Cymbella subcistula Krammer, 2002

Joh, Gyeongje, 2025, The seven species of the genus Cymbella (Bacillariophyta), previously unknown from the inland water of South Korea, Journal of Species Research 14 (1), pp. 6-19 : 15

Cymbella subcistula Krammer ( Figs. 54-63 )

Reference. Krammer, 2002, p. 93, pl. 83, figs. 1-9, pl. 84,

figs. 1-9, pl. 85, figs. 5-8.

Description. Valves strongly dorsiventral, dorsal margin highly arched, ventral margin nearly straight to slightly concave with slightly tumid middle portion. Apices broadly rounded. Valves 34-83 μm long, 13-17 μm wide. Raphe nearly in midline, lateral, becoming slightly reverse-lateral near the proximal ends and filiform near the distal ends. Striae, 8-9 in 10 μm in the middle, and areolae on a stria 18-23 in 10 μm. Isolated stigmata 2-3 in the central parts of the ventral side of the valve.

Diagnoses. Cymbella subcistula exhibits a close relationship in valve dimension and morphology with those of both C. neocistula Krammer and C. cymbiformis C. Agardh. Although variations exist in raphe and central area morphology, as well as in the density of striae and strial areolae, between C. neocistula and other species ( Krammer, 2002; White, 2011), distinguishing between them is challenging because of overlapping ranges. Cymbella neocistula exhibits a lower number of stigmata than that of C. cymbiformis . Additionally, the stigmata of C. neocistula are positioned in proximity to the central striae, and its valve width tapers significantly towards the apices ( Krammer, 2002). In addition to morphological variations, the extent of the dorsal curvature of the valve was assessed. Cymbella subcistula exhibited a pronounced taper towards the apices of the valve, which was more robust than that of C. neocistula . The central area on the dorsal side of C. cymbiformis exhibited notable differences compared with those of the other species. In C. cymbiformis , the central area is not well-developed and the areolae on each stria are more densely packed. Additionally, C. cymbiformis typically has a higher number of stigmata, with 2-3 observed, whereas other species mostly have only one.

Occurrence. In the present survey, Cymbella subcistula was frequently observed as periphytons in the Ian Stream located in Sangju. This species was originally described in West Germany by Krammer (2002). It is occasionally abundant in the mesotrophic waters of temperate zones; however, to date, there have been a limited number of documented occurrences. It has been found in aquatic habitats in the mountainous regions of Macedonia ( Levkov et al., 2005), mountain lakes in China ( Liu et al., 2007), and reservoirs in Ukraine ( Genkal et al., 2020).

Figures

Figs. 54-63. Cymbella subcistula. 54-58. Valve view under LM, showing the size variations of valves. 59-63. Valve view under SEM. 59- 61. Transapically elongated areolae, deflection of the proximal raphe ends and stigmata isolated from the proximal ends of striae. 62, 63. In the internal view of valve, areolae in alveolus and three stigmata on the ventral side. Scale bars=10 μm(54-58); 5 μm(59, 62); 2μm (60, 61, 63).