AMPHIDINIUM CRYOPHILUM SP. NOV. (DINOPHYCEAE) A NEW FRESHWATER DINOFLAGELLATE. I. SPECIES DESCRIPTION USING LIGHT AND SCANNING ELECTRON MICROSCOPY1

Amphidinium cryophilum sp. nov. was found in the fall of 1979 in a small pond near Madison, Wisconsin. During the ensuing winter, it became the dominant phytoplankter. Cell numbers remained high despite a thick layer of ice and snow. After the ice melted in the spring the organism disappeared from plankton samples. A successful culture of A. cryophilum was established only when isolates were incubated at 5–7° C. It is compared with two morphologically similar species, A. amphidinioides (Geitler) Schiller and Gymnodinium inversum Nygaard. Amphidinium cryophilum is distinguished from the former by its pigmentation (golden-yellow vs. blue-green), the location of the cingulum, and its lack of an eyespot. It differs from the latter in cell shape, the route of the sulcus and position of the nucleus.