Zooxanthellae are single-celled protozoans  (dinoflagellates) that are able to live in symbiosis with certain marine invertebrates. In freshwater habitats, similar endosymbionts are known as zoochlorellae. 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.