Paragymnodinium verecundum sp. nov. (Gymnodiniales, Dinophyceae), a new species of mixotrophic dinoflagellate from Japan

Paragymnodinium verecundum Yokouchi & Horiguchi sp. nov. was isolated from sand samples collected at Kitsunezaki, Miyagi Prefecture, Japan, and described based on observations using light and scanning and transmission electron microscopy, together with molecular analysis. The cells were 9.4–17.1 μm long and 5.7–13.6 μm wide and showed a benthic lifestyle. They possessed a sulcal extension‐like furrow on the episome, an eyespot, nematocysts and multiple chloroplasts with separated thylakoids. They ingested prey cells by engulfment, indicating a mixotrophic nutrition. These characters are shared with a known species of the genus Paragymnodinium, P. stigmaticum. However, unlike P. stigmaticum, P. verecundum showed negative phototaxis and possessed a pusule. Additional differences in the ultrastructure of the nematocysts and the flagellar apparatus indicated that these two dinoflagellates are different species. A phylogenetic analysis based on concatenated 18S and 28S ribosomal DNA sequences recovered P. verecundum in a robust clade of species of Paragymnodinium within the clade Gymnodinium sensu stricto. The sister to P. verecundum was P. stigmaticum, but they were clearly separated. Therefore, this dinoflagellate was concluded to be a new species in the genus Paragymnodinium.