any of numerous one-celled, aquatic organisms bearing two dissimilar flagellae and having characteristics of both plants and animals. Most are microscopic and marine. Botanists place them in the algal division Pyrrophyta, and zoologists claim them as members of the protozoan order Dinoflagellida. Dinoflagellates range in size from about 5 to 2,000 micrometres (0.0002 to 0.08 inch). Nutrition among dinoflagellates is plantlike, animal-like, or mixed; some species are parasitic or commensal. About one-half of the species are photosynthetic, but even among these many are also predatory. The group is an important component of phytoplankton in all but the colder seas and is an important link in the food chain. Dinoflagellates also produce some of the luminescence sometimes seen in the sea.

The dinoflagellate cell is banded by a median or coiled groove, the annulus, which contains a flagellum. A longitudinal groove, the sulcus, extends from the annulus posteriorly to the point at which a second flagellum is attached. The nuclei of dinoflagellates are larger than those of other eukaryotes. So-called armoured dinoflagellates are covered with cellulose plates, which may have long, spiny extensions; some species lacking armour have a thin pellicle (protective layer). Photosynthetic dinoflagellates have yellowish or brownish plastids (pigment-containing bodies) and may store food in the form of starches, starchlike compounds, or oils.

Although sexual processes have been demonstrated in a few genera, reproduction is largely by binary or multiple fission. Under favourable conditions dinoflagellate populations may reach 60,000,000 organisms per litre of water (water bloom). Such dense growths, called blooms, can result in the red tides that discolour the sea and may poison fish and other marine animals. Some dinoflagellates produce toxins that are among the most poisonous known.

For additional information on specific dinoflagellate genera, see Ceratium; Gymnodinium. See also red tide.