Diploneis hoevsgoelensis, Jovanovska & Levkov & Edlund, 2015

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 : 206-214

Diploneis hoevsgoelensis sp. nov. ( Figs 1–40 )

Valves are broadly lanceolate to rhombic-elliptic becoming circular with smaller cell size. Valve apices are bluntly round ( Figs 1–8 , 24–30 , 36, 39, 40 ). Valve length is 43.0–106.5 μm and valve breadth is 25.0–44.5 μm. The axial area is linear to lanceolate, expanding toward the round to transapically elongated central area. Internally, a thick rhombic silica plate covers the whole length of the longitudinal canal ( Figs 14 , 35 ). Externally, the central area is round to transapically elongate, 4.5–9.5 μm wide ( Figs 9 , 36, 39 ). Externally, the longitudinal canal is lanceolate and expanded in the middle of the valve with two to five rows of areolae narrowing into two to one at the valve apices ( Figs 1–3 , 24, 25 , 36 ). From outside the areolae of the longitudinal canal are covered with cribrate occlusions similar to the external opening of the striae, from which they are separated by a thin hyaline line hardly visible on SEM (arrow on Figs 9 , 36 ). Internally, the longitudinal canal is closed with a thick silica plate throughout the whole length. The silica plate that encloses the canal is raised above the level of the raphe, leaving the raphe in a deep “depression” ( Figs 14–16 , 31, 32, 35 ). Externally, the raphe is straight, with expanded drop-like proximal ends that are bent to the same side of the valve, positioned within a small depression ( Figs 8, 9, 13 , 36, 37 ). The distal raphe ends terminate with short terminal fissures that are bent to the same side of the valve ( Figs 8, 10 , 36, 38, 39 ). Internally, the raphe is straight and placed in the depression formed by the longitudinal canal with simple proximal and distal raphe ends that are slightly elevated ( Figs 14–16 , 31, 32, 35 ). The striae are parallel in the middle becoming radiate towards the valve apices, 8–10 in 10 μm. The alveolate striae are composed of one row of round to rectangular areolae, 6–10 in 10 μm. Externally, the areolae are covered with cribra ( Figs 11, 12 ), becoming larger towards the valve margins ( Figs 8, 10, 13 , 36, 38 ). Internally, each alveolus opens through a single elongate aperture covered with perforate siliceous layer (arrow on Fig. 11 ; Figs 14, 17, 18 , 31–35 ). In the broken valves where the perforated silica layer is corroded and/or destroyed the alveolus becomes clearly visible, which opens to the exterior through one row of areolae ( Figs 17 , 33, 34 ). Herein a detailed illustration of initial valve is presented ( Figs 19–23 ). The raphe system and the other morphological features are not well developed ( Figs 19, 20 ). The initial valve length is 91.0 μm and the valve breadth is 21.0 μm. The striae are parallel in mid-valve becoming slightly radiate towards the valve apices, 9 in 10 μm. Each alveolate stria is composed of one row of areolae, externally covered with weakly developed cribrate occlusions ( Figs 21, 22 ).

Type:— MONGOLIA, Lake Hövsgöl (Hövsgöl National Park), Port of town of Hanh , at 1688 m elevation. Coordinates : 51°30.407’ N ; 100°39.236’ E., marl from 5 m depth (accession number: M280 A, M.B. Edlund Collection, Science Museum of Minnesota, collected by Mark B. Edlund and Nergui Soninkhishig, 19 July 1998) ( Slide M280A, ANSP GC-36351 , GCM-24053), holotype, designated here ; example specimens on Figs 2, 3 ; ( Slide 919080, CAS, isotype, designated here; Slide 919076, CAS, paratype, designated here) .

Etymology:— The species name refers to the type locality, Mongolia’s ancient Lake Hövsgöl.

Observations: — Diploneis hoevsgoelensis seems to be quite variable in valve outline: lanceolate-rhombic to elliptic-circular with bluntly rounded ends ( Fig. 40 ). In all other characters these two shape trajectories are rather indistinguishable, and therefore we consider the population to contain two morphologically different races (morphodemes). With cell size reduction both morphodemes have the tendency to become elliptical in valve outline, and therefore a clear morphological cutoff is difficult at smaller cell sizes when using the outline to distinguish them as separate entities. In order to clarify the identity and potential separation of the morphodemes, detailed morphometric analyses, including shape analysis, should be included in future studies. Such increased attention can potentially provide important insights into the phenotypic plasticity or evolutionary trajectories of D. hoevsgoelensis . To attempt to reduce the subjectivity in morphologically based analyses, a molecular study and culture study are also justified in order to reveal the key drivers for shape variation in D. hoevsgoelensis . Such an approach could potentially clarify the identity of the proposed morphotypes within D. hoevsgoelensis . Interestingly, both morphodemes are found sympatric in Lake Hövsgöl collections, and populations of D. hoevsgoelensis have also been observed in Lake Baikal, having more or less similar morphological variations (Kulikovskiy et al. in prep.). In order to test the conspecificity of Baikal and Hövsgöl populations a comparative analysis is needed.

Diploneis curiosa is morphologically similar to D. hoevsgoelensis . In the original description, Metzeltin et al. (2009) gives very narrow size range for D. curiosa [80–120 μm], without providing LM illustrations. Therefore, it is difficult to compare D. hoevsgoelensis with D. curiosa . Further analyses on the type material of D. curiosa are necessary in order better characterize the identity of this taxon. Taking the valve outline into account D. hoevsgoelensis can easily be compared with Diploneis balcanica Ognjanova-Rumenova & Butczkó (2010: 171, figs 14–28). The measurements of D. hoevsgoelensis fit the original description given for D. balcanica . Interestingly, Ognjanova-Rumenova & Butczkó (2010) illustrates two different valve shapes when describing D. balcanica : a rhombic and an elliptical form. The rhombic form can potentially be allied with D. hoevsgoelensis . However, the differences in the structure of the longitudinal canal and the presence of apertures along the whole length of the raphe distinguish D. balcanica (Ognjanova-Rumenova & Butczkó 2010: 169; fig. 18). Diploneis clevei Fontell (1917 : pl. 1, fig. 2) and D. hoevsgoelensis have uniseriate striae, while Diploneis finnica (Ehrenberg) Cleve (1891 : pl. 2, fig. 11) and Diploneis duplopunctata Fontell (1917 : pl. 1, fig. 4) have biseriate striae throughout the whole length. According to Metzeltin et al. (2009), the biseriate pattern can be noticed in larger specimens in D. curiosa (see p. 658, fig. 2), a feature not observed in D. hoevsgoelensis . The structure of the longitudinal canal (lanceolate and wide throughout in D. clevei vs. lanceolate, expanded in the middle of the valve, which gradually narrowing at the valve apices in D. hoevsgoelensis ) clearly distinguishes these two species. Diploneis hoevsgoelensis can be compared with Diploneis baicalensis Skvortzow & Meyer (1928: 11 , pl. 1, fig. 31), from which it differs by the narrower longitudinal canal. The drawing of Diploneis elliptica var. ostracodarum f. baicalensis Skvortzow & Meyer (1928 : pl. 1, fig. 30) also closely fits D. hoevsgoelensis , from which it slightly differs in the stria density (7 in 10 μm vs. 8–10 in 10 μm) and in the valve width (54.0 μm vs. 25.0–44.7 μm). Similarities in the valve outline can be observed in Diploneis skvortzovii Skabichevskii (1936 : pl. 1, fig. 7), although the difference in striae structure (uniseriate vs. biseriate) separates them.

Ecology and Distribution: —M248A; M276A; M280A; M281A; M329A; M330A; M331A: distributed in shallow waters of central and northern Lake Hövsgöl in sediments and on marl and rocks.

Figures

FIGURES 1–7. Diploneis hoevsgoelensis morphotype 1, LM valve views. Fig. 2. Specimen from the holotype slide M280A. Scale bar = 10 μm.

FIGURES 8–13. Diploneis hoevsgoelensis morphotype 1, SEM external valve views. Fig. 8. Whole valve. Fig. 9. View of the central area with proximal raphe endings; white arrow showing the narrow hyaline area between the longitudinal canal and the striae. Fig. 10. Distal raphe endings with deflected terminal fissures. Fig. 11. The openings of the alveolus, external cribrate and internal perforate silica layer (see white arrow). Fig. 12. Close view of the structure of the depressed cribrate areolae. Fig. 13. External and internal view of the whole frustule. Scale bars = 10 μm (Figs 8, 13); 5 μm (Figs 9, 10); 1 μm (Figs 11, 12).

FIGURES 14–18. Diploneis hoevsgoelensis morphotype 1, SEM internal valve views. Fig. 14. View of the whole valve. Fig. 15. Central area with simple proximal raphe endings. Fig. 16. Distal raphe endings. Fig. 17. Structure of the single elongate opening of the alveolus. Fig. 18. Alveoli covered with thin layer of perforate silica. Scale bars = 10 μm (Fig. 14); 5 μm (Figs 15, 16); 2 μm (Fig. 17); 1 μm (Fig. 18).

FIGURES 19–23. Diploneis hoevsgoelensis morphotype 1, SEM external view of initial valve. Fig. 19. View of entire valve. Fig. 20. Central area with proximal raphe endings. Fig. 21. Structure of the alveolate striae. Fig. 22. Close view of the cribra. Fig. 23. Distal raphe endings. Scale bars = 20 μm (Fig. 19); 5 μm (Fig. 20); 2 μm (Figs 21, 23); 1 μm (Fig. 22).

FIGURES 24–30. Diploneis hoevsgoelensis morphotype 2, LM valve views. Figs 28, 30. Specimens from the holotype slide, M280A. Scale bar = 10 μm.

FIGURES 31–35. Diploneis hoevsgoelensis morphotype 2, SEM internal valve views. Fig. 31. View of the valve central area. Fig. 32. Distal raphe endings. Fig. 33. Broken valve, showing the structure of the alveolate striae. Fig. 34. Close view of the structure of the single openings of the alveoli. Fig. 35. View of the whole valve. Scale bars = 10 μm (Fig. 35); 5 μm (Figs 31–34).

FIGURES 36–40. Diploneis hoevsgoelensis morphotype 2, LM and SEM external valve views. Fig. 36. View of the entire valve, white arrow showing the narrow hyaline area between the longitudinal canal and the striae.Fig. 37. Proximal raphe endings. Fig. 38. Distal raphe endings with terminal fissures. Fig. 39. View of the whole valve. Fig. 40. LM view of D. hoevsgoelensis morphotype 1 and morphotype 2, showing the differences in the valve shape. Scale bars = 10 μm (Figs 36, 39, 40); 5 μm (Figs 37, 38).