Dinoflagellates are an important group of phytoplankton that produce oxygen in marine and freshwater. Some species form symbiotic relationships with larger animals, including corals (zooxanthellae), jellyfish, sea anemones, nudibranchs and others. Sometimes dinoflagellates grow out of control, to more than a million cells per milliliter, causing an algae bloom or red tide. Because some dinoflagellates produce toxins, when there are too many in the water, they can creep up the food chain, killing animals and making people sick. Learn more about red tides.