Encyonema liujiangensis Y.-L. Li sp. nov. (Figs 1–20)

Description:

LM (Figs 1–18): Valves distinctly asymmetrical and dorsiventral, semielliptical-lanceolate, dorsal margin broadly or strongly arched, ventral margin slightly elevated in the middle, curving significantly towards the ends, approximating the curvature of the dorsal margin. Many specimens are slightly heteropolar about the transapical axis. Both ends gradually tapered, typically acutely rounded but occasionally approaching broadly rounded. The headpole slightly less acute with a shorter half-axis from the central nodule, whereas the footpole more acutely rounded with a longer half-axis. Valve length 8.5–17.5 um, width 3.0–4.0 um, valve length/width ratio varying between 2.6–4.2, median 3.2 (n=30). Raphe positioned nearly along the central part of the valve, nearly straight and linear. Terminal raphe ends slightly expanded and curved towards the ventral margin. Central raphe ends strongly deflected towards the dorsal margin. Axial area narrow and extending along the apical axis. Central area small, almost absent, with little to no shortened striae along its borders. The striae nearly parallel in the mid-valve, becoming slightly radiate towards the ends, density 9–11 in 10 um in the mid-valve, up to 13–18 in 10 um at the ends. Areolae not resolvable.

SEM: Valve exterior (Figs 19–25): The terminal fissures curved towards the ventral side, forming a sickle-shaped structure (Figs 19, 20). The central raphe ends slightly expanded and deflected towards the dorsal side (Figs 21–25). The middle part of the valve may lack shortened striae (Figs 21, 22). Striae uniseriate, composed of a single row of areolae, predominantly slit-like or short lineolate at the ends of the valve, gradually becoming more elliptical or dot-like in shape towards the central area (Figs 22–25). Apical pore field and isolated pore absent (Figs 19–25).

Valve interior (Figs 26–32): The terminal raphe ends offset, terminating in small, knob-like helictoglossae (Figs 26, 27). The intermissio is absent (Figs 28–32). Striae formed by uniseriate, short lineolate or dot-like areolae, delimited by slightly oblique orientated vimines (Figs 26–32; arrow A in Fig. 27). Small beam-like projections extend above the intercostal ribs (Figs 26–32; arrow B in Fig. 28). A small elliptical non-penetrating depression present at the central region of the valve (Figs 28, 29; arrow C in Fig. 28). A pseudoseptum present at the footpole (Figs 26, 29–32; arrow D in Fig. 32).

Type:— CHINA. Guangxi Zhuang Autonomous Region: Liujiang River, upper tributary of unknown name, freshwater, 24° 44.56′ N, 107° 55.02′ E, elevation 675 m a.s.l., 8th July 2018, stones collected by Prof. Wei Liu (Holotype LJ- 1 in Coll. Wei Liu, Yunnan University, Kunming, China = Fig. 1; Isotype YUNGL20220713, Harbin Normal University, Harbin, China).

Etymology:— liujiangensis, referring to the type locality from which the new species was obtained.

Ecology: — Encyonema liujiangensis, is known only from the Liujiang River, in a littoral habitat with low COD Mn (0.9 mg L- 1), BOD 5 (4.05 mg L- 1) and NH 4 + -N (0.066 mg L- 1), and moderate DO (8.47 mg L- 1). At the type locality E. liujiangensis was uncommon (<1% abundance); the related species in decreasing order of abundance were Achnanthidium minutissimum (Kützing) Czarnecki (1994: 157) (16.1%), Nitzschia palea (Kützing) Smith (1856: 89) (14.8%), Halamphora montana (Krasske) Levkov (2009: 207) (6.5%), Navicula trivialis Lange-Bertalot (1980: 31) (6.3%), Humidophila contenta (Grunow) Lowe, Kociolek, J. R. Johansen, Van de Vijver, Lange-Bertalot & Kopalová (2014: 357) (5.4%), Encyonopsis leei Krammer (2003: 514) (2.7%), Nitzschia valdecostata Lange-Bertalot & Simonsen (1978: 58) (2.4%), Cocconeis placentula Ehrenberg (1838: 194) (2.1%), Luticola mutica (Kützing) Mann (Round et al. 1990: 670) (2.1%) and other unidentified species of Gomphonema Ehrenberg (1832: 87) and Cymbella Agardh (1830: 1).