Abstract
A new, planktonic, marine dinoflagellate of the genus Heterocapsa, Heterocapsa minima sp. nov. is described. The thecal tabulation is p.p., c.p., 5′, 3a, 7″, 6c, a.s., r.s., l.a.s., l.p.s., p.s., 5′″, 2″″ and is identical to that of other members of the genus. The spherical nucleus is posterior to the cingulum and numerous chloroplasts are parietally arranged. Heterocapsa minima is the smallest member of the genus and is distinguishable by its size and morphology. It is present throughout the year in the Celtic Sea and may reach densities of 62,400 cells 1-1 in bloom conditions.