This paper presents a comprehensive examination of the taxonomy of the
genus Ostreopsis Schmidt. The morphology of six species of marine din
oflagellates, Ostreopsis siamensis Schmidt 1902, Ostreopsis lenticular
is Fukuyo 1981, Ostreopsis ovata Fukuyo 1981, Ostreopsis heptagona Nor
ris, Bomber, et Balech 1985, Ostreopsis mascarenensis Quod 1994, and O
streopsis labens Faust et Morton 1995 from three geographical regions
(Japan, Southwest Indian Ocean, and the Caribbean) and three marine ha
bitats (sand, water column, and macroalgal surfaces) are described fro
m scanning electron micrographs. Differences in the following morpholo
gical characteristics differentiated the species: cell shape and size,
and ornamentation of the epitheca, cingulum, and hypotheca. The theca
l plate formula of the six Ostreopsis species is P-0, 3', 7'', 6C, 6S?
, V-p, R(p), 5''', 1p, 2'''', with differences in thecal plate size an
d shape. The cingulum in ventral view has two prominent structures: a
ventral plate (V-p) with a ventral pore (V-0) and a ridged plate (R(p)
) that distinguishes Ostreopsis species from any other dinoflagellate
taxa. This paper also includes ecological and toxicity information reg
arding the six Ostreopsis species.