Thirty clones of Thoracosphaera heimii (Lohmann) Kamptner have been isolated and cultured from oceanic waters of the western North Atlantic and Gulf of Mexico. Observations on the life cycle and morphology of one clone isolated from the Sargasso Sea are reported herein. The calcareous cell wall is present in the coccoid, vegetative life phase. Reproduction is accomplished by the formation of transitory aplanospore or planospore stages or occasionally by binary fission of weakly calcified cells. The planospore is non-thecate and Gymnodinium-like with an undulating transverse flagellum and a whip-like longitudinal flagellum. All life stages possess chloroplasts and a nucleus with continually condensed chromosomes. The planospore morphology and the dinocaryotic nucleus demonstrate that T. heimii is a dinophyte and not a coccolithophorid. The taxonomic affinity and classification of T. heimii within the Dinophyceae is discussed and a new order Thoracosphaerales Tangen, ord. nov. is proposed for primarily coccoid marine dinoflagellates that possess a calcified cell wall in the vegetative life phase.
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