Alternate life history stages in Amphidinium klebsii (Dinophyceae, Pyrrophyta)

October field collections from a salt water marsh maintained in the laboratory revealed stages of cell fusion and nuclear cyclosis in Amphidinium klebsii Kofoid & Swezy. Cells were observed with one enlarged nucleus and two epicones oriented at 90° to 180° relative to each other. Such a configuration is not typical of dividing cells, suggesting that these cells represent planozygotes. Over time, the two epicones fuse. The product of the fusion is a pigmented cell exhibiting the normal vegetative cell morphology but possessing two trailing flagella. These latter cells enlarge in size and exhibit nuclear cyclosis (rotation of the contents of the nucleus). This rotation has been linked to meiosis I in other dinoflagellate zygotes. Within this same field collection, cells enclosed by a smooth cyst wall composed of a single layer of electron-translucent material were observed. These encysted cells exhibited the same morphology as free-swimming forms of Amphidinium observed in the collection. Transfer of cysts into fresh media produced free-swimming Amphidinium cells which were capable of subsequently forming cysts. Representatives of Amphidinium klebsii are therefore able to cycle between a free-living stage and an encysted stage. Although mitosis in Amphidinium normally occurs in the motile stage, cysts contained up to eight daughter cells, suggesting that the cysts are vegetative division cysts.