Ultrastructure and phylogeny of a new species of mixotrophic dinoflagellate, Paragymnodinium stigmaticum sp. nov. (Gymnodiniales, Dinophyceae)

A new dinoflagellate, Paragymnodinium stigmaticum Yokouchi, Onuma & Horiguchi sp. nov., was described from sand samples collected at Sumiyoshi beach, Sado Island, Niigata Prefecture, Japan, based on observations at the light, scanning and transmission electron microscope levels together with molecular characterisation. The cells were 8.5-15.2 mu m long and 6.3-12.4 mu m wide. The episome and hyposome were subequal and hemispherical; although, the episome could also be conical. The cingulum was wide, well excavated and descended one quarter to a half of its own width. The sulcus was straight and widened slightly as it reached the antapex. A slightly curved sulcal extension-like furrow (SEF) ran from the right end of cingulum toward the apex. Cells possessed five to 10 yellowish brown chloroplasts, a red eyespot and small nematocysts. The dinoflagellate ingested prey cells despite the possession of chloroplasts, indicating a mixotrophic mode of nutrition. Paragymnodinium stigmaticum shared many morphological features and nutritional strategies with a known species of the genus Paragymnodinium, Paragymnodinium shiwhaense, including the possession of nematocysts, small polygonal amphiesmal vesicles and a mixotrophic mode of nutrition. However, it is clearly distinguished from P. shiwhaense by its feeding mechanism, its chloroplast ultrastructure, the presence of an eyespot, the absence of a plate-like structure in the amphiesmal vesicles and a benthic lifestyle (P. shiwhaense is planktonic). A phylogenetic analysis inferred from concatenated small and large subunit ribosomal DNA sequences recovered P. stigmaticum in a robust clade with P. shiwhaense that was included in the clade Gymnodinium sensu stricto. Therefore, it is reasonable to conclude that this dinoflagellate from Japan is a new species in the genus Paragymnodinium.