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Abstract. The dinoflagellate genera Oxytoxum and Corythodinium that account for more than fifty species are widespread in warm oceans. These genera have been considered synonyms and thecal plate designations varied among authors. Several planktonic and sand-dwelling genera have been placed within the Oxytoxaceae. We obtained the first molecular data based on small subunit (SSU) rRNA gene sequences of Oxytoxum and Corythodinium, including the type species (O. scolopax and C. tessellatum) and C. frenguellii and C. cristatum. The three species of Corythodinium branched together a strong support [bootstrap (BP) of 98%]. This formed a sister clade with moderate support (BP 75%) with O. scolopax that supported the generic split. Oxytoxaceae should exclusively remain for Oxytoxum and Corythodinium, as an independent group, unrelated to any other known dinoflagellate. Oxytoxum was characterized by spindle-shaped cells with an anterior narrow epitheca, an apical spine and little cingular displacement. Corythodinium exhibits relatively broad cell shapes, with wider epitheca and greater cingular displacement, and an obovate or pentangular anterior sulcal plate that noticeably indented the epitheca. This suggested the need of new combinations for species that were described as Oxytoxum and possessed the characteristics of Corythodinium.
Key words: 18S ribosomal RNA, armored Dinophyta, Atlantic Ocean, Mediterranean Sea, phytoplankton, SSU rDNA phylogeny, taxonomy, thecate Dinoflagellata.
INTRODUCTION
The genera Oxytoxum and Corythodinium account for more than fifty species, most often reported in the phytoplankton assemblages of warm temperate and tropical seas (Kofoid 1907, Schiller 1937; Rampi 1939, 1941, 1951; Gaarder 1954; Balech 1954, 1971a,b; Taylor 1976, Gómez et al. 2008). The genera Oxytoxum and Pyrgidium were described by Stein (1883) having five postcingular plates and the first one (1 " ') being shorter and narrower than the others. He proposed Oxytoxum for four elongated species with a pointed apex and antapex or spines, and Pyrgidium for broader cells with more blunt ends. However, the generic split was not clear cut and further authors merged both genera into Oxytoxum (Schiller 1937). Based on plate dissections, Balech (1954, p. 115) reported the tabulation of O. constrictum as 3 ', 2a, 6 ", 5c, 4-5s, 5 "1 ', and Oxytoxum scolopax as 3 ', 2a, 3 ", 5c, 4-5s, 5 "1 "". Unfortunately, there was an error in the plate formula that was reported as...