Didymosphenia Guide
Credit: Sarah Spaulding, Danielle Pite
  1. Valves large and robust
  2. Stigmata one to several
  3. Apical porefield at footpole, large

Frustules of Didymosphenia are asymmetric to the transapical axis and symmetric to the apical axis (although some populations may be slightly to strongly asymmetric to the apical axis). One to several stigmata are present, a feature that may be variable within a given species. A large apical porefield is present at the footpole. The terminal raphe fissure is deflected prior to reaching the apical porefield, a feature that is key to showing that Didymosphenia shares a lineage with the cymbelloid diatoms. In contrast, the terminal raphe fissure of gomphonemoids bisects the apical porefield. Frustules are wedge shaped in girdle view. A marginal ridge of silica extends along the valve, terminating at the headpole in small spines.

Didymosphenia geminata is common in North America and in the Upper Great Lakes. It is locally abundant in some lakes and streams, at times producing high biomass. The large volume of mucilaginous stalks of D. geminata may cover surfaces and foul water intake pipes, reaching nuisance proportions. It is invasive in New Zealand and expanding its range in regions of the Northern Hemisphere. This genus is more closely allied to the cymbelloid diatoms than to the gomphonemoid groups, as has been previously reported. Lake Baikal, in Siberia is considered a hotspot of diversity for Didymosphenia.