Didymosphenia geminata

Valves are large, robust and slightly asymmetric to the apical axis. One end of the valve forms a distinct, capitate headpole. The other end of the valve, the footpole, is less capitate to bluntly rounded. The axial area is relatively linear and the central area is an expanded oval. Stigmata number 2-5, with the number of stigmata positively related to valve size. Internally, the stigmata possess convoluted internal occlusions. The raphe is lateral, with expanded proximal raphe ends and deflected distal raphe ends. The distal raphe is deflected near the porefield at the footpole, but does not pass through the porefield. The porefield is large, distinct and composed of fine porelli. The areolae are complex; externally the areolae are within ornamented depressions, internally the areolae open with chambers. The striae are radiate for most of the valve, except near the footpole, where they become nearly parallel. Striae near the valve center are irregular and many do not reach between the central area and valve margin. Under the SEM, a ridge of silica is present between the valve face and margin. This ridge may be expanded into small spines at the headpole.

Didymosphenia geminata is the only species within the genus that has been documented in the lower 48 states, but D. clavaherculis has been documented from Alaska (Metzeltin and Lange-Bertalot, 1995; Pite et al. 2009). Some populations may show greater asymmetry to the apical axis, while others may be nearly linear. The sister taxa to the genus Didymosphenia include Cymbella mexicana and Cymbella janischii (Kociolek and Stoermer 1988).

The range in length posted here is based on measurement of over 90,000 valves over a one year period from South Boulder Creek (Bishop, M.S. Thesis, University of Colorado, 2014).