Nitzschia amphibioides Hustedt 1942

Nitzschia amphibioides Hustedt (Figs 99–112; 113–138) Basionym: Nitzschia amphibioides Hustedt 1942, Internationale Revue der gesamten Hydrobiologie und Hydrographie vol. 42, p. 132, fig. 283–288. Lectotype: slide 397/40a (BRM), Finder Reference (Zeiss) 346.4–7. Celebes. 54. Towoeti-See (Lake Towuti, Sulawesi, Indonesia), Hustedt Collection (BRM). Lectotype designated by Simonen (1987, p. 291, fig. 283). LM (Figs 99–112): Valves linear-lanceolate to linear, with cuneate ends (Figs 99–112). Small specimens are more lanceolate (Figs 102, 107), the larger ones more linear (Figs 109–111). The ends are typically cuneate (Figs 103, 105, 108). Valve face striae parallel, composed of single rows of circular areolae (Figs 99–101). Near the apices, the striae exhibit a slight divergence in their directions (Figs 104, 106, 112). Fibulae are clearly visible, unevenly to somewhat evenly distributed. Their shape varies from rectangular (Figs 100, 103, 106, 108, 109) to pointed (with the sharp end towards the valve face: Figs 99, 101, 102, 105, 107, 110–112) and/or branched (tooth-root-like), extending along two or three virgae (Figs 99, 102, 104, 105, 110–112); they can be short or may extend up to half the valve width (Figs 102, 104, 105, 110–112). Portulae can be detected as a smooth line parallel to the raphe margin (arrows in Figs 103, 109). There is no distinction of central fibulae or any signs of raphe interruption. SEM (Figs 113–138): The frustules show nitzschioid symmetry (Figs 113, 117, 119) and are rectangular in girdle view (Fig. 113). The areolae are large, circular or elliptical (Figs 120, 123), and occluded internally by hymenes, with their pores in a hexagonal arrangement. (Figs 123, 125, 126). Externally, the areolae bear structures associated with and apparently linked to the internal hymenes, in the form of reticulate cribra. These structures are significantly below the valve surface, often making them difficult to observe. However, they are frequently detectable in the areolae of the raphe canal, where the thicker median bar tends to be apical rather than transapical. (Fig. 118). Between the areolae, coarse warts are scattered on the valve face (Figs 117, 119, 120). The virgae are very robust. Internally they project above the vimines (Figs 121–123, 125–127), appearing as thin strips between areolae (Figs 132, 135). Externally, the vimines are much wider and only slightly less developed and prominent than the virgae (Figs 113–119). The virgae also bear coarse warts (Figs 119, 120). The keel is marginal, with slightly raised ridges, and the raphe is continuous from pole to pole (Figs 117, 129, 133). The raphe canal areolae are slightly offset from the valve face/mantle areolae and are usually single and round (Figs 117–120, 130, 134), can also occur in pairs (Figs 114–115, 137). These areolae have a raised rim of silica surrounding their external apertures (Figs 117–120, 130, 134). Fibulae occur on every second or third virga, extending across the valve face for up to half its width (Figs 123, 128). On the mantle side, the fibulae often split into two, each half connecting with a mantle virga (Figs 126–128). The terminal raphe endings are bent or hooked and lie on the mantle apices (Figs 116, 138). The mantle on the raphe side has striae composed of three areolae, more rarely two, four or five (Figs 113–116, 129–131, 133). The first areolae may show a thickening of silica along its breadth and sometimes apart from the other areolae. On the opposite side of the valve from the raphe, the mantle contains only one areola (Figs 114, 119, 136–138). It is separated from the valve face by a slight ridge (Figs 116, 119, 120). Coarse warts may be scattered on the mantle, between the areolae (Figs. 124, 130, 131, 134). The girdle is incompletely known, the only band remaining in the specimens examined being the valvocopula. This has a row of unevenly distributed small poroids close to the mantle (Figs 114, 119, 136, 137), which fades out towards the apices (Figs 119, 138); it also bears scattered warts throughout (Figs 114, 128, 134, 137). Measurements obtained in this study: apical axis 60–114 µm, transapical axis 8–10 µm, pervalvar axis ca. 10 µm, 5–6 fibulae in 10 µm, 12–13 striae in 10 µm, 12–14 areolae in 10 µm, and 13–18 pores in 10 µm in the valvocopulae. The original description (Hustedt 1942) gave 32–115 × 9–10 µm, with 5–7 fibulae, 11.5–14 striae and 12–16 areolae in 10 µm. Remarks: Based on observations of specimens with different levels of erosion, it seems that the areola structure in N. amphibioides is similar to that of N. amphibia, with a coarse external meshwork (cribrum) appressed to and reinforcing an internal hymen. In the valve interior shown in Fig. 126, the occlusions are quite strongly eroded, revealing the form of the cribra, which, in valve face areolae, have a transapical bar as in N. amphibia. The occlusions are more complete in Fig. 125 and careful observation of the original photographs reveals that the hymenes have hexagonally arranged pores.