<i>WOLOSZYNSKIA HALOPHILA</i> (BIECHELER) COMB. NOV.: A BLOOM‐FORMING COLD‐WATER DINOFLAGELLATE CO‐OCCURRING WITH <i>SCRIPPSIELLA HANGOEI</i> (DINOPHYCEAE) IN THE BALTIC SEA¹

Molecular analyses and subsequent morphological reinvestigation of clonal isolates germinated from cysts previously assigned to Scrippsiella hangoei (Schiller) Larsen revealed considerable differences to vegetative cell isolates of this cold‐water dinoflagellate from the northern Baltic Sea. The presence of hexagonal platelets on the cell surface and a characteristic acrobase on the episome agree with the description of Gymnodinium halophilum Biecheler. However, the arrangement of amphiesmal vesicles in more than nine latitudinal series indicates allocation of this dinoflagellate to Woloszynskia Thompson. We therefore reassign G. halophilum to Woloszynskia halophila . This species exhibits ultrastructural characteristics similar to Polarella glacialis Montresor et al. and symbiontic Gymnodinium Stein, such as stalked pyrenoids and a central eyespot consisting of multiple layers of crystal‐filled vacuoles. A close relationship between these dinoflagellates is also supported by 28s rRNA sequence data. The preference for high salinities identifies W. halophila as a marine species. The spiny resting cysts of W. halophila are identical to the cysts formed during the massive encystment events previously attributed to S. hangoei in the Baltic Sea. This suggests that W. halophila is a significant contributor to the dinoflagellate spring blooms in the Baltic Sea. Scrippsiella hangoei clones, in turn, produce noncalcareous and smooth‐walled cysts when crossed with a complementary mating type.