Aulacoseira muzzanensis

Frustules are cylindrical and form colonies that often consist of separation cells only. Linking valves are rare. Valves are 9-24 µm in diameter, with a mantle height of 5-13 µm. The ratio of the mantle height to valve diameter is 0.4-1.2. The mantle has straight sides and the valve face is flat or vary slightly convex or concave. The rows of areolae are almost straight and parallel to pervalvar axis in separation valves, but often curved to the right in linking valves, 7-15 in 10 µm. The mantle areolae are square, but often appear oval in LM. Areolae in some valves are coarse and disorganized. The straight rows of areolae often appear as biseriate in LM. The valve face has small scattered areolae. Linking spines are located at the end of each pervalvar costa. They are short, triangular or bifurcated. Separation spines originate from two pervalvar costae. Most separation spines are 0.5-2 µm long, but some spines are longer, and 1 -2 are almost equal in length to the valve mantle. These long spines have corresponding grooves on the mantle of sibling valves. The ringleiste is solid and moderately shallow. At least one coiled rimoportula is positioned on the mantle at the distance of 1-3 areolae from the collum, usually in the groove, and another one is positioned close to the valve face.

The structure of the frustule is essentially the same as in A. granulata with all characters overlapping between two species. Until further studies clarify the taxonomy of this species complex, we recommend differentiating these two species using an arbitrary rule: if the ratio of the mantle height to valve diameter in at least some cells in a colony is less than 0.8, or if it is between 0.8 and 1.2, but areolae are coarse and disorganized, a specimen is identified as A. muzzanensis. If this ratio in all cells is greater than 0.8, or it is between 0.8 and 1.2, but areolae are organized in regular rows, it is identified as A. granulata.

Note that Krammer (1991) provided LM and SEM images of the syntype slide from the Lago di Muzzano, Ticino (mounted by Meister 16.9.1906, coll Hustedt slide A1/50). These images include specimens that are similar to the original Meister illustration (shown below), as well as specimens that conform to North American specimens.