The genus Symbiodinium encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known to science. These unicellular algae commonly reside in the endoderm of tropical cnidarians such as corals, sea anemones, and jellyfish, where they translocate products of photosynthesis to the host and in turn receive inorganic nutrients (e.g. CO2, NH4+) (Fig. 1). They are also harbored by various species of sponges, flatworms, mollusks (e.g. giant clams), foraminifera (soritids), and some ciliates. Generally, these dinoflagellates enter the host cell through phagocytosis, persist as intracellular symbionts, reproduce, and disperse to the environment (note that in most mollusks, Symbiodinium are inter- rather than intra-cellular). Cnidarians that are associated with Symbiodinium occur mostly in warm oligotrophic (nutrient-poor) marine environments where they are often the dominant constituents of benthic communities. These dinoflagellates are therefore among the most abundant eukaryotic microbes found in coral reef ecosystems.
Symbiodinium are colloquially called "zooxanthellae" (or "zoox"), and animals symbiotic with algae in this genus are said to be "zooxanthellate". The term was loosely used to refer to any golden-brown endosymbionts, including diatoms and other dinoflagellates. Continued use of the term in the scientific literature should be discouraged because of the confusion caused by overly generalizing taxonomically diverse symbiotic relationships.
Read more about Symbiodinium: Intracellular Symbionts, Natural Services and Economic Value, Symbiodinium and Coral Bleaching, Molecular Systematics of Symbiodinium, Culturing Symbiodinium, Life Cycle