The life stages of dinoflagellates in the genus Symbiodinium. (A) Electron micrographs of a Symbiodinium mastigote (motile cell) with characteristic gymnodinioid morphology (S. natans; Hansen & Daugbjerg 2009) and (B) the coccoid cell in hospite. As free-living cells the mastigote allows for short-range dispersal and can exhibit chemo-taxis toward sources of nitrogen. Once within the host, these symbionts rapidly proliferate in and in many cases dominate the cytoplasm of the host cell.