Skeletonema ardens Sarno et Zingone

Figs 1–11

References: Sarno et al. 2007, p. 162, figs 3 A–K; Gu et al. 2012, p. 252, figs 3–6.

Description: Cells are usually arranged in small, delicate, fairly straight to very slightly curved and short chains (2–6 cells per chain) (Figs 1–3), although solitary small cells were also found (Fig. 5). There are regularly one or two chloroplasts per cell (Figs 1–3). The valves are flat to convex, with a mantle relatively high (Figs 4–9), and small spines (or knobs) are scattered over the valve surface, especially on the mantle (Figs 4, 5, 7–9, small arrows in Figs 7, 8). The valves show irregular areolae: rectangular, polyedric or elongate areolae (Figs 5, 7–9, 11). Because cingulum bands may appear collapsed, few details of the cingular bands show the valvocopula with longitudinal, elongate poroids and ribs, whereas copulae have rows of tiny poroids (Figs 7, 8).

All fultoportulae (terminal -TF- and intercalary -IF- fultoportulae) are open, but have a high variation in length (1.3–3.8 μm), especially TF (Figs 4–11). The fultoportulae of terminal valves (TF) may be parallel or have an angle and may extend out of the chain axis (Figs 4, 5), whereas their tips have conspicuous and very often long spines, which may be variable in length (Figs 4–11); some terminal fultoportulae (TF) appeared more truncated (Figs 8, 11). Each IF connects only one IF of the sibling valve (Figs 6, 7). Each fultoportula has three satellite pores (Fig. 11). The rimoportulae of terminal valves (TR) occur in the valve margin, between two fultoportulae (Figs 4–10), are usually tubular and may be longer than the fultoportulae (Figs 4–6, 9), or about the same length (Figs 7, 10), although a very short rimoportula was found at one terminal valve (Fig. 8); extremely long terminal rimoportulae (and fultoportulae) were detected in some few cells (Figs 4, 5), and it is suggested to be also represented in Fig. 1 (small arrow). Rimoportulae of intercalary valves are also tubular, and have the same length of the fultoportulae (Fig. 7).

Measurements: cell diameter 2.3–4.6 μm, areolae density 46–58 in 10 μm (other morphometric data are also provided in Table 1).