The name means 'fire plankton', derived from the luminous colors of the genus. These are unicellular organisms, able to synthesize both types of chlorophyll. Pyrrophyta are algae incorporating some dangerous species which can cause oxygen starvation in bodies of water, 'black tides'. Some species can also cause red tides in areas of high population density. There are two forms of pyrrophyta called dinoflagellates and criptomonads. Dinoflagellates are free swimmers with two flagellates, criptomonads have one. Some forms have common characteristics with animal unicellular organisms, and classification is sometimes blurred by definitions.