DinoXagellate diversity and distribution

DinoXagellates are common to abundant in both marine and freshwater envi- ronments. They are particularly diverse in the marine plankton where some cause "red tides" and other harmful blooms. More than 2,000 extant species have been described, only half of which are photosynthetic. They include autotrophs, mixotrophs and grazers. They are biochemically diverse, varying in photosynthetic pigments and toxin production ability. Some are important sources of bioluminescence in the ocean. They can host intracellular symbionts or be endosymbionts themselves. Most of the photosynthetic "zooxanthellae" of invertebrate hosts are mutualistic dinoXagellate symbionts, including all those essential to reef-building corals. Roughly 5% are parasitic on aquatic organisms. The fossil record, consisting of more than 2,500 species, shows a rapid radiation of cysts, starting in the Tri- assic, peaking in the Cretaceous, and declining throughout the Cenozoic. Marine species with a benthic, dormant cyst stage are conWned to the continental shelf and fossil cysts can be used as markers of ancient coastlines. Northern and southern hemispheres contain virtu- ally identical communities within similar latitudes, separated by a belt of circumtropical species. A few endemics are present in tropical and polar waters. Some benthic dinoXagel- lates are exclusively tropical, including a distinct phycophilic community, some of which are responsible for ciguatera Wsh poisoning. In lakes chemical and grazing eVects can be important. Predatory dinoXagellates co-occur with their prey, often diatoms.