Description of the new heterotrophic dinoflagellate Stoeckeria changwonensis n. sp. from Korean coastal waters and emended description of the genus Stoeckeria and the type species Stoeckeria algicida

• The new heterotrophic dinoflagellate Stoeckeria changwonensis was described.
• Its SSU, ITS, and LSU rDNA sequences were 2.2–16.8% different from S. algicida.
• A clade of S. changwonensis was clearly divergent from a clade of S. algicida.
• Size and position of its cingular plates were different from those of S. algicida.
• We emended the description of S. algicida to include previously unreported details.

The planktonic heterotrophic dinoflagellate Stoeckeria changwonensis n. sp. isolated from the coastal waters off Korea is described from living and fixed cells by light, scanning electron (SEM), and transmission electron microscopy (TEM). For comparison with closely related species, the ultrastructure of Stoeckeria algicida, the only Stoeckeria species previously known, using TEM was also examined. Furthermore, DNA sequence data were collected from the small subunit (SSU), the D1–D3 large subunit (LSU), internal transcribed spacer regions (ITS1 and ITS2), and 5.8S of the ribosomal rDNA of S. changwonensis and compared to S. algicida. The SSU, ITS1-5.8S-ITS2, and the D1–D3 LSU rDNA sequences of the new dinoflagellate were ca. 2.2%, 16.8%, and 5.4% different, respectively, from those of S. algicida, but very similar to those of an unidentified dinoflagellate strain from Florida, USA, known as “Shepherd's Crook”. In the phylogenetic tree based on D1–D3 LSU rDNA sequences, S. changwonensis formed a clade with this unidentified strain, which was clearly divergent from a clade comprising S. algicida and another unidentified “Shepherd's Crook” strain. Morphological analysis showed that S. changwonensis had thin plates with a Kofoidian plate formula of Po, cp, X, 4′, 2a, 7″, PC, 6c, 5+s, 5‴, 0p, and 2″″, which conformed to the genus Stoeckeria. The size and overall shape of this dinoflagellate were similar to those of S. algicida. However, the size and position of each cingular plate relative to the surrounding plates in S. changwonensis were clearly different from those of S. algicida. The length of c2, c3, c4, c5, and c6 of S. changwonensis were similar, unlike in S. algicida, in which the lengths of c3 and c4 greatly exceeded that of c2, c5, and c6. In addition, plate c6 of S. changwonensis was sufficiently long to contact the 6″ plate, again in contrast with what happens in S. algicida. Using TEM, an eyespot in the sulcal areas of both Stoeckeria changwonensis and Stoeckeria algicida, which has not been seen in other species of the family Pfiesteriaceae was herein reported for the first time. On the basis of morphological and phylogenetic criteria, it is proposed that this is a new species of the genus Stoeckeria. In addition, the description of S. algicida has been emended to include previously unreported details regarding the relative size and position of the cingular plates, the presence of eyespots, and the ultrastructure of its cells examined using TEM.