Some species glow in the dark in a process called bioluminescence. These species contain a compound called luciferin (the same compound found in fireflies). The glow increases markedly if the algae cells are agitated, as when a ship churns through the water.
About half the species of dinoflagellates are photosynthetic; the other half are predators that attack bacteria, algae, and even fish.
Dinoflagellate neurotoxins can concentrate in the bodies of shellfish and fish that eat the algal cells, in turn causing people who eat these seafoods to come down with illnesses such as paralytic shellfish poisoning and ciguatera (a combination of gastrointestinal, neurological, and cardiovascular disorders.)
So-called “red tides” occur when enormous blooms of trillions of dinoflagellates are triggered by an upwelling of nutrients from the water’s depths during warmer seasons. The population of dinoflagellates can jump to more than 20 million cells per liter of sea water along some coasts during these blooms, turning the water a reddish hue