Gymnodinium danicans cells are round from the ventral side, dorso-ventrally flattened. Length 13 16 microns, width 12 15 microns, breadth (lateral) approximately 5 microns. Cingulum begins 0.5 0.6 of the way down the cell, relatively wide, 2 - 3 microns, proximal end 1 2 microns lower than distal. Sulcus 2 3 microns wide, with prominent edges. Apical groove not present. A red, usually triangular shaped eyespot, approximately 3 microns long, in or just beside the sulcus. Longitudinal flagellum arising at the anterior end of the sulcus. Nucleus round, central, often to the left of the cell, 3 5 microns diameter. Chloroplasts yellow-green to yellow-brown, peripherally positioned, usually rounded to oval, 3 4 microns diameter. Fast swimming.
