Genus of one-celled aquatic protists. Members are dinoflagellates with both plant and animal characteristics. The cell is flattened horizontally and contains yellow, brown, or green pigments. Ceratium species are covered by a theca, or armour, composed of many textured plates that form one horn in the front and usually two in the back; these horns slow the sinking of the organism. The body form varies according to the salt content and temperature of the surrounding water: the spines tend to be short and thick in cold, salty water and long and thin in less salty, warmer water. Members form an important part of the plankton found in northern seas.
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