Zooxanthella

Zooxanthellae are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans. They are typically dinoflagellate algae, although other algae such as diatoms can also be zooxanthellae. They are often acquired by direct ingestion, and subsequently multiply in the host's tissues, providing it with various nutrients. Most are autotrophs and provide the host with energy from photosynthesis. Their population in the host tissue is limited by controlling the amount of food and light they receive and by expulsion of excess cells. There are also zooxanthellae transmitted by the coral eggs.

Hermatypic (reef-building) corals have zooxanthellae and are largely dependent on them, limiting their growth to the photic zone. The symbiotic relationship is probably responsible for the phenomenal success of corals as reef-building organisms in tropical waters. However, when corals are subjected to high environmental stress, they can lose their zooxanthellae by either expulsion or digestion and die, changing from their normal colour to their white 'skeletons' in a process known as coral bleaching.

Other organisms which may have zooxanthellae include jellyfish, clams, sea slugs, and radiolaria. There are several different species of zooxanthellae, typically grouped together as the genus Symbiodinium, which appears to be monophyletic.