On Levanderina fissa gen. & comb. nov. (Dinophyceae) (syn. Gymnodinium fissum, Gyrodinium instriatum, Gyr. uncatenum), a dinoflagellate with a very unusual sulcus

Gymnodinium fissum was described by Levander in 1894 from the Baltic Sea near Helsinki, and we argue, on the basis of morphological and molecular studies of material from the type locality, and on cultures from the Aland islands, Puerto Rico, Portugal and United States identified as Gyrodinium instriatum, Gymnodinium instriatum, Gyrodinium uncatenum and Gyrodinium sp., that all these taxa are conspecific. They are morphologically and genetically distinct from Gymnodinium and are described here as Levanderina fissa gen. & comb. nov. This species also includes Gyrodinium pavillardii. Levander observed chloroplasts in the cell and on some occasions diatoms, probably the first report of mixotrophy in a dinoflagellate. Biecheler in 1952 described the process of food uptake in Gyr. pavillardii, feeding it with ciliates and other dinoflagellates. Prey was taken up through the posterior part of the sulcus, some prey items being almost as large as the host. Our observations showed that the longitudinal flagellum, in contrast to what has been described in all other dinoflagellates possessing a longitudinal sulcal furrow, is not located in the furrow but in a separate, internal tube beneath the sulcal furrow. The tube opened to the exterior dorsally near the posterior end of the cell, and the sulcus appeared to be used for food uptake only. The cytoskeleton of L. fissa was complex and included a large number of muscle-like fibres. Food uptake using the sulcus involved major changes of cell shape, which requires the presence of a highly flexible cytoskeleton. Levanderina fissa was not morphologically or genetically close to any other dinoflagellate for which molecular sequences were available. The detailed structure of the apical furrow or acrobase comprised three rows of elongate vesicles, one row forming the bottom of a furrow. The new term apical structure complex (ASC) is introduced as a general term to replace apical furrow or acrobase, none of which adequately describes all the many known types. The ASC in Levanderina may be characteristic of most if not all species of the Gymnodiniales (an apomorphy of the order?) and different from the types present in the Suessiales, the other order of mainly thin-walled dinoflagellates.