Flagellated protists, some of which contain chloroplasts derived from secondary endosymbiosis. They have two flagella: one which emerges from the posterior end of the cell (the longitudinal flagellum) and the other which encircles the waist (the transverse). There is usually a groove (cingulum) in the cell wall at the waist in which the transverse flagellum sits. Present in marine and freshwater environments.
Many species are photosynthetic, including those marine dinoflagellates which live symbiotically within the cells of corals. They are called zooxanthellae, and supply nutrition to the coral animals. Rising sea temperatures and other forms of stress can cause the zooxanthellae to be expelled from the coral polyps, a phenomenon called “coral bleaching”.
Some species are also capable of bioluminescence, giving off light when disturbed, e.g. in the wake of a ship. The luminescence may also follow a circadian pattern.
Empty dinoflagellate tests can sometimes be found among sediment.
This dinoflagellate may be a Gymnodinium.
The flagella are used for swimming, but one is hidden from view in the cingulum.
Another species swims in this video, but too swiftly for a certain identification: