Zooxanthellae are
dinoflagellate algae, found as
endosymbionts of various marine animals and protozoa. They are acquired by direct ingestion, and subsequently multiply in the host's tissues, providing it with energy and various nutrients. Their population is limited by controlling the amount of food and light they receive and by digestion of excess cells. Most tropical
corals have zooxanthellae, and are largely dependent on them, limiting their growth to the
photic zone. Other organisms which may have zooxanthellae include
jellyfish,
clams,
sea slugs[?], and
radiolaria. There are several different species, typically placed in the genus
Symbiodinium, which appears to be
monophyletic.