Slow, though anything but a runt: Gymnodinium undulatum Wołosz. is a cheerful species of Borghiella (Borghiellaceae, Phytodiniales)

Abstract Borghiellaceae are a small group of dinophytes that frequently lack distinctive morphological traits, which is why integrative approaches including DNA sequence information is of particular importance for their species delimitation. An undulated margin of the posterior cell half is rarely documented in dinophytes such as Gymnodinium undulatum and has been considered a teratological trait (‘abnormalcy’) in the past. Unialgal strains were established from field material collected near Brombach (Bavaria, Germany). Molecular sequences of the rRNA complex were gained for phylogenetics, and strain GeoM*889 was analysed in detail using light and electron microscopy. The predominant flagellated cells were highly polymorphic but showed consistently a right undulated margin of the posterior cell half. Molecular sequence data indicated a placement of the organism within Borghiellaceae, and Borghiella cf. tenuissima was retrieved as closest relative. Therefore, we perform a taxonomic transfer of the studied material to Borghiella undulata , comb. nov. , which is hereby documented for the first time reliably after its description just 100 years ago. Despite very different morphologies, the sister species differ in three ITS positions only, which makes it impossible to differentiate between them in metabarcoding studies when the V4 region of the SSU is used as genetic marker. The less frequent smaller flagellated cells are discussed as dispersal units rather than gametes .