Morphology and Molecular Phylogeny of the Planktonic Dinoflagellate <i>Mesoporos perforatus</i> : A New Lineage of <i>Prorocentrum</i> (Prorocentrales, Dinophyceae)

The dinoflagellate order Prorocentrales comprises two genera: Prorocentrum, which has been extensively studied, and the little-known Mesoporos. We established a monoclonal culture of Mesoporos perforatus from Long Island Sound, USA. The high variability in cell size (12-27 μm) and outline, together with irregular chloroplast morphology, suggests that previously described congeneric species may represent synonyms. The principal generic diagnostic character, commonly referred to as a pore, is not a true perforation but a conical depression located at the center of the theca; its function remains unknown. Cell morphology (cell size and shape, thecal ornamentation, and short apical spine) suggests affinities with P. cordatum and related species. However, a first detailed study of the periflagellar structure reveals differences such as platelet 8 separating platelets 2 and 3, a feature unreported in Prorocentrum species. Our first report of molecular data (SSU, LSU rDNA, and ITS regions) places M. perforatus as a species nested within Prorocentrum sensu stricto, with P. nux as the closest relative. Accordingly, we consider Mesoporos a synonym of Prorocentrum, and this species should be referred to as Prorocentrum perforatum (Gran) Krachmalny 1993, despite the absence of a true thecal perforation.