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

ABSTRACT

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.