013 <http://www.eoearth.org/article/Zooxanthellae>