Valve morphology and systematic position of Navicula walkeri (Bacillariophyceae), a diatom endemic to Oregon and California (USA)

Variation in valve size and ultrastructure is documented for Navicula walkeri, a freshwater diatom species endemic to Oregon and central California (USA). In LM, this large diatom has longitudinal lines on either side of the axial area, as well as lineolate striae. External proximal raphe ends recurve toward the same side as the deflected distal ends. A large central nodule and bulbous areas at the terminus of each raphe branch are visible internally. The edges of an axial plate form the image of longitudinal lines. The suite of features present in N. walkeri suggests it is part of Navicula sensu stricto but occupies an isolated position within the group. Navicula sensu stricto is not an entirely homogeneous assemblage, and further refinements of the systematic affinities of its members may be warranted.