A morphological study of the giant diatoms ardissonea formosa and synedra bacillaris

We have made detailed SEM studies of the diatoms Ardissonea formosa (Hantzsch) Grunow and Synedra bacillaris (Grunow) Hustedt collected as epiphytes from seagrass leaves. The siliceous architecture of A. formosa clearly conforms in all respects to the detailed generic circumscription of Ardissonea De Notaris as put forth by Round et al. (1990), with the most notable characters being its robust longitudinal ribs, thick internal plate (i.e. the valve is fully chambered), and the valvocopula bearing an apical notch. While the morphology of S. bacillaris is similar to that of A. formosa, the former completely lacks an internal siliceous plate (i.e. the valves are not chambered) and there are other differences as regards the shape of the valve areolae and fine structure of the connecting bands. The morphological data gathered thus far clearly show that Ardissonea sensu Hustedt (1932) is heterogeneous and must be broken up. Differences in morphology between S. bacillaris and other non-chambered species of the Ardissonea cluster indicate that a generic separation based on the presence or absence of an internal plate may be an overly simplistic solution; as many as three new genera may have to be erected for these forms.