Cymbella compactiformis B. Liu & D. M. Williams 2020

Cymbella compactiformis B. Liu & D.M. Williams (Figs. 1-17) Reference. Liu et al., 2020, p. 101, figs. 31-57. Description. Valves moderately to strongly dorsiventral, semi-lanceolate in outline, but semi-rhomboidal in smaller specimens, dorsal margin strongly arched, and ventral margin with slightly tumid middle portion. Apices broadly rounded to subrostrate, and somewhat protracted. Valves 40-109 μm long, 14-21 μm wide. Raphe lateral, reverse-lateral near the proximal end and the distal ends. Striae 6-9 in 10 μm in the middle, and areolae on a stria apically elongated or lineate, 16-20 in 10 μm. Isolated stigmata 2-5 in the middle of the ventral side of the valve. Diagnoses. In C. compactiformis, the valve outline is not cymbiform. Additionally, the apices of the valve are thick and the valve is notably wider than those of other species in its vicinity. This species exhibits morphological similarities with those of C. compacta Østrup (Krammer, 2002) and C. paenetruncata Li & Gong (Gong et al., 2013). However, it differs from C. compacta in terms of valve width (14-21 vs. 11-15 μm) and stria density (6-9 vs. 10-14 in 10 μm). Furthermore, it is significantly distinct from C. paenetruncata in its valve dimensions of length and width. Cymbella compactiformis occurred in South Korea has a significantly lower density of areolae compared to Chinese specimens (16-20: 19-22 in 10 μm). Occurrence. Cymbella compactiformis was frequently observed as periphytic and benthic diatoms in the Korean fresh water: the mid-reach of the Namhan River in Danyang, and the Bukhan River in Gapyeong, the mid-stream of the Ian Stream in Sangju, the upper-reaches of the Dongchang Stream in Cheongdo, Lake Bomun in Gyeongju, and Lake Jangseong in Jangseong. Among the seven species, it exhibited the highest frequency of observation and a broad distribution. This species was initially designated in a river in the Hunan Province of the subtropical climate zone of China (Liu et al., 2020), and it was also recently reported in some rivers in Japan (Izumino et al., 2023). This species appears to be endemic, being restricted to the region of Northeast Asia.