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A new species of Dinoflagellata—Protoperidinium euxinum Krachmalny sp. nov. (Protoperidiniaceae, Peridiniales) was described from the plankton of Odessa Bay (Black Sea). The species is morphologically close to P. steinii, P. mediterraneum and P. joergensenii. It differs from the listed species in elongated pentagonal body shape, conical epitheca with straight or concave lateral sides, inverse trapeziform hypotheca, and asymmetrical, laterally compressed plate 2a. Dimensions: 73.3–99.0 µm in length and 38.8–51.6 µm in width. Pictures and photomicrographs of the novel species are presented, along with detailed description and discussion.

The application of scientific names is determined by means of nomenclatural types, and every name has to be typified properly. The concept has limitations for unicellular organisms, because original material frequently consists of drawings and/or inadequately preserved physical material. Peridinium cinctum is an abundant freshwater microalga and variable in both morphology and genotype. Morphological variation is mainly expressed in its epithecal conformation: shape deviations of plates, plate rearrangements, plate fusion and plate additions. Different epithecal conformations were traditionally described as either varieties of P. cinctum or were established as closely related species. Despite this, relations between varieties, ribotypes and geographic locations were oversighted, and the full spectrum of plate variation in P. cinctum is still not well represented. For this reason, we sampled localities in Germany and Poland, from which varieties of P. cinctum were described a century ago. We cultivated monoclonal strains, exhibiting two distinct ITS ribotypes, and assessed their epithecal variation of morphology. Based on ca 2,500 observations of individual cells we report a plethora of both plate and suture deviations from the archetypical epithecal conformation of P. cinctum. Morphologies corresponding to previously described varieties were rare, even at their type localities. Nevertheless, we found morphologies consistent with protologues in four cases and use this material for epitypification. These varieties are now linked to specific DNA sequences, allowing reliable application of scientific names for future studies.

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Kryptoperidiniaceae are a small group of dinophytes hosting a tertiary endosymbiont derived from a diatom (Tomas et al. 1973, Horiguchi & Pienaar 1994). Those so called ‘dinotoms’ (Imanian et al. 2011) include Dinothrix , Durinskia , Galeidinium , Kryptoperidinium (= Phyllodinium ) and some species currently assigned to “ Peridiniopsis ” and “ Peridinium ” (Tamura et al. 2005, Horiguchi & Takano 2006, Hansen et al. 2007, Zhang et al. 2011). Besides ‘possessing a diatom endosymbiont’ as a highly derived trait, the monophyly of Kryptoperidiniaceae is also supported by a unique type of eyespot that has possibly derived from the original chloroplast (Moestrup & Daugbjerg 2007). In molecular trees, Kryptoperidiniaceae constitute a well supported monophyletic group (Kretschmann et al. unpubl.), but it is not finally resolved at present, whether they are embedded in the Thoracosphaeraceae or constitute their sister group (Gottschling & McLean 2013). Regarding habitat preference, molecular trees further indicate at least two independent marine→freshwater transitions in the Kryptoperidiniaceae during the late Paleogene at the latest (Žerdoner Calasan et al. unpubl.).

The vast majority not only of dinophytes, but also of Peridiniales, live in the marine environment, and some 350 (out of ca 2.500) species are present in freshwater habitats (Mertens et al. 2012). Many freshwater Peridiniales constitute small and only distantly related species groups embedded in predominantly marine clades (e.g., Apocalathium , Chimonodinium , Naiadinium in the Thoracosphaeraceae; species assigned to “ Peridiniopsis ” in the Kryptoperidiniaceae; Zhang et al. 2011b, Gottschling & Sohner 2013, Annenkova et al. 2015; Fig. 1). However, the most distinguished freshwater group of the Peridiniales are the Peridiniaceae including the type species, Peridinium cinctum , and other frequently encountered species such as Peridinium bipes , Peridinium volzii and Peridinium willei .

Peridinium acuminatum (Peridiniales, Dinophyceae) was described in the first half of the 19 th century, but the name has been rarely adopted since then. It was used as type of Goniodoma , Heteraulacus and Yesevius , providing various sources of nomenclatural and taxonomic confusion. Particularly, several early authors emphasised that the organisms investigated by C.G. Ehrenberg and S.F.N.R. von Stein were not conspecific, but did not perform the necessary taxonomic conclusions. The holotype of P . acuminatum is an illustration dating back to 1834, which makes the determination of the species ambiguous. We collected, isolated, and cultivated Scrippsiella acuminata , comb . nov . (strain GeoB 427) from the type locality off Kiel, Germany (Baltic Sea). We barcoded the species of the Thoracosphaeraceae using rRNA sequences and investigated the morphology of the strain using light and electron microscopy. As taxonomic result, we designate an epitype for Peridinium acuminatum , as no conflict with C.G. Ehrenberg’s protologue can be stated. It is indistinguishable from Scrippsiella trochoidea (likewise described from the Kiel Fjord) that we consider a later heterotypic synonym. Our study contributes to the disentanglement of dinophyte taxonomy in a very challenging case, and we trust that C.G. Ehrenberg and S.F.N.R. von Stein investigated different species under the epithet ‘ acuminatum ’. The complex nomenclature and taxonomy of Goniodoma , and its type species Goniodoma acuminatum , is discussed in the Electronic Supplement. We consider Pyrrhotriadinium , with the type species Pyrrhotriadinium polyedricum (Gonyaulacales), well suited to harbour all gonyaulacalean taxa so far assigned to Goniodoma and Heteraulacus as well.