Symbiodinium is one of at least eight genera of dinoflagellate "algae" that occur as endosymbionts in various marine invertebrates and protists, forming mutualistic (mutually beneficial) symbioses with their hosts (Baker 2003). Symbiodinium, the best studied of the symbiotic dinoflagellates, are commonly (but not exclusively) found in shallow water tropical and subtropical cnidarians and in this context are often referred to as zooxanthellae ("little yellow animals", a reference to their typically golden-brown color). Among the diverse cnidarians known to host Symbiodinium are representatives of the class Anthozoa (including anemones, scleractinian corals, zoanthids, corallimorphs, blue corals, alcyonacean corals, and sea fans) and several representatives from the classes Scyphozoa (including rhizostome and coronate jellyfish) and Hydrozoa (including milleporine fire corals). Symbiodinium have also been identified from some non-cnidarians, including some gastropod and bivalve mollusks, foraminiferans, sponges, and a giant heterotrich ciliate.
Associations between particular Symbiodinium zooxanthellae and particular hosts are clearly nonrandom--i.e., there is some specialization of particular hosts on particular Symbiodinium species and specialization of particular Symbiodinium on particular host species. However, considerable flexibility is evident. It now appears that many (perhaps even most or all) hosts are able to associate with more than one type of Symbiodinium, and Symbiodinium appear to be even less specific than their hosts (i.e., a single Symbiodinium type has the potential to associate with a variety of hosts). The ability of a particular host species to associate with different Symbiodinium, which may perform differently in different ecological settings (e.g., functioning more efficiently in corals in shallow, high-light situations versus deep water low-light conditions) may allow host species to thrive in a much broader range of ecological conditions than would be possible if they were limited to associating with a single dinoflagellate species. Symbiotic dinoflagellates such as Symbiodinium are "keystone species" in coral reefs (i.e., species that have an impact on the community that is extremely large relative to their fraction of the total biomass of the community).
(Baker 2003 and references therein)