Nitzschia semirobusta Lange-Bertalot 1993

Nitzschia semirobusta Lange-Bertalot (Figs 139–160; 161–177) Nitzschia semirobusta Lange-Bertalot 1993, Bibliotheca Diatomologica vol. 27, p. 149, pl. 120, figs 3–21, pl. 122, fig. 7, pl. 123, figs 1–7. Holotype: Preparation Am-S 92, collection Lange-Bertalot (at FR, the Senckenberg Institute). Punta d. Roca, Masaya, Nicaragua, coll. M. Pumm, 24 June 1988. LM (Figs 139–160): Frustules with nitzschioid symmetry, rectangular in girdle view (Figs 139, 144, 146–148, 156, 157). Valves lanceolate with cuneate apices (Figs 149–155, 158–160). Striae comprising simple uniseriate rows of circular areolae, mostly parallel on the valve face but may be slightly curved or bent towards the ends (Figs 149–155, 158–160). Fibulae long, unevenly to somewhat evenly distributed, with central fibulae sometimes (Figs 140, 154) but not always spaced further apart from each other (Figs 139, 141–153, 155, 159). The fibulae are somewhat uneven in length, pointed, and reach one-third to two-thirds of the valve width towards the other margin (Figs 142, 143, 145, 158–160). In girdle view the fibulae are visible, straight and pointed, and extend almost to the valve margin (Figs 139, 148). Between the central fibulae, the raphe canal (i.e. the margin, when seen in valve view) may be very slightly constricted (Figs 140, 157, 158, 160). The portulae are visible, as a smooth line parallel to the raphe margin (Fig. 160 arrow). SEM (Figs 161–178): Coarse areolae occur on the valve face (Figs 161, 167, 171), occluded internally by hymenes with hexagonally arranged pores (Fig. 177). Externally, the hymenes are supported by a coarser cribrum, which sometimes has a transapically directed median thickening (Figs 164, 168–170). Virgae robust, elevated and connected by thin vimines internally (Figs 174–176). Externally, however, the virgae and vimines are almost equally developed (Figs 161, 171). The areola occlusions in the raphe canal are more external than on the valve face and mantle (Figs 162, 166, 168). The raphe canal areolae are also narrower and therefore appear more elliptical than the areolae on valve face and mantle (Figs 161, 164, 166). The raphe is interrupted at center (internally Fig. 178), with deflected, slightly depressed and drop-shaped endings (Figs 168, 172); the raphe fissure is bordered externally by fine ‘lines’ (extremely narrow ridges bordered by tiny pits: Figs 168–170, 172). The terminal raphe endings are bent or hooked, lying on the mantle (Figs 170, 173). The keel is marginal, somewhat raised from the valve face, with a single row of areolae opening into the raphe canal on either side of the raphe. The margin opposite to the raphe presents a ridge elevated from the valve face (Figs 161, 165, 167). In internal view, the fibulae extend from the mantle and continue along the virgae on the valve face, occupying one-third to two-thirds of the valve width (Figs 174, 176, 178), each with a knob-like structure below the raphe (Fig. 175 arrows). Portulae are present connecting the raphe canal to the cell lumen, their shapes varying from round to ovalsubrectangular. (Figs 174–176, 178). Short striae are present in both sides of the mantle. On the mantle closest to the raphe, the striae are composed of three or four areolae, with the areolae near the raphe having the same spacing as the others (Fig. 163, 167; in Figs 165 and 166 part of mantle is hidden by the third and fourth girdle bands). The areolae nearest to the valvocopulae may be subdivided into two or three smaller ones (Figs 163, 164). On the side of the mantle distant from the raphe, the striae contain one areola (Fig. 173), which again may be subdivided into two (Fig. 165). Each cingulum contains four bands, all open (Figs 165, 173). The valvocopula and third band are wide, whereas the second and fourth are very narrow (Fig. 165). Scattered warts occur all over the first and third bands (Figs 164, 166). Measurements obtained in this study: apical axis 6.5–23 µm, transapical axis 4–5 µm, pervalvar axis 4–5.5 µm, 6–10 fibulae in 10 µm, 14–18 striae in 10 µm, and 12–20 areolae in 10 µm. These figures extend the ranges given by Lange-Bertalot (1993, p. 149), who gave 12–15 × 4–6 µm, with 5–9 fibulae and 13–17 striae in 10 µm.