Brachysira microcephala

This entry is being evaluated following Van de Vijver et al. (2021) as belonging to B. neoexilis.

Valves are lanceolate to rhombic-lanceolate with protracted, capitate apices. Some specimens are weakly asymmetric to the transapical axis, or heteropolar. Striae are radiate, except may be parallel at the apices. The central area is small, asymmetrically rounded and, in some specimens, undistinguishable from the axial area. The axial area is narrow and straight. The raphe is positioned within two ribs that lie on the external face of the valve The ribs terminate within the central area and at the apices. Raphe is straight, and filiform with undifferentiated proximal fissures and T-shaped distal fissures. Areolae are variable in shape, most are elongate rectangular openings, and are arranged to form the appearance of wavy hyaline areas. At the apices, each stria consists of a single elongated areola. In the SEM, small papillae (silica nodules) may be located on the external valve face between the striae. A rib is present along the valve margin and apex, separating the valve face from the mantle. Striae on the mantle are long uniseriate slits aligned with striae on the valve face. In SEM, a small helictoglossa is present. Internally, the areolae are covered by delineated hymenes.

Confusion regarding the taxonomy and nomenclature of B. microcephala, B. neoexilis and B. vitrea (sensu Patrick and Reimer, 1966) is examined by Patrick and Reimer (1966), Wolfe and Kling (2001) and Shayler and Siver (2004).

Note that the subcapitate to capitate form illustrated here more typical of this species. It is referred to as Morphotype 2 (Shayler and Siver 2004). Morphotype 1 is distinguished by apiculate rounded to weakly subcapitate apices. Morphotype 1 was found in habitats with a lower pH than habitats with Morphotype 2 (Siver et al. 2004). We consider these two morphotypes as separate entities and we suggest that further taxonomic clarification. According to Compère (1986; 1988) and Wolfe and Kling (2001), B. microcephala would encompass both morphotypes.

Second, the last paragraph of his description creates an impression that morphotype 1 in Shayler and Siver is a morphotype of B. microcephala, while in fact, it is a combination of B. brebissonii and B. arctoborealis.