Aulacoseira granulata var. angustissima

Frustules are cylindrical and form colonies. Valves are 2.5-4 µm in diameter, with a mantle height of 8-20 µm. The ratio of the mantle height to valve diameter is greater than 3.2. The mantle has straight sides and the valve face is flat. The mantle areolae are square. The rows of areolae on the mantle are curved to the right (dextrorse), but often are almost straight and parallel to pervalvar axis in separation valves. There are 10-20 rows in 10 µm. The valve faces of most valves do not have areolae, but a single ring of areolae might be found on the valve face of separation valves. Linking spines are located at the end of each pervalvar costa. They are short and bifurcated. Separation spines originate from two pervalvar costae. Most separation spines are 2-6 µm long and broadly triangular, but 1 or 2 spines per valve are very long, almost equal in length to the valve mantle. The ringleiste is solid and moderately shallow.

Kilham and Kilham (1975) reported that A. granulata var. angustissima represents a morphological variation within the life cycle of A. granulata. We, however, observed a break in the distribution of valve diameters and mantle height/valve diameter ratios in natural populations of A. granulata sensu lato from the US. Because of the discontinuity in size, we recommend separating the variety angustissima from the nominate variety until additional studies clarify the taxonomy of this species complex. We suggest that specimens with a valve diameter 4 µm or less and mantle height/valve diameter ratio greater than 3.2 are be identified as A. granulata var. angustissima. We suggest that specimens with either valve diameter greater than 4 µm, or mantle height/valve diameter ratio lower than 3.2, or both, are placed in the nominate variety, A. granulata.

Note that it is not clear, whether A. granulata var. angustissima from North America represents the same biological species as the type of this taxon from Africa, but the name has been used by North American workers in the past and is here recommended to be used to maintain consistency.