Diploneis aestuarii Hustedt, 1939

Kim, Bongho & Park, Jong Seong Khim and Jinsoon, 2021, Taxonomy and distribution of two small negligible diatoms of Plagiogrammopsis minima and Diploneis aestuarii from Northeast Asian tidal flats, Journal of Species Research 10 (3), pp. 237-245 : 241-243

Diploneis aestuarii Hustedt, 1939 ( Figs. 4 , 5 )

Reference. Hustedt, 1939, p. 612, figs. 41, 42; Simonsen, 1987, p. 254, figs. 376: 21-26.

Taxonomic remarks. The members of the genus Diploneis Ehrenberg ex Cleve are mostly marine or brackish species, with only a small number being freshwater species. Valve faces of the Diploneis species are to be divided into two parts, firstly longitudinal canal and also the mar- ginal part ( Jovanovska et al., 2015). The size of our specimens is slightly larger compared to the description of Diploneis aestuarii in Hustedt (1939) where the frustules were 15-20 μm in length and 6-7 μm in width. The density of striae, shape of frustules, square central nodule, and narrowly linear longitudinal canal are well matched with the original description of D. aestuarii . The majority of Diploneis species reported from Korea ( Noh and Choi, 1992; Cho, 2015) are much larger in size compared to D. aestuarii and readily distinguishable. The size range of Diploneis rimosa Pennesi et Caputo sp. nov. (Figs. 98- 107) ( Pennesi et al., 2017) is similar to D. aestuarii both in terms of length (12-21 μm) and width (6.3-9.7 μm) with transapical striae of 16-20 in 10 μm. However, D. aestuarii has a much longer valve length compared to valve width and characteristic in having a central nodule with four areolae as C-shaped foramen.

LM observation. Outline of the valves is linear-ellip- tic, slightly constricted in the central shape with bluntly rounded apices, 10-21.3 μm in length, 5.3-7.3 μm in width. Raphe-sternum along with longitudinal canal is narrowly linear. The central nodule is rarely distinct but small and square. Transapical striae are almost parallel in the central region and gradually radiate to both apices. The density of striae is 15-18 in 10 μm, and areolae are inconspicuous ( Fig. 4A- N ).

SEM observation. External view of the transapical striae is shown under LM but not observed on the valve face ( Fig. 5A- D ). Areolae, as C-shaped foramen, were aligned both along the valve margin and raphe sternum ( Fig. 5A ). Raphe system is very narrow ( Fig. 5B ), and terminal raphe endings were bent to the same direction ( Fig. 5C, D ). Central raphe endings were bent in the opposite direction, and the area around the central nodule was deeply depressed with four areolae ( Fig. 5B ). Internally areolae are rounded perforations of cribra ( Fig. 5E, G, H ). Internal terminal fissures were bent to the same side ( Fig. 5G, H ) as the central raphe endings ( Fig. 5F ).

Distribution. Diploneis aestuarii was first reported from the German coast by Hustedt (1939). Simonsen (1987) observed the species from Rheiderland Shore, Bingum in East Frisia Island, Germany. The species is also wide- spread along the European coasts of Atlantic Ocean and western Baltic Sea ( Witkowski et al., 2000). Diploneis aestuarii was also reported from a saltern in Sinan, Korea ( Bae et al., 2020), and Isahaya tidal flat in the Ariake Sea, Japan ( Ohtsuka, 2005). The present study would be second report of the occurrence of the species from Korea, and the very first report of detailed morphology of the species. In the present study, D. aestuarii was observed from 10 locations in Korea, five locations in China, and one location in Japan ( Fig. 1 ).

Figures

Fig. 4. Light microscope (LM) photographs of Diploneis aestuarii: A-N. Size range in valve view. Scale bar=10μm.

Fig. 5. Scanning electron microscope (SEM) photographs of Diploneis aestuarii: A. External view of the whole valve; B. Arrow indicates central nodule with four areolae. Large Arrowhead indicates central raphe endings which are bent to the opposite direction; C, D. Large ar- rowhead indicates C-shaped foramen areolae.Arrows indicate terminal raphe endings which are bent to the same direction; E. Internal view of the whole valve; F. Arrow notes central raphe endings; G, H. Large arrowhead indicates rounded perforations of cribrum. Arrows indi- cate internal terminal fissures.

Fig. 1. The distribution of Plagiogrammopsis minima and Diploneis aestuarii in the Northeast Asia.