Shionodiscus karianus sp. nov. (Thalassiosirales, Thalassiosiraceae), a new planktonic diatom from the Kara Sea and notes on some morphological characters of Shionodiscus taxa

The genus Shionodiscus (Thalassiosirales, Thalassiosiraceae) includes species that have strutted processes with long inward but reduced outward extensions, a labiate process typically located on the valve face, distant from the margin. Shionodiscus species inhabit the marine environment, and the genus includes both living and fossil species. In the course of our study of plankton samples from the Kara Sea, we found specimens belonging to Shionodiscus. Based on LM, SEM, and TEM investigations, a new species, Shionodiscus karianus sp. nov., is described. This species is distinguished from others in the genus based on areola densities, as well as the fine structure and location of the rimoportula and the central fultoportula and the ultrastructure of the areolae and cingulum. In this paper, we provide a morphological description of this taxon as well as a comparison with similar taxa. Previous records of Shionodiscus species in the Russian Arctic seas are discussed. Finally, we summarize and evaluate information on the important morphological characters, such as rimoportula position and fultoportula structure, for Shionodiscus.