All Taxa

Fragilariaceae: Asterionella

Cells elongate, linear in valve view with expanded spatulate or capitate poles, the basal (attaching) pole wider than the apical pole. Unusually, the poles of the frustules are also expanded in girdle view, which is therefore not rectangular, in contrast to the planktonic species of Tabellaria that (like Asterionella) also form stellate colonies.

Heteropolar, isovalvar, bilaterally symmetrical in valve view.

Striae uniseriate, parallel and perpendicular to the apical axis; areolae small, round, occluded externally. A rimoportula is present near each end. One or both poles with areas of tiny unoccluded round pores (apical pore fields), through which mucilage is secreted. Small conical spines occur along the margin of the valve face and around the pore fields.

Axial area very narrow, no central area.

Several open girdle bands, each with one or two rows of pores. Numerous small, plate-like (discoid) like chloroplasts.

Forming stellate colonies that are freely suspended in the water column. Cells are connected by mucilage pads secreted from the base poles of the cells.