Ultrastructure and molecular phylogeny of Mesodinium annulatum sp. nov. (Mesodiniidae, Cyclotrichiida), a new member of the Mesodinium rubrum / Mesodinium major complex

The species complex <i>Mesodinium rubrum</i> / major, common red tide-forming ciliates, has been intensively studied with regards to its ecological roles in global marine ecosystems and the evolutionary aspects of its “stolen” organelles (kleptoplasty and karyoklepty). Nonetheless, the taxonomy of the species within the complex remains unclear. A new marine Mesodinium species isolated from Gomso Bay, Korea, was cultivated under mixotrophic conditions by providing <i>Teleaulax amphioxeia</i>, a red cryptomonad, as prey. Cells of the new isolate consisted of two portions separated by two types of polykinetids. The number of polykinetid associated with the equatorial ciliary belt was approximately 38, and each consisting of two rows of up to 18 alternating kinetosomes each. There was an equal number of cirral polykinetids, each consisting of 16 kinetosomes organized into four longitudinal rows having five, five, four, and two kinetosomes, respectively (in anti-clockwise direction). The two kinds of kinetids and their associated microtubules and fibers were structurally similar to those of <i>M. rubrum</i> from Denmark. However, the Korean <i>Mesodinium</i> species was characterized by its broad posterior portion, 20–22 tentacles, and a cytopharyngeal annulus. Molecular phylogeny based on internal transcribed spacer sequences placed the Korean isolate in clade B of the <i>M. rubrum</i> / major species complex, rather than in clade F representing the neotype of <i>M. rubrum</i>. Based on morphological, ultrastructural, and molecular data, we propose the Korean strain as a new marine <i>Mesodinium</i> species, <i>M. annulatum</i>.