The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. They are characterized by two flagella, one girdling the cell and the other trailing the cell. Some dinoflagellates exist in coral, in a symbiotic relationship. These dinoflagellates are termed the zooxanthellae. Other dinoflagellates occur in such high numbers that the water is colored red, a phenomenon known as a red tide.