The broadly lanceolate axial area is the best feature for differentiating F. pennsylvanica from species of similar size and outline. Fragilaria gracilis, F. rumpens, and Synedra famelica all have a straight, narrow axial area and valves are wider. Fragilaria rumpens and S. famelica have parallel striae. Synedra famelica has coarse areolae that can be resolved in LM. In SEM, F. gracilis has spines on the striae.

LM scalebar = 10 µm
Fragilaria pennsylvanica LM1 Fragilaria pennsylvanica LM2 Fragilaria pennsylvanica LM3 Fragilaria pennsylvanica LM4 Fragilaria pennsylvanica LM5 Fragilaria pennsylvanica LM6 Fragilaria pennsylvanica LM7
Length Range
12.8-25.6 µm
Width Range
1.8-2.2 µm
Striae in 10 µm

Synedra famelica

Kütz. 1844

Synedra famelica LM7 Synedra famelica LM6 Synedra famelica LM5 Synedra famelica LM4 Synedra famelica LM3 Synedra famelica LM2 Synedra famelica LM1
Length Range
18-65 µm
Width Range
2.4-3.1 µm
Striae in 10 µm

Also compare to

Fragilaria gracilis