Content area
Full Text
ABSTRACT: Peridinium is a genus of freshwater thecate dinoflagellate. Because it was one of the earliest named genera (Ehrenberg 1832), many species placed in it were later removed to other genera. Genera continue to be extracted and Peridinium, while more closely defined, still harbors groups of species unlike the type species, P. cinctum. It is the goal of this paper to remove one of the most dissimilar groups, the Umbonatum Group. Peridinium cinctum has no apical pore, three apical intercalary plates and five cingular plates. Species in the Umbonatum Group have an apical pore, two apical intercalary plates and six cingular plates warranting their separation into a new genus, Parvodinium.
OHIO J SCI 108 (5): 103-107, 2008
INTRODUCTION
Freshwater dinoflagellates are a group of algae found mostly in open water habitats. As members of the phytoplankton they are food for zooplankton and may form blooms during the temperate summer. Most are recognizable as dinoflagellates by their golden brown color and shape, but further taxonomic identity may be challenging. Many reports of dinoflagellates, as part of a list of taxa, include only "Peridinium sp." The genus Peridinium was originally established for cells with a cell wall divided into plates and with a transverse groove (Fig. 1), and was distinguished from a similar genus (Glenodinium) by the absence of an eyespot (Ehrenberg 1830, 1838). Stein (1883) refined the description by illustrating the plate tabulation pattern of P. cinctum (O.F.Müller) Ehrenberg, the type species (Fig. 2). It has long been recognized that species in Peridinium showed a great deal of variability. Thecate dinoflagellate taxa are primarily defined by the number and arrangement of plates in the epitheca, hypotheca, sulcus and cingulum, the latter three considered more conservative (Balech 1980). Balech (1974) used differences in the number and shape of cingular plates to separate Protoperidinium from Peridinium, and other taxa first described as species of Peridinium have been moved to other genera including Gymnodinium, Gonyaulax, Ceratium, Thompsodinium and Glochidinium based on differences from the plate pattern of P. cinctum. Peridinium has come to be defined as having a plate formula of 4' 2-3a, 7'', 5''' 2'''', with species based on the presence/absence of an apical pore, the two alternatives for number of apical intercalary plates, plate arrangements, size,...