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Scientific classification
Domain: Eukaryota
Kingdom: Chromalveolata
Superphylum: Alveolata
Phylum: Dinoflagellata
Class: Dinophyceae
Order: Suessiales
Family: Blastodiniaceae
Genus: Symbiodinium

Zooxanthellae (plural, pronounced /ˌzoʊ.əzænˈθɛliː/) are flagellate protozoa that are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans such as the scleractinian corals and the tropical sea anemone, Aiptasia.

Zooxanthella live in other protozoa (foraminiferans and radiolarians) and in some invertebrates. Most are autotrophs and provide the host with energy in the form of translocated reduced carbon compounds, such as glucose, glycerol, and amino acids, which are the products of photosynthesis .[1] Zooxanthellae can provide up to 90% of a coral’s energy requirements.[2] In return, the coral provides the zooxanthellae with protection, shelter, nutrients (mostly waste material containing nitrogen and phosphorus) and a constant supply of carbon dioxide required for photosynthesis. Available nutrients, incident light, and expulsion of excess cells limits their population.

Hermatypic (reef-building) corals largely depend on zooxanthellae, which limits that coral's growth to the photic zone. The symbiotic relationship enables corals' success as reef-building organisms in tropical waters. However, under high environmental stress, corals die after losing their zooxanthellae either by expulsion or digestion.


Coral bleaching

Normal coral (left) and bleached coral (right).

Coral bleaching occurs when zooxanthellae densities within coral tissue become low or the concentration of photosynthetic pigments within each zooxanthella decline. Color loss also comes from reduced concentrations of Green Fluorescent Proteins (GFP) from the cellular pigments of the cnidarian itself. The result is a ghostly white calcareous skeleton. The coral will then die unless conditions improve enough to allow the zooxanthellae to return.

Zooxanthellae directly or indirectly experience the stress that their containing corals undergo. Exposure to air during low tides and damage from solar radiation in shallow water environments are two of the ecological stressors coral and zooxanthellae face. Temperature changes now provide the most stress to the zooxanthellae-coral relationship. A 1-2 degree Celsius temperature rise for 5–10 weeks and a 3-5 degree decline for 5–10 days have produced a coral bleaching event. Such temperature changes induce cell adhesion dysfunction which detaches zooxanthellae from their cnidarian endodermal cells.


There is considerable disagreement as to whether there are a single or many species of Symbiodinium. DNA testing shows differences between the symbionts from different corals, but the issue is whether or not these are significant enough to represent different species.

Some see several different species of zooxanthellae within Symbiodinium, which appears to be monophyletic.[3] Symbiodinium is related to Gymnodinium simplex, Gymnodinium beii, and Polarella glacialis.[3]

Examples of zooxanthellae are Cryptomonas (see cryptomonad) and Chrysidella.

Life cycle

The genus, Symbiodinium, was defined by Hugo Freudenthal in 1970, after his identification of the life cycle of zooxanthella from Cassiopea.[citation needed] At that time he proved that symbodinia had a motile stage which resembled a "gymnodinioid" dinoflagellate. Being both symbiotic and a dinoflagellate, he named the genus Symbiodinium, and the species epithet microadriaticium, after its resemblance to a similar free-living species.

Dr. Freudenthal demonstrated that zooxanthellae go through a vegetative stage, a cyst stage, and a motile stage as part of their life cycle. Note that both hermatypic (zooxanthellae containing) and ahermatypic corals may be reef-building.

Coral acquisition

Polyps can acquire Zooxanthella by direct ingestion. However, their hosts do not digest them. In other cases, zooxanthellae may be transmitted by coral eggs and planulae.

Zooxanthella reproduce asexually by budding (one individual splitting into multiple descendants).

Other animal relationships

Other organisms which can host zooxanthellae include jellyfish, clams, foraminifera, sea slugs i.e. nudibranchs such as Pteraeolidia ianthina, and also ciliates and radiolaria.


Rudman, W.B., 2000 (October 10) What are Zooxanthellae?. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Riddle, D., 2006 (January) Lighting by numbers: ”types” of Zooxanthellae and what they tell us. Advanced Aquarist’s Online Magazine. Available from

Buchheim, J., 1998 Coral Reef Bleaching. Odyssey Expeditions – Marine Biology Learning Center Publications. Available from

External links



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