Molecular phylogeny and synonymy of Balechina gracilis comb. nov. (=Gymnodinium gracile), a widespread polymorphic unarmored dinoflagellate (Dinophyceae).

Gymnodinium gracile, described from the coasts of Denmark in 1881, is one of the first described unarmored dinoflagellates. Individuals that morphologically fit with the original description were isolated from the English Channel (North-East Atlantic). The SSU rRNA gene sequences were identical to the sequences identified as Balechina pachydermata and Gymnodinium amphora from the Mediterranean Sea and Brazil. We propose the transfer of Gymnodinium gracile into the genus Balechina as B. gracilis comb. nov. These sequences constitute an independent lineage, clustering with numerous environmental sequences from polar to tropical waters. The widespread distribution, the high plasticity in size, shape and coloration and the difficulties in discerning the fine longitudinal striae have contributed to the description of numerous synonyms: Amphidinium vasculum, Balechina pachydermata (=Gymnodinium pachydermatum), Gymnodinium achromaticum, G. abbreviatum, G. amphora, G. dogielii, G. lohmannii (=G. roseum sensu Lohmann 1908), G. situla, and Gyrodinium cuneatum (=G. gracile sensu Pouchet 1885).