Dinoflagellate protozoan. Coloured scanning electron micrograph (SEM) of a Ceratium sp. dinoflagellate. Dinoflagellates are unicellular protozoans. About 90% are found in marine environments as plankton.They are characterised by a complex covering called an amphiesma, which is made of a continuous outer and inner membrane. The amphiesma can be perforated. Another major characteristic is the presence of two flagella (not visible here). One flagellum beats back and forth, while the other is flat and ribbon-like, and works as a rudder to steer and rotate. They range in size from 2 millimetres to less than a hundredth of a millimetre across. Magnification: x1600 when printed at 10 centimetres wide. Specimen courtesy of Mike Allen, Plymouth Marine Laboratory.