Excessive proliferation of new dinoflagellate generic names: the case of Torquentidium (Ceratoperidiniaceae, Dinophyceae)

New genera of unarmoured dinoflagellates represented by a single or very few species have largely increased in the last two decades. An example is the new genus name Torquentidium H.H. Shin, Z. Li, K.W. Lee & Matsuoka 2019 (Shin et al. , Eur. J. Phycol . 54: 249-262) recently proposed for species that were transferred into Pseliodinium in 2018. The SSU- and LSU rRNA gene sequences of the type species of Pseliodinium and Torquentidium only differed by 0.3% and 2%, respectively, and they clustered together as a monophyletic group with high support. Based on these tree topologies and/or the sequence divergences, each dinoflagellate species would be classified in its own genus. The morphological character proposed to distinguish Pseliodinium and Torquentidium is that the cingulum encircled the cell 1 and 1.5 times, respectively. This kind of arbitrary numeric morphometric character, proposed one century ago, is currently considered invalid for the generic split of unarmoured dinoflagellates. For example, Polykrikos geminatum (with ~1.5 cingular turns) clusters between species of Polykrikos (1 turn). Within the Ceratoperidiniaceae, the arrangement of the amphiesmal vesicles in apical structure complex was used for distinction at the genus level (i.e., Kirithra ), while this character is identical for Pseliodinium and Torquentidium . For the first time, an unarmoured dinoflagellate genus contains species with smooth cell covering ( T. convolutum , T. helix ) and species with longitudinal striae ( T. pirum ). However, the original description as Gymnodinium pirum in 1895 erroneously illustrated two distinct species, and since then nobody has ever reported surface markings in T. pirum . The morphological and molecular criteria used for the split of Torquentidium from Pseliodinium are unsupported, contributing to the excessive proliferation of monotypic genera. Torquentidium pirum and T. convolutum are synonyms, and Torquentidium is a superfluous generic name and junior synonym of Pseliodinium .