Cymbella distalebiseriata B. Liu & D.M. Williams

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 : 8

Cymbella distalebiseriata B. Liu & D.M. Williams

( Figs. 18-23 )

Reference. Liu et al., 2018, p. 41, figs. 1-40.

Description. Valves moderatelyly dorsiventral, dorsal margin moderately arched, the ventral margins slightly convex to tumid. The ends of valve either rounded or somewhat pointed in smaller forms, appearing to be slightly protract- ed. Valves 40-58 μm long, 11-13 μm wide. Raphe lateral, becoming reverse-lateral near the proximal ends and filiform near the distal ends. Striae uniseriate in most areas of the valve, but biseriate at the apices, 5-8 in 10 μm in the middle, and 8-11 in 10 μm in the apices. In some specimens, striae on the entire valve surface biseriate (eg., Fig. 18 ). Areolae on a stria apically elongated or lineate, 24- 26 in 10 μm. Stigmata 2-3 in the middle of the ventral side of the valve.

Diagnoses. Cymbella distalebiseriata and C. liyangensis exhibit similarities in valve morphology, with many overlapping characteristics. However, they exhibit difference in areolar density. The former possesses 25-30 areolae in 10 μm, whereas C. distalebiseriata has 19-22 ( Liu et al., 2018; Zhang et al., 2018). Even during their collection in Japan, it was reported that the two species exhibited comparable valve morphologies ( Ohtsuka et al., 2021). Consequently, these two species were regarded as species complex rather than distinct entities in Japan, given that the only distinguishing factor between them was the variation in areola density. Additionally, it was observed that the density of striae displayed continuous variations within the samples. However, C. distalebiseriata has protracted apices that is somewhat subrostrate, making it different from C. liyangensis , which simply has round apices, making it difficult to regard the two species as the same entity.

Occurrence. In South Korea, this species was discovered in the mid-stream of the Ian Stream, a tributary of the Nakdong River in Sangju, and in the lower Danyang Stream in Danyang, which serves as a tributary of the Namhan River. The specimens were derived from filamentous chlorophytes as epiphytes and epilithons. Cymbella distalebiseriata was initially identified in the Shenxi River in Hunan, China ( Liu et al., 2018), and has also been documented in two rivers located in western Japan ( Ohtsuka et al., 2021).

Figures

Figs. 18-23. Cymbella distalebiseriata. 18-21. Valve view under LM, the size variation of valves. 22, 23. Valve view under SEM. The valve exhibiting a flat surface and a distinct ridge between the valve face from the mantle. 23. Estimating the number of stigmata on the external view of the valve is challenging; however, it can be better assessed by examining the internal view. Scale bars=10 μm(18-22); 2 μm(23).