Nitzschia amphibia Grunow 1862

Nitzschia amphibia Grunow (Figs 2 –49; 50–79) Nitzschia amphibia Grunow 1862, Verhandlungen der Kaiserlich-Königlichen zoologisch-botanischen Gesellschaft in Wien, vol. 12, p. 574, pl. 28, fig. 23. Lectotype (designated here): Grunow sample 188 (including slide 188a, W0164849 and slide 188b, W0164850), Grunow collection (W) from the shore of the Neusiedlersee (Lake Neusiedl) at “Holling”, referred to by Grunow (1862: 574): see below “Notes on the lectotype of N. amphibia ” and Figs 1–3 Valves from Grunow sample 188 showing size reduction in the type population are illustrated in Figs 5–20 (micrographs kindly provided by B. Van de Vijver). LM (Figs 2 –68): Frustules presenting nitzschioid symmetry (Figs 4, 56–58), rectangular in girdle view (Figs 32, 33, 51, 57, 65). Valves are lanceolate or linear-lanceolate (Figs 5, 11, 18, 39, 52), mostly with parallel sides (Figs. 12, 23, 50, 59, 67). Smaller specimens tend to be more lanceolate (Figs 19, 34, 35, 55) or have only a short region in the center where the margins are parallel (Figs 18, 26, 49). Larger specimens have a much longer parallel region (Figs 5, 8, 23). Margin may present a slightly constricted region at the center (Figs 7, 14, 24, 25, 64). The apices are generally cuneate (Figs 11, 17, 27, 38). Striae uniseriate, easily visible, parallel, sometimes slightly curved towards the valve ends (Figs 13, 14). The areolae can be visible, though discreetly, and appear circular. (Figs 9, 12, 13 –22, 36, 48, 68). Fibulae are short and vary in shape, somewhat unevenly distributed. (Figs. 16–20, 40–42) The central fibulae are often further apart from each other than other fibulae (e.g. Figs 6, 17, 37, 43). In many cases fibulae branch along two or three virgae, extending towards valve face no more than 1/3 of valve width, resembles tooth roots (Figs 9, 13, 14, 18, 30, 52); Less often, they are rectangular, can be adjacent in numbers of up to three (Figs 8, 10, 17, 20, 31, 39, 67), and may taper. (Figs 7, 9, 53). A smooth line can be seen running parallel to the raphe margin, corresponding to the openings of the raphe canal into the cell lumen. (Figs 29 arrow, 12, 28, 59, 61–63). SEM (Figs 69–98): Areolae circular or elliptical (Figs 69–71), occluded internally by hymenes with pores in a hexagonal arrangement (Fig 79); externally, the hymenes are accompanied by a cribrum with a reticulate structure and often a median transapical bar-like thickening (Figs 80, 81, 97). Virgae are thick and elevated from the valve face internally and are connected by thinner vimines (Figs 72, 73, 89). Keel marginal, narrow, sometimes somewhat raised from the valve face (Figs 84, 90). Raphe canal walls areolate, usually with two paired areolae opposite each stria on the valve face and mantle. These areolae are somewhat apart from the striae and are smaller and spaced closer to each other than the areolae on the valve face and mantle striae. (Figs 70, 92); Raphe canal areolar occlusions appear to be more external than in other areolae (Figs 83, 91). Internally, the fibulae can be seen extending laterally across up to three interstriae and slightly onto the valve face (Figs. 74, 78). The raphe is interrupted at the center and its endings are straight, slightly expanded and drop-shaped (Figs 70, 84). The terminal raphe endings are bent or hooked at the mantle apices (Figs 82, 85). Portulae can be seen internally (Figs 86–89), each being a circular to elliptical or subrectangular aperture (Figs 74–78). Short striae are present on both sides of the mantle. The mantle on the raphe side has striae composed of 2–4 areolae, with the areolae closest to the raphe being doubled or paired (Figs 92–94). On the mantle opposite the raphe, each striae contains only one areola (Figs 95–98). There seem to be four bands in a complete epicingulum. The valvocopula is a wide and open band with a row of round, unevenly distributed, small poroids next to the mantle (Figs 93–98), whereas the other bands (copulae) are nonporous. The third band is also wide, though lesser than the valvocopula. Both larger elements bear small, unevenly distributed warts (Figs 97, 98). The second band is very narrow and delicate and open at one pole, but the extent to which it fills the gap between the ends of first and third bands at the other pole is unclear; the same uncertainty surrounds the almost equally narrow fourth band, which is visible in Fig. 98. Measurements: apical axis 6–70 µm, transapical axis 3.5–6 µm, pervalvar axis 3.8–5.5 µm, 6–10(–12) fibulae in 10 µm, 13–18 striae in 10 µm, and 16–24 areolae in 10 µm. The maximum length given here corresponds well to the maximum of the initial cells observed by Geitler (1969) in the only study published to date on the life cycle of the species. Remarks: Despite observations of many complete and partly disrupted frustules and thecae, some details of the girdle structure remain to be confirmed. Many previous authors have illustrated and commented on individual features of the N. amphibia valve; we have not attempted to give credit to all of them here – they are listed in the bibliographies by Gaul et al. (1993), available online at https://www.jstor.org/stable/4065006, and the update by Henderson & Reimer (2003).