Balechina and the new genus Cucumeridinium gen. nov. (Dinophyceae), unarmored dinoflagellates with thick cell coverings

The genus Balechina (=subgenus Pachydinium ) was established for heterotrophic gymnodinioid dinoflagellates with a thick cell covering. The type species, B. pachydermata (= Gymnodinium pachyderm‐atum ), showed numerous fine longitudinal striae, whereas B. coerulea (= G. coeruleum ) showed ~24 prominent longitudinal surface ridges or furrows and a distinctive blue pigmentation. We have investigated the morphology and molecular phylogeny of these taxa and the species Gymnodinium cucumis , G. lira and G. amphora from the western Mediterranean, Brazil and Japan. Sudden contractions at the cingulum level were seen in B. pachydermata , which also showed a high morphological variability which included morphotypes that have been described as Amphidinium vasculum , G. amphora , G. dogielii and G. gracile sensu Kofoid and Swezy. Molecular phylogeny based on small subunit rRNA gene sequences revealed that Balechina coerulea , G. cucumis and G. lira formed a clade distantly related to the clade of the type species, B. pachydermata , and G. amphora . We propose the new genus Cucumeridinium for the species with longitudinal ridges and a circular apical groove ( Cucumeridinium coeruleum comb. nov., C. lira comb. nov. and C. cucumis comb. nov.), and Gymnodinium canus and G. costatum are considered synonyms of C. coeruleum . The genus Balechina remains for the species with a double‐layer cell covering, bossed surface with fine striae, and an elongated elliptical apical groove. At present, the genus is monotypic containing only B. pachydermata .