Aulacoseira islandica

Frustules are cylindrical, most often observed in girdle view as long filamentous colonies attached at the valve face to sister cells by linking spines. The valve face is flat and mantle is straight (8.9-18.3 µm high), becoming slightly curved near the valve face. The ratio of the mantle height to valve diameter greatly varies. Pervalvar rows of uniformly circular areolae form striae that run parallel to the mantle sides, occasionally curving slightly to the right (dextrorse). Interstriae terminate at the base of a spine. Separation spines are short and tapered, about as wide as a single areola. Linking spines are small and spathulate. Pervalvar rows of areolae occur 12-14 in 10 µm.

Valve polymorphism is common in A. islandica (Stoermer et al. (1981). Cell silicification and striae coarseness vary within populations, and attributed to silica availability (Stoermer et al. 1985). Early publications referred to filaments with only thickly silicified valves as alpha forms, filaments with only thinly silicified cells as beta forms, and filaments with valves with both types of silicification as gamma forms (Müller 1906). With more lightly silicified specimens occurring under low silica conditions, many authors regard this lightly silicified variety, often called A. islandica ssp. helvetica to be a morphotype (Stoermer et al. 1985, Genkal et al. 1986, Krammer and Lange-Bertalot 1991, Siver and Kling 1997). The images presented here document the moderately silicified form of A. islandica.

Alternatively, others have proposed that polymorphic variation is associated with seasonal life history strategies (Jewson et al. 2010). In the closely related species, Aulacoseira baicalensis (K. Meyer) Simonsen, longer and more heavily silicified forms were produced seasonally upon stratification. These cells functioned as resting cells capable of perennating in cooler waters of 50-150 m deep until autumn overturn.

Aulacoseira islandica also varies in striae and the spine structure. Two types of striae have been observed: 1) coarse striae comprised of complex areolae, which may be uniseriate or biseriate, and 2) fine striae that are uniseriate (Stoermer et al. 1981, Chou 1996). Furthermore, areolae may vary from round to slit-like (Siver and Kling 1997). In fact, the two types of areolae have been observed within the same filament (Siver and Kling 1997).

Both linking and separation spines differ within A. islandica. Separation spines are sharply tapered or lanceolate, while linking spines are spathulate (Chou 1996). The areolae and spine polymorphisms occur across the spectrum of silicification polymorphism (Stoermer et al. 1981).