Rhoicosphenia Guide
Credit: Marina Potapova
  1. One valve with complete raphe
  2. One valve with small raphe branch (rudimentary raphe)
  3. Pseudosepta at both poles
  4. Frustule curved in girdle view

Frustules of Rhoicosphenia are clavate in valve view and heterovalvar. In girdle view, valves are flexed, and curved. As a result of the asymmetry to the transverse axis, the valve face is curved. One valve has a complete raphe with two distinct raphe branches. The other valve has a very small raphe branch on the footpole. Pseudosepta are present and distinct at both poles. In contrast with species in Gomphonema, valves lack apical pore fields.

Cells grow on short stalks in streams and wave zones of lakes. Rhoicosphenia is common in western North America. For many years, only a single extant freshwater species was found in North America, R. abbreviata (an earlier homonym of R. curvata). Now, a number of species are recognized, within what had been R. abbreviata (Levkov et al. 2010, Thomas et al. 2014).

Morphology of the genus has been documented by Mann (1982a, 1982b).