TAXONOMIC INTERPRETATION OF THE RIMOPORTULA OF FRESHWATER GENERA IN THE CENTRIC DIATOM FAMILY THALASSIOSIRACEAE

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Abstract
It is difficult to delineate Cyclotella, Cyclostephanos and Stephanodiscus—the three most taxonomically diverse, predominantly freshwater genera of the Thalassiosiraceae. Cyclostephanos was originally distinguished by a Stephanodiscus-like exterior of fasciculate striae and raised radiating costae, and a Cyclotella-like interior, with chambers near the margin. After the creation of Cyclostephanos, Cyclotella and Stephanodiscus were each diagnosed by absence of characters. However, this study suggests that most Cyclostephanos and Stephanodiscus species can be classified on the basis of their unusual rimoportula morphology. Over half of the Cyclostephanos species known, including the lectotype, have the rimoportula underneath a spine; we call this the ‘subspina’ species group. The rimoportula of all known Cyclostephanos species lacks a long external tube, regardless of its location, although some may have a small raised annulus around the external opening. In contrast, the lectotype of Stephanodiscus and all but one other known species have a rimoportula with a long external tube at the valve face—valve mantle junction. The rimoportula of Cyclotella is never associated with a spine and, like that of Cyclostephanos, lacks a long external tube. Thus rimoportula morphology can be sufficient, if not necessary, evidence for classification of a species as either Cyclostephanos or Stephanodiscus. Cladistic analysis indicates that the rimoportula morphologies of the vast majority of Stephanodiscus and of the Cyclostephanos ‘subspina’ group represent derived conditions within each taxon and so Stephanodiscus and the Cyclostephanos ‘subspina’ group may be considered natural groups. Stephanodiscus invisitatus is here transferred to Cyclostephanos because it has the rimoportula position and morphology of that genus. Cyclotella can still be distinguished only by the lack of characters.