Aulacoseira granulata

Frustules are cylindrical, join face-to-face and form filamentous colonies. Valves are 4-17 µm in diameter, with a mantle height of 4-20 µm. The ratio of the mantle height to valve diameter is usually greater than 0.8 but less than 5. 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 and number 8-18 in 10 µm. The valve face has small scattered areolae more densely distributed around the valve face margin. Linking spines are located at the end of each pervalvar costa. Linking spines are short, triangular or bifurcated. Separation spines originate from two pervalvar costae. Most separation spines are 2-6 µm long, but a few, usually 1-2 spines per valve, are very long, almost equal in length to the valve mantle. The ringleiste is solid and moderately shallow. Coiled rimoportulae are usually positioned along a stria on the mantle at the distance of 1-3 areolae from the collum and and similarly along a stria, but much closer to the valve face.

Density and arrangement of the mantle areolae varies considerably among populations and even among cells within a single colony depending on environmental conditions (Stoermer et al. 1981). For the populations found in the Great Lakes the following informal classification of morphotypes was proposed by Stoermer et al. (1981): status α specimens have coarse areolae (8-12 rows in 10 µm, 8-12 areolae in 10 µm in a row) and usually straight rows in separation valves, but spiral in linking valves; status γ specimens have fine areolae (13-18 rows in 10 µm, 13-18 areolae in 10 µm in a row) have only spiral rows of areolae; status β specimens have both types of valves within a colony.

Populations with long thin cells approximately 3-4 µm in diameter have been described as A. granulata var. angustissima (Otto Müller) Simonsen, but Kilham & Kilham (1975) showed that this, as well as another variety A. granulata var. ionensis (Grunow) Simonsen represent morphological variations within the life cycle of A. granulata. We 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, and for that reason recommend separating the variety angustata from the nominate variety until additional studies clarify the taxonomy of this species complex. No break is observed, however, in the distribution of valve diameters and mantle height/valve diameter ratios between A. granulata and A. muzzanensis, and all other characters appear to overlap between two species. Until further studies clarify the taxonomy of this species complex, we recommend differentiating these two species using this decision 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 and 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.

Colonies are usually straight, but sometimes spiral, with curved cells. Such populations have been described as separate forms of A. granulata (A. granulata f. curvata (Hustedt) Simonsen) and A. granulata f. spiralis (Hustedt) Czarnecki & Reinke, but there is no evidence that this trait has taxonomic significance.