Stentor sp. and dinoflagellates. Light microscope footage of ciliate protozoan Stentor sp. (centre) amongst dinoflagellates (motile green cells). Stentor species are filter-feeding freshwater protozoa that can reach lengths up to 2 millimetres, and are among the largest known unicellular organisms. Dinoflagellates are also unicellular protozoans. About 90 per cent are found in marine environments as plankton but they also occur widely in fresh water. They range in size from 2 millimetres to less than a hundredth of a millimetre across.