1991. Fine structure and biology of Cryothecomonas gen.nov. (Protista incertae sedis) from the ice biota. Can. J. Zool. 69: 1048-1070. The morphology and ultrastructure of four species of Cryothecomonas gen.nov. (Protista incertae sedis) in material from the Weddell Sea, Antarctica, and the Isefjord, Denmark, are described. These heterotrophic flagellates, which were initially observed in association with sea ice, display a unique combination of morphological characteristics. At present it is impossible to assign the new genus to an existing higher taxonomic level of protistan flagellates. Cryothecomonas species are furnished with a close-fitting multilayered theca. The two naked anterior flagella emerge through narrow thecal funnels. A transitional helix is part of the flagellar transition zone. A conspicuous cytostome is located in a posterior (lateral) position. Food uptake is mediated through the extension of cytostomal pseudopodia. The nucleus is anteriorly located and contains a conspicuous nucleolus and distinct areas of chromatin. Mitochondria1 cristae are tubular. Cryothecomonas species feed on cells in the size range 2-4.5 k m (e.g., algal flagellates). Data are presented on the abundance of Cryothecomonas armigera sp.nov. in Antarctic waters.

The small (< 15 μm) hypnozygote of an autotrophic athecate dinofiagellate found in association with Antarctic sea ice had an external covering composed of approximately 60 plates, each of which was bounded by sutural ridging and possessed an intratabular process. A cingulum and sulcus were also evident. The ultrastructure of the cyst was increasingly dominated by storage bodies as the cyst matured, and the cell wall thickened from 0.2 to 0.8 μm over 2 months. This cyst has been encountered often but usually at low abundances (103−104 cells·L−1); however, the maximum abundances observed (106 cells·L−1) indicate that the formation of this cyst may play an important part in the ecology of sea ice communities.