Fragilaria Guide
Credit: Eduardo Morales
  1. Frustules rectangular to lanceolate, in girdle view
  2. Frustules joined by spines to form colonies
  3. Single rimoportula present
  4. Girdle bands open

Fragilaria frustules are rectangular to lanceolate in girdle view. The pattern of ornamentation on the valve face is variable, but a central sternum is generally present. Frustules are joined by small marginal spines to form ribbon like (band-shaped) colonies. A single rimoportula is present, usually positioned at a distal end. Small, apical porefields are also present. Girdle bands are open. Living cells contain plastids composed of 2 plates, positioned against the valve face.

Fragilaria is often abundant in the plankton of lakes, including the species F. crotonensis, F. capucina, and F. vaucheriae. Like Asterionella formosa, F. crotonensis is a species that is considered to have a world wide distribution and is also considered to be introduced by human activities.

Note that the concept of the genera Fragilaria, Synedra, and now, Ulnaria, have changed substantially. For example, a broad concept of Fragilaria is applied in some floras (Krammer and Lange-Bertalot 1991), while a much narrower concept is applied by others (Williams and Round 1987). Later works present a more narrow circumscription of the genus (Lange-Bertalot and Ulrich 2014).