Asterionella Guide
Credit: Megan Otu, Sarah Spaulding
  1. Valve linear-lanceolate with capitate ends
  2. Spines present on valve margins
  3. Cells linked by mucilage pads forming characteristic stellate colonies
  4. Porefield one, or two
  5. Rimoportula(e) at headpole and/or footpole

The valves of Asterionella are symmetric to the apical axis and asymmetric to the transapical axis. In valve view, the valves are linear-lanceolate in shape, with capitate ends. Living cells attach by mucilage pads at the basal ends, or footpoles, to form characteristic stellate colonies. Living colonies present cells in girdle view, while processed samples break up the colonies so that single valves typically lie in valve view. In processed samples, small spines are visible on the valve margins.

Asterionella is a characteristic planktonic species and the stellate colonies are buoyant and resistant to sinking. It is considered to have one of the slowest sinking rates of the freshwater planktonic diatoms. During blooms of A. formosa, cells may become heavily infested with chytrid fungal parasites, which are colorless, spherical structures that attach to the frustules by hyphae. Asterionella formosa is common and considered to have a worldwide distribution. The taxon is known to be introduced by humans into new habitats and also to increase with anthropogenic eutrophication of lakes.