Peridinium is a genus that has been used in the past to cover a wide range of freshwater and marine dinoflagellates. For a long time, the standard diagnosis of Peridinium was that it contained species with four apical plates (the ring of plates at the front of the cell when it is moving), seven precingular plates (the ring of plates in front of the cingulum), five postcingular plates and two antapical plates (Carty 2008). However, the genus has been divided by differences in the shape and arrangements of the plates making up the theca into a number of species groups, and more recent studies have concurred that these species groups are not all closely related to each other. While support remains low in most phylogenetic studies of dinoflagellates, and many species remain to be analysed, indications are that all of the marine species and many of the freshwater species are not true Peridinium (Horiguchi & Takano 2006; Logares et al. 2007). As it currently stands, the probably monophyletic Peridinium sensu stricto includes two species groups, the P. cinctum and P. willei groups, and is exclusively freshwater. As well as the characters mentioned above, true Peridinium species have three apical intercalary plates between the apical and precingular plates, five cingular plates, and ridges on all the plates forming an areolate pattern. They are also united by a distinct combination of which plates in the front section of the organism break off when the theca is shed during cell division (Craveiro et al. 2009). The two species groups differ in the exact arrangement of the plates anterior to the cingulum: in the P. willei group they are symmetrical relative to the dorsal-ventral axis, vs asymmetrical in the P. cinctum group. Slightly surprisingly, though the presence or absence of an apical pore was one of the first characters used to subdivide the genus Peridinium, Peridinium sensu stricto includes both species with (such as P. bipes) and without (such as P. cinctum and P. willei).
Peridinium species are photosynthetic, with a much-lobed chloroplast that ramifies through the cell. One species, identified by Hickel & Pollingher (1988) as P. gatunense, has been intensely studied as the creator of annual blooms in Lake Kinneret in Israel.
Carty, S. 2008. Parvodinium gen. nov. for the Umbonatum Group of Peridinium (Dinophyceae). Ohio Journal of Science 108 (5): 103-107.
Craveiro, S. C., A. J. Calado, N. Daugbjerg & Ø. Moestrup. 2009. Ultrastructure and LSU rDNA-based revision of Peridinium group Palatinum (Dinophyceae) with the description of Palatinus gen. nov. Journal of Phycology 45: 1175-1194.
Hickel, B., & U. Pollingher. 1988 Identification of the bloom-forming Peridinium from Lake Kinneret (Israel) as P. gatunense (Dinophyceae). British Phycological Journal 23 (2): 115-119.
Horiguchi, T., & Y. Takano. 2006. Serial replacement of a diatom endosymbiont in the marine dinoflagellate Peridinium quinquecorne (Peridiniales, Dinophyceae). Phycological Research 54: 193-200.
Logares, R., K. Shalchian-Tabrizi, A. Boltovskoy & K. Rengefors. 2007. Extensive dinoflagellate phylogenies indicate infrequent marine–freshwater transitions. Molecular Phylogenetics and Evolution 45 (3): 887-903.