Morphological and genetic variability of Skeletonema dohrnii and Skeletonema japonicum (Bacillariophyta) from the northwestern Sea of Japan

Species of the genus Skeletonema are one of the most common marine planktonic diatoms of the temperate zone of the World Ocean. Despite the worldwide distribution of the genus, there are only fragmentary data on its ecology and quantitative characteristics of habitats, as well as limited information on the species diversity in the northeastern Pacific Ocean. In the present study, 13 strains of Skeletonema were obtained from planktonic and benthic samples of microalgae collected during the period from 2009 to 2017 from the northwestern Sea of Japan. We identified Skeletonema dohrnii and S. japonicum based on light and electron microscopic observations and molecular genetic analysis of 28S rDNA sequences. A detailed description, as well as new data on water temperature, salinity and species distribution, is provided for each species and their morphological features are discussed. The presence of several rimoportula processes on the terminal valve of Skeletonema species is shown for the first time. Our results support the usage of the topological criterion for the delimitation of the S. dohrnii-marinoi, as opposed to the distance-based criterion.