Ceratium, genus of single-celled aquatic dinoflagellate algae (family Ceratiaceae) common in fresh water and salt water from the Arctic to the tropics. As dinoflagellates, the organisms have two unlike flagella and have both plant and animal characteristics; their taxonomic placement as algae is contentious. Members of the genus form an important part of the plankton found in temperate-zone seas, and several are known to cause red tides and water blooms.
The cell contains chromatophores with yellow, brown, or green pigments. The theca, or armour, is composed of many textured plates that form one anterior horn and usually two posterior horns, which may help to slow the sinking of the cells. The spines tend to be shorter and thicker in cold salty water and longer and thinner in less-salty warmer water.