Guide to Fragilaria

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).