A coccolithophore (or coccolithophorid, from the adjective) is a unicellular, eukaryotic phytoplankton (alga). They belong either to the kingdom Protista, according to Robert Whittaker's Five kingdom classification, or clade Hacrobia, according to the newer biological classification system. Within the Hacrobia, the coccolithophorids are in the phylum or division Haptophyta, class Prymnesiophyceae (or Coccolithophyceae). Coccolithophorids are distinguished by special calcium carbonate plates (or scales) of uncertain function called "coccoliths", which are also important microfossils. However, there are Prymnesiophyceae species lacking coccoliths (e.g. in genus "Prymnesium"), so not every member of Prymnesiophyceae is a coccolithophorid. Coccolithophores are almost exclusively marine and are found in large numbers throughout the sunlight zone of the ocean.