These are unarmoured, solitary or chain-forming cells. The cingulum is usually equatorial or premedian, with or without descending (left) displacement; if displaced, then less than 20% of cell length (cf. Gyrodinium) (Steidinger & Tangen, 1997). See Larsen (1994, 1996) for a discussion of the separation of Gymnodinium and Gyrodinium. Individual species are differentiated by their size, shape and contours, chain formation, the occurrence and shape of the apical groove, the presence of striae, ribs or furrows, cingulum premedian, median or postmedian, the displacement of the cingulum, the extent to which the sulcus extends onto the epitheca, the presence of chloroplasts, lateral compression of the body and colour (Steidinger & Tangen, 1997). Identification is difficult, and we include here a series of images representing the range of morphologies seen.