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.