Gymnodinium. Coloured scanning electron micrograph (SEM) of Gymnodinium. Gymnodinium is a genus of dinoflagellates. It is one of the few naked dinoflagellates, or species lacking armor (cellulosic plates). Gymnodinium is a toxic bloom former that causes red tides. Brevetoxins produce neurotoxic shellfish poisoning in humans when consumed. Magnification: x4000 when printed 10cm wide. Specimen courtesy of Mike Allen, Plymouth Marine Laboratory.