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Zooxanthellae sp.Photo by Scott R. Santos.


Higher order taxa

Eukaryota; Alveolata; Dinophyceae


Symbiodinium microadriaticum Symbiodinium californium Symbiodinium mustcatinei

Description and Significance

Zooxanthellae species are members of the phylum Dinoflagellata. However, this is not a taxonomic name. Instead, it refers to a variety of species that form symbiotic relationships with other marine organisms, particularly coral. The most common genus is Symbiodinium. Not all Zooxanthellae are endosymbionts; some are free-living. Typically, Zooxanthellae form relationships with organisms simply because they inhabit the same area. However, there are other ways for organisms to acquire Zooanthellae endosymbionts. In the sea anenome Anthopleura ballii, Zooxanthellae are inherited maternally. This, however, is a rare phenomenon.

Genome Structure

There is not yet an extensive body of research on the numerous genome structures within the Zooxanthellae category.

Cell Structure and Metabolism

Zooxanthellae in Xenia. [ http://www.seaslugforum.net/factsheet.cfm?base=zoox2 W. B. Rudman, "Zooxanthellae in Cnidarians."]

Zooxanthellae are unicellular organisms with a spherical shape. They have two flagella, although these are lost if the organism is acquired by a host. This is called the coccoid state.

Zooxanthellae are mixotrophic organisms. They are mainly photosynthetic organisms (photoautotrophic). However, some species can also obtain food by ingesting other organisms.

Asexual reproduction by division is the most common form of reproduction. Zooxanthellae typically spend their entire life on the organism to which they are attached. The exception is when coral bleaching occurs, and the Zooxanthellae are expelled from the coral.


​​​​Zooxanthellae are known for their symbiotic relationships with coral. Zooxanthellae often suffer from bacterial infections that attack corals. For example, the bacteria that causes Yellow Band/Blotch Disease (YBD) in Montastraea species actually affects the Zooxanthellae endosymbionts rather than the actual organism. Many bacterium interfere with the photosynthetic processes of these organisms. Zooxanthellae can help host coral harvest light. This helps the host meet its carbon and energy needs. In addition, Zooxanthellae give host corals their color. The research of Levy et. al. (2003) indicates that corals with continuously extended tentacles have denser populations of Zooxanthellae. Coral bleachings are caused by a disruption in these relationships. Symbiotic relationships with corals and other organisms are common in tropical waters with a low abundance of nutrients. These relationships are significantly less common in temperate waters.

The Adaptive Bleaching Hypothesis (ABH) suggests that if the loss of Zooxanthellae occurs due to environmental change, the host organism forms a new symbiotic relationship with a different type of Zooxanthellae. These new endosymbionts are blelieved to be better adapted to the new environment. Other research on the adaptations of coral and Zooxanthellae suggest that corals that have been damaged due to high temperatures contain an abundance of Zooxanthellae that are thermally tolerant (Baker et. al. 2004). The symbiont changes during the stress period. It is suggested that these corals will be resistant to future thermal stress because they now have an endosymbiont that will better help them manage these environmental conditions. Rowan (2004) also shows that corals adapt to high temperatures by hosting Zooxanthellae that are specifically adapted to such conditions.

In addition to living in coral, Zooxanthellae can inhabit clams, nudibranches, flatworms, octocorals, sea anenomes, hydrocorals, mollusks, zoanthids, sponges, Foraminifera, and jellyfish.

Left: A coral with Zooxanthellae. Right: A bleached coral that has lost its Zooxanthellae.National Ocean Service.


Baker, Andrew C., Craig J. Starger, Tim R. McClanahan, and Peter W. Glynn. "Corals' adaptive response to climate change." Nature. 12 August 2004; 430:741.

Banin, Ehud, Sanjay K. Khare, Fred Naider, and Eugene Rosenberg. "Proline-Rich Peptide from the Coral Pathogen Vibrio shiloi That Inhibits Photosynthesis of Zooxanthellae." Appl Environ Microbiol. 2001 April; 67(4): 1536–1541.

Ben-Haim, Yael, Maya Zicherman-Keren, and Eugene Rosenberg. "Temperature-Regulated Bleaching and Lysis of the Coral Pocillopora damicornis by the Novel Pathogen Vibrio coralliilyticus." Appl Environ Microbiol. 2003 July; 69(7): 4236–4242.

Cervino, James M., Raymond L. Hayes, Shawn W. Polson, Sara C. Polson, Thomas J.Goreau, Robert J. Martinez, and Garriet W. Smith. "Relationship of Vibrio Species Infection and Elevated Temperatures to Yellow Blotch/Band Disease in Caribbean Corals." Appl Environ Microbiol. 2004 November;70(11): 6855–6864.

Davy, Simon K. and John R. Turner. "Early Development and Acquisition of Zooxanthellae in the Temperate Symbiotic Sea Anemone Anthopleura ballii (Cocks)." Biol. Bull. 205: 66-72. (August 2003)

Heatherwick, Pete and Sue Heatherwick. "Guide to the Great Barrier Reef." Accessed 5 July 2005.

Ho, Leonard. "Zooxanthellae." March 8, 1998. Accessed 5 July 2005.

Levy, O., Z. Dubinsky and Y. Achituv. "Photobehavior of stony corals: responses to light spectra and intensity." The Journal of Experimental Biology 206, 4041-4049 (2003).

Kinzie, III, Robert A., Michelle Takayama, Scott R. Santos, and Mary Alice Coffroth. "The Adaptive Bleaching Hypothesis: Experimental Tests of Critical Assumptions." Biol. Bull. 200: 51-58. (February 2001).<nobr> </nobr></a>

National Ocean Service.

Rowan, Rob. "Thermal adaptation in reef coral symbionts." Nature. 12 August 2004;430:742.

Rudman, W.B. "What are Zooxanthellae?" Sea Slug Forum. Australian Museum, Sydney. October 10, 2000. Accessed 5 July 2005.

Rudman, W.B. "Zooxanthellae in Cnidarians." Sea Slug Forum Austrailan Museum, Sydney. October 10, 2000. Accessed 5 July 2005.

Sea World/Busch Gardens Animal Information Database. Accessed 5 July 2005.

Zooxanthellae Mania. Accessed 5 July 2005.