Reclassification of Cochlodinium convolutum as Ceratoperidinium convolutum (Kofoid & Swezy) comb. nov. (Gymnodiniales), based on apical groove and phylogeny

Abstract

Although the transfer of Cochlodinium convolutum to genus Ceratoperidinium was proposed based on phylogeny based on large subunit (LSU) rDNA sequences, the morphological details and taxononomic position of C. convolutum remain unclear. We re-examined morphologically uncertain species C. convolutum for its unequivocal assignment. Morphology of C. convolutum, which is morphologically similar to C. helix and C. helicoides, was frequently collapsed during the observation. This indicates that turn number, displacement of cingulum and outline of the cell are unreliable characteristics for identification of this species. Scanning electron microscopy showed that the dense reticulation (small vesicle) is observed in the surface of cell, and that apical groove is characterized by a circular loop, with both ends in contact with the sulcus (sulcal extension). The shape of the apical groove differs from those of other Cochlodinium species, excluding C. strangulatum, whereas it is consistent with that of Ceratoperidinium falcatum. In addition, phylogenetic analysis based on small (SSU) and large subunit (LSU) rDNA sequences revealed that C. convolutum is distantly related to Cochlodinium species such as C. polykrikoides and C. fulvescens, and placed within a well-defined Ceratoperidinium clade. Based on the shape of the apical groove and phylogeny, C. convolutum should be transferred to Ceratoperidinium as Ceratoperidinium convolue-examined morphologically uncertain species C. convolutum for its unequivocal assignment. Morphology of C. convolutum, which is morphologically similar to C. helix and C. helicoides, was frequently collapsed during the observation. This indicates that turn number, displacement of cingulum and outline of the cell are unreliable characteristics for identification of this species. Scanning electron microscopy showed that the dense reticulation (small vesicle) is observed in the surface of cell, and that apical groove is characterized by a circular loop, with both ends in contact with the sulcus (sulcal extension). The shape of the apical groove differs from those of other Cochlodinium species, excluding C. strangulatum, whereas it is consistent with that of Ceratoperidinium falcatum. In addition, phylogenetic analysis based on small (SSU) and large subunit (LSU) rDNA sequences revealed that C. convolutum is distantly related to Cochlodinium species such as C. polykrikoides and C. fulvescens, and placed within a well-defined Ceratoperidinium clade. Based on the shape of the apical groove and phylogeny, C. convolutum should be transferred to Ceratoperidinium as Ceratoperidinium convolu