Zooxanthella

Zooxanthellae are single-celled dinoflagellates that are able to live in symbiosis with marine invertebrates such as corals, jellyfish, and sea anemones. Most known zooxanthellae are in the genus Symbiodinium but some are known from the genus Amphidinium, and other taxa, as yet unidentified, may have similar endosymbiont affinities. In freshwater habitats, similar endosymbionts are known as zoochlorellae.

Zooxanthellae are photosynthetic organisms, which contain chlorophyll a and chlorophyll c, as well as the dinoflagellate pigments peridinin and diadinoxanthin. These provide the yellowish and brownish colours typical of many of the host species. During the day, they provide their host with the organic carbon products of photosynthesis, sometimes providing up to 90% of their host's energy needs for metabolism, growth and reproduction. In return, they receive nutrients, carbon dioxide, and an elevated position with access to sunshine.

Dinoflagellates are microscopic single-celled planktonic organisms that used to be classified as algae but are now considered to be in a large and diverse group known as Chromalveolata. They can be free-living or can be present in large number in the tissues of a host. The motile, mastigote stage in the water column has two flagella and a characteristic method of swimming, while inside a host it metamorphoses into a spherical, non-motile, coccoid stage without any flagella.

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