Visit us at

Diatoms of the United States is now known as Diatoms of North America.
This site has been redesigned and updated, and will be closing soon.


O. Muller 1895      Category: Epithemioid
TYPE SPECIES: Rhopalodia gibba (Ehrenberg) O. Müller




Smallest to largest - 3 taxa sorted by maximum length

Image Credit: Ditmar Metzeltin, Marina, Potapova, Pat Kociolek

CLASS: Bacillariophyceae
  ORDER: Rhopalodiaceae
    FAMILY: Rhopalodiaceae

  1. Symmetry dorsiventral
  2. Raphe positioned in dorsal keel
  3. Valve faces nearly planar

Frustules of Rhopalodia have a strong dorsiventral symmetry. The raphe is positioned on the dorsal side of each valve in a shallow keel. The valve faces are nearly planar, that is, each frustule is shaped like 1/2 of an orange. The central endings of the raphe are expanded. The raphe is supported internally by costae. In contrast to Epithemia, Rhopalodia lacks complex cingula (girdle bands).

Cells of Rhopalodia grow in nitrogen poor habitats. Like Epithemia, cells typically include endosymbiotic cyanobacteria that fix atmospheric nitrogen.

Cite This Page:
Spaulding, S., and Metzeltin, D. (2011). Rhopalodia. In Diatoms of the United States. Retrieved May 22, 2018, from

Contributor: Sarah Spaulding | Ditmar Metzeltin - March 2011