Red tide is the appearance of the sea when the red dinoflagellates (example: Gonyaulax ) undergo rapid multiplication. Depending on the pigments present in their cells they derive the colour, red, blue, brown, green or yellow and among these the red dinoflagellates very often, multiply rapidly and become so numerous that they discolor the coastal waters, hence the name red tides. These organisms also deplete the oxygen in waters and release toxins which may cause illness in other aquatic animals and humans. The first option is dinoflagellate. The second one is Euglena, third one is Amoeba and the fourth one is Paramoecium.