A marine photosynthetic protist possesses stiff cellulose plates and two flagella, one longitudinal and one transverse.
These features identify a dinoflagellate. Red dinoflagellates such as Gonyaulax may multiply so rapidly that the sea appears red. Toxins released by such large populations may kill marine animals, including fishes.
Ref: NCERT Class 11 Biology Chapter 2: Biological Classification Protista - Chrysophytes and Dinoflagellates