Zooxanthellae

Zooxanthellae (; sing.zooxanthella) is a colloquial term for photosyntheticsingle-celled organisms capable of symbiosis with diverse marineinvertebrates such as corals, jellyfish, demosponges, and nudibranchs. Most known zooxanthellae are in the dinoflagellategenusSymbiodinium, but some are known from the genus Amphidinium, and other taxa, as yet unidentified, may have similar endosymbiont affinities. The genus name "Zooxanthella" (meaning literally "little yellow animal") was originally given by Karl Brandt in 1881 to (a mutualist of the radiolarianCollozoum inerme) which has been placed in the Peridiniales. Another group of unicellular eukaryotes that partake in similar endosymbiotic relationships in both marine and freshwater habitats are green algae zoochlorellae.

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