Dinoflagellates representing at least eight genera in four (or five) classical dinoflagellate orders occur as endosymbionts in marine invertebrates and protists (Baker 2003). These eight genera, which are not all closely related, form mutualistic (mutually beneficial) symbioses with a wide range of hosts. Particularly well studied and of especially far-reaching ecological importance are the symbiotic relationships between certain dinoflagellates and their scleratinian coral hosts. These endosymbiont dinoflagellates are photosynthetic and the energy they capture through photosynthesis and transfer to their hosts is critical to the maintenance and growth of coral reefs.
A healthy coral reef might easily contain >1010 algal symbionts per m2 (Baker 2003). Despite their tremendous abundance, however, as a result of their tiny size the total dinoflagellate biomass is very small relative to the entire biomass of a coral reef community. Thus, given the critical importance of these dinoflagellate symbionts to the health of the corals and, by extension, to the well being of the diverse invertebrates, fish, and other organisms dependent on the coral, Baker (2003) suggests that symbiotic dinoflagellates in coral reefs are "keystone species" (i.e., species that have an impact on an ecological community that is extremely large relative to their fraction of the total biomass of the community).
The best studied of the symbiotic dinoflagellates are those in the genus Symbiodinium, which 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) (Baker 2003). Symbiodinium have also been identified from some non-cnidarians, including some gastropod and bivalve mollusks, foraminiferans, sponges, and a giant heterotrich ciliate (Baker 2003 and references therein). 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) (Baker 2003 and references therein). 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 (Baker 2003 and references therein).
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