Dinoflagellates are any of numerous one-celled aquatic organisms bearing two dissimilar flagella and having characteristics of both plants and animals. Most are marine, though some live in freshwater habitats. 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 bioluminescence sometimes seen in the sea. Under certain conditions, several species can reproduce rapidly to form water blooms or red tides that discolor the water and may poison fish and other animals. Some dinoflagellates produce toxins that are among the most poisonous known. Dinoflagellates range in size from about 0.0002 to 0.08 inc. Most are microscopic, but some form visible colonies. Nutrition among dinoflagellates is autotrophic, heterotrophic, or mixed; some species are parasitic. About one-half of the species are photosynthetic; even among those, however, many are also predatory. 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 million organisms per litre of water. As their name implies, flagella are present. Indeed, the term dinoflagellate means whirling flagella. Typically, there are two flagella. One of these circles around the body of the cell, often lying in a groove called the cingulum. The other flagellum sticks outward from the surface of the cell. Both flagella are inserted into the dinoflagellate at the same point. The arrangement of the flagella can cause the organism to move in a spiral trajectory. Some dinoflagellates contain cell walls of cellulose that lie between the two surface membranes that cover the organism. These walls function as protective armor.