Diploneis aff. heteromorphiforma Metzeltin, Lange-Bertalot & Nergui, 2009

Jovanovska, Elena, Levkov, Zlatko & Edlund, Mark B., 2015, The genus Diploneis Ehrenberg ex Cleve (Bacillariophyta) from Lake Hövsgöl, Mongolia, Phytotaxa 217 (3), pp. 201-248 : 236-237

Diploneis aff. heteromorphiforma Metzeltin, Lange-Bertalot & Nergui (Figs 206–210, 213–219)

The valves are linear-elliptical with slightly convex margins and round ends (Figs 206–210). The valve length is 16.0–29.0 μm, and the valve breadth is 8.5–11.5 μm. The axial area is narrow, lanceolate, and expands toward the round to elongate central area. From inside, a thick lanceolate silica plate covers the longitudinal canal (Fig. 216). The central area is round to elongate, 2.5–4.5 μm wide. The longitudinal canal is narrow, linear to lanceolate, expanded in mid-valve, composed in parts with two rows of areolae and in parts with one row of areolae (Fig. 214). The external openings of the canal areolae are covered with cribra, similar to those in the striae and from which they are separated by a thin hyaline area (Figs 214, 215). Internally, the longitudinal canal is closed with thick silica plate throughout its length, forming a “depression” where the raphe is located (Fig. 216). Externally, the raphe is simple and straight with slightly curved and expanded central pores (Figs 213, 214). The distal raphe branches are deflected, and the terminal fissures are slightly bent into a drop-like shape (Figs 214, 215). Internally, the raphe is straight with simple and slightly expanded proximal ends, inserted in a slightly elevated sternum inside the wide “depression” formed by the longitudinal canal (Fig. 216). The distal raphe endings are slightly curved (Fig. 218). Only the proximal raphe endings reach the height of the canal itself (Fig. 217). The striae are radiate and uniseriate becoming biseriate toward the valve margins, 9–11 in 10 μm. The striae areolae are round and occluded externally with cribra, 25–30 in 10 μm. The areolae are barely visible with LM observations, leaving the impression that each alveolate striae is composed of two rows of areolae (Fig. 206). In highly corroded valves it seems that from the valve exterior the alveolus opens though a single opening covered with cribrate occlusions (Fig. 219). The alveoli each open to the valve interior via a single continuous opening (Figs 216–218). However, from the inside, it appears that each alveolus initially opens toward the valve exterior via three areolae aligned in staggered transapical rows (Figs 217, 218), then transition to the cell exterior through a layer of one or two cribrate occlusions that must each cover multiple internally staggered areolae (213–215). The external openings of the areolae (uniseriate becoming biseriate) and the LM observations (biseriate throughout) is misleading as to the detailed structure of the alveolate striae and their areolae.

Observations:— Diploneis aff. heteromorphiforma closely resembles D. heteromorphiforma , described from Zyyn byrkh River in Khentii province in Mongolia ( Metzeltin et al. 2009). Morphologically D. aff. heteromorphiforma is almost identical to D. heteromorphiforma , but differs in size range (length: 16.0–29.0 μm vs. 22.0–43.0 μm; wide: 8.5– 11.5 μm vs. 10.0–15.0 μm) and in the shape of the axial area (narrow-lanceolate vs. wide to narrow-lanceolate). Stria density is an additional character that separates D. heteromorphiforma [“8–9 (not 10)”] from D. aff. heteromorphiforma (9–11 in 10 μm).

When describing D. heteromorphiforma, Metzeltin et al. (2009) illustrated two different morphotypes [narrow, lanceolate axial area (pl. 108, figs 1–3, 16) vs. wide, lanceolate axial area (pl. 108, figs 4–15)], and designated the wide axial area form as the type (pl. 108, fig. 4). However, when illustrating type specimens they referred to both narrow and wide axial area specimens (pl. 108, figs 1–3, 6, 10–12, 14). Due to this, it is rather difficult to characterize D. heteromorphiforma , and an emended typification is in order to clarify the identity of the given morphs. Furthermore, Metzeltin et al. (2009) were not consistent in designating the type locality for D. heteromorphiforma . In the description they gave Zyyn byrkh River as a type locality, while in the illustrations they point to Jargalant River (pl. 108, fig. 4). Additionally, Metzeltin et al. (2009) argue that the population from Zyyn byrkh River is characterized with morphological variation within the axial area, explaining the variation as possible for abnormal clones or maybe a separate species. Such morphological variations are observed neither in Lake Hövsgöl nor in the Arkhangai province. Furthermore, in the description Metzeltin et al. (2009) compared D. heteromorphiforma to a non-existent D. heteromorpha Hustedt , when they should have referred to Diploneis dimorpha Hustedt (1937 : pl. 1, fig. 18–19). Keeping in mind that D. dimorpha is characterized by heterovalvar frustules and has a distribution restricted to marine and brackish environments, it is rather unlikely that D. dimorpha should be compared with D. heteromorphiforma and D. aff. heteromorphiforma ( Hustedt 1937, Witkowski et al. 2000, Witon et al. 2006). The heterovalvar frustule is nicely visible in Simonsen’s illustrations (1987, pl. 680, figs 12–14). In order to support the comparisons and therefore to observe the heterovalvar feature, the Hustedt type material of D. dimorpha (O4/78) was analysed and included in this study (Figs 211, 212). According to the comparisons, no Mongolian population seems to be allied with the type of D. dimorpha . When describing D. heteromorphiforma, Metzeltin et al. (2009) mentioned Genkal’s report for D. dimorpha ( Lange-Bertalot & Genkal 1999: 194, pl. 43, figs 12, 13), characterized with frustules that have isomorphic valves and suggested their separation into a new species. According to the comparisons, D. aff. heteromorphiforma appears to be different from both D. heteromorphiforma morphotypes ( Metzeltin et al. 2009) but closely allied to Genkal’s description of D. dimorpha ( Lange-Bertalot & Genkal 1999) . Further analysis is therefore preferable in order to clarify the morphological variation within the D. heteromorphiforma complex and possibly to ascertain any phylogenetic relationships with D. dimorpha .

The original drawings (iconotype) of D. robusta Cleve-Euler (1953 , fig 622 a–c) are very similar to D. aff. heteromorphiforma , whose numerical features unambiguously fit the measurements given in the original description of D. robusta . However, additional analyses such as observation of D. robusta type material are needed in order to test the similarity, or the possible conspecificity of these species.

Ecology and Distribution: —M068A; M166A; M167A: found in a shallow epilithic collection from southern Lake Hövsgöl and also in streams of Arkhangai province .