Navicula craticuloides Y.Li & Metzeltin, 2015

Gong, Zhijun, Metzeltin, Ditmar, Li, Yanling & Edlund, Mark B., 2015, Three new species of Navicula (Bacillariophyta) from lakes in Yunnan Plateau (China), Phytotaxa 208 (2), pp. 135-146 : 136-137

Navicula craticuloides Y.Li & Metzeltin , nov. spec. ( Figs. 1–11 )

Valvae elliptico-lanceolatae apicibus late rotundatis leniter protractis. Longitudo 78–88 μm, latitudo 21–23 μm. Raphe anguste lateralis poris centralibus modice expansis paulo deflexis ad latus secundum valvae fissuris terminalibus distincte hamatis. Area axialis anguste, linearis minime dilatata proximaliter. Area centralis transverse expansa rectangulata. Striae transapicales modice radiantes ad margines versus undulantes in medio tertio valvae tum paulatim subparallelae transientes denique convergentes, 8–9, sub apices 9–10 in 10 μm sed ad aream centralem 2–3 striis irregulariter abbreviatis conspicue distantius sitis inter se. Lineolae curtae apparentes inter costas transapicales alias virgas 28–32 in 10 μm. Similes quoad structuram sunt nonnullae species sub sectionis Navisantigua habitat in lacubus antiqunis terbariis vel depositae fossiles.

Type:— CHINA. Yunnan: Lake Fuxian, 24º 24.870’ N, 102º 50.590’ E, 1715 m a.s.l. Y. Li, May 2005 (holotype: praep. Fuxian 46– 1 in coll. Li Yanling, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, P. R. China, illustrated in Fig. 1 ).

Description:— Valves are elliptical-lanceolate ( Figs. 1–5 ). The broadly rounded ends are gently protracted. Length is 78–88 μm, breadth is 21–23 μm. Raphe branches are narrow-lateral with moderately expanded central ends that are slightly deflected to the secondary side of the valve, terminal fissures are widely hooked. The axial area is narrow, linear, and hardly or slightly extended proximally. The central area is regularly expanded transversely rectangular in shape of a semifascia. Striae are moderately radiate in the central third part of the valve, and become gradually subparallel and are finally convergent towards the poles. Due to the domed valve face, the striae appear wavy towards the margins. Striae density is 8–9 and near the ends not higher than 9–10 in 10 μm. The 2–3 shortened striae at the central area are considerably more widely spaced. Lineolae are comparatively very short apically in contrast to the broader transapical costae, and are 28–32 in 10 μm. The short areola foramina together with an apically orientated pattern of aligned frets give an appearance as the outside of some Craticula Grunow (1868: 20) species, but the interal sternum with a twisted raphe rib conforms clearly to Navicula s. str. SEM observations of the sternum show it is neither depressed nor conspicuously expanded at the central nodule ( Figs. 6–8 ). Central pores are large, almost straight, broadly expanded and drop-like. The central area is almost symmetric ( Fig. 7 ). The terminal fissure is a prolongation of the raphe slit from its terminal pore to the valve mantle with a clear curve to the secondary side ( Fig. 8 ). Uncorroded intact hymens occlude the areolae on the inside of the valve ( Figs. 9–11 ). Sternum on valve interior is narrow ( Figs. 9–11 ).

Etymology:— The epithet refers to the pattern of structures of the external valve surface in SEM, which roughly resemble the genus Craticula .

Distribution:— Known only from Lakes Fuxian and Erhai.

Comments:— Navicula craticuloides is distinguished from N. aurora by its central area and striae density. The central area and striae density of N. aurora are irregularly rectangular and 6–10 in 10 μm, respectively. N. craticuloides belongs to the subsection Navisantiqua Lange-Bertalot (2001: 223) . This group contains, in particular, species living in ancient lakes originating from the upper Tertiary (Neogene) like Lakes Ohrid and Prespa (the Balkans) or species found in fossil deposits from that period.

Figures

FIGURES 1–5. Navicula craticuloides, Light microscopy, valve views showing size diminution series. Fig. 1 is of the holotype.

FIGURES 6–11. Navicula craticuloides. SEM, external views. 6. Entire valve view. 7. Central portion of the valve showing broadly expanded drop-like. 8. View of the end with prolongation of the raphe slit from a terminal pore to the valve mantle with a clear curve to the secondary side. 9–11. Navicula craticuloides. SEM, internal views. 9. Internal valve view. 10. Central portion of the valve with a twisted raphe rib. 11. Uncorroded intact hymens occlude the areolae and helictoglossa slightly offset from raphe terminates.