Definition
Dinoflagellata is a group of single-celled eukaryotes, mostly marine plankton. They are characterized by two flagella: one encircling the cell like a belt, and another trailing behind. Many are photosynthetic, but some are predators or parasites. They are responsible for "red tides," harmful algal blooms that can kill fish. They're basically the rockstars of the microscopic world. Unlike diatoms, they can actively swim.