Navicula sinicomeniscus Li & Metzeltin, 2023

Xiao, Qiao-Zhi, Guo, Ji-Shu, Tang, Zheng-Bin, Kociolek, John Patrick & Li, Yan-Ling, 2023, Navicula sinicomeniscus sp. nov. (Bacillariophyta; Naviculaceae), a new species from southwestern China, Phytotaxa 591 (4), pp. 241-252 : 243

Navicula sinicomeniscus Li & Metzeltin sp. nov. (LM: Figs 1–10 , SEM: 11–16)

LM: Valves linear-lanceolate, with cuneate ends and bluntly rounded apices (also the ends of largest specimens are never protracted). Length 55–75 µm, width 16–18 µm. Length/width ratio 3.3–4.1, median 3.7 (n=30). Raphe filiform, almost straight or weakly curved, and occasionally appears lateral. Central raphe ends expanded and inconspicuously deflected to the secondary side of the valve with drop-like central pores. Terminal raphe fissures rather small and bent towards the same side of the valve. Axial area moderately narrow, linear, slightly expanded proximally. Central area comparatively much less expanded, transversally widened, asymmetrical, shape variable, with one or few obviously irregularly shortened central striae. Striae radiate more strongly towards the center, subapical striae almost parallel, density 7–8 in 10 µm. Voigt faults appear indistinct. Areolae coarse and visible, 22–24 in 10 µm.

SEM: External views: Raphe almost straight to slightly undulate with central raphe ends dilated, drop-shaped and slightly inclined to the secondary valve side. The terminal fissures clearly hooked in the same direction. Raphe sternum prominently developed, narrow, raised along the raphe, more obviously towards the center (arrow A in Fig. 11 ). Areolae slit-like ( Figs 12, 13 ), with one row of apical areolae, arranged horizontally, positioned on the valve apex and mantle (arrow B in Fig. 12 ). Striae uniseriate, with two obviously shortened striae near the central area (arrows C in Fig. 13 ) and two occasionally intermittent striae (arrows D in Fig. 13 ).

Internal views: Striae uniseriate, irregularly shortened in central area, composed of round-elliptical, roundrectangular areolae, separated by robust silica ribs. Sternum broad and raised, not interrupted, scarcely expanded in the central pore and inclined to the secondary valve side; lacking central area besides the sternum (black arrows in Figs 14, 16 ). Raphe continuous, lacking an intermissio. Terminal raphe ends terminate in small and knob-like helictoglossae ( Fig. 15 ).

Type: CHINA. Yunnan Province: Fuxian Lake, Fuxian 46–1, 24°24.870′ N, 102°50.590′ E, elevation 1720 m asl, Prof. Yan-Ling Li, 28th March 2005 (holotype Praep. Fuxian 46– 1 in Coll. Yan-Ling Li, Yunnan University, Kunming, China = Fig. 4 ).

Etymology: the epithet refers to the most similar established taxon and the country ( China) of the type location.

Ecology: Navicula sinicomeniscus was found in a low altitude freshwater lake with moderately alkaline pH (8.71) and very low concentrations of nutrients (TN 0.14 mg ̛L- 1, TP 0.02 mg ̛L- 1). The dominant diatom species discovered in this area include Aulacoseira granulata (Ehrenberg) Simonsen (1979: 58) (1.7 %), Cyclotella rhomboideo-elliptica Skuja (1937: 49) (6.9 %), Cyclotella krammeri Håkansson (1990: 263) (46.4 %), Fragilaria crotonensis Kitton (1869: 110) (7.8 %), Stephanodiscus minutulus (Kützing) Cleve & Möller (1882: 300) (1.1 %), Stephanodiscus hantzschii Grunow (in Cleve & Grunow 1880: 121) (8.6 %), and some unidentified species of Achnanthidium Kützing (1844: 152) (6.7 %) and an Amphora Ehrenberg ex Kützing (Kützing 1844: 107) sp. (4.3 %). These entities comprise more than 83 % of all diatoms enumerated in the samples.

Figures

FIGURES 1–10. Navicula sinicomeniscus sp. nov. LM. Valve views, showing size range and variability of the holotype population.

FIGURES 11–13. Navicula sinicomeniscus sp. nov. SEM, external views. 11. External view of an entire valve. The arrow A points to developed, narrow, raised sternum. 12. Valve apex, with striae with long slit-like areolae and undulate distal raphe fissure bent onto the mantle before the apex. The arrow B points to a row of visible, horizontal areolae positioned on the valve mantle at the apices. 13. External view of valve center, showing the drop-like shaped proximal raphe endings and interrupted sternum. The arrows C and D point to shortened and interrupted striae, respectively.

FIGURES 14–16. Navicula sinicomeniscus sp. nov. SEM, internal views. 14. Internal view of entire valve with an obviously raised raphe sternum. 15. Internal view of valve apex with prominent helictoglossa. 16. Valve center showing not interrupted raphe, shortened striae. The black arrows in Figs 14 and 16 point to broad and raised, not interrupted sternum.