Gyrosigma dongtingense Bing Liu & Rioual

Yi, Man-Qi, Liu, Bing, Rioual, Patrick, Lei, Qin & Ma, Yi-Dan, 2024, Gyrosigma dongtingense sp. nov. (Bacillariophyta) from Dongting Lake, China, Phytotaxa 644 (2), pp. 113-123 : 114-117

Gyrosigma dongtingense Bing Liu & Rioual sp. nov. ( Figs 1–28 )

Description: —LM ( Figs 1–10 ). Cells solitary and usually lie in valve view on a slide. Valve outline is of two types: one is S-shaped ( Figs 1, 2 , 11, 15 ) and the other is Ƨ-shaped (reverse S-shaped) ( Figs 3, 4 , 13, 16 ). Each valve lanceolate with bent, cuneate apices ( Figs 1–4 ). Valve dimensions: 161–218 μm long and 24–29 μm wide (n=34). Each half-valve has a convex side and a concave side (labeled in Fig. 3 ). Each half of the raphe sternum has two curvatures (labeled in Fig. 4 ). Central area small, roughly a quadrangle, and rotates several degrees about the valve longitudinal axis ( Figs 1–4 , 6, 9 , 17 ). Terminal area with a hyaline triangle area and located on the concave valve side ( Figs 5, 7, 8, 10 , 23–26 ). Transverse and longitudinal striae are distinct and perpendicular to each other. Transverse stria density is 12–14 in 10 μm and longitudinal stria density is 16–18 in 10 μm.

SEM ( Figs 11–28 ): Valves vault ( Figs 11, 13 ) and are of two types, either S-shaped ( Figs 11, 14 ) or Ƨ-shaped ( Figs 13, 15 ). The raphe sternum is almost located at the midline of each valve. Both external proximal raphe fissures are deflected towards the convex side of each corresponding half-valve, i.e., the two external proximal raphe fissures on the epivalve and the ones on the hypovalve in the same cell are isomorphic ( Figs 11–14 ). One axial costa (thickened rib) is produced along the better-developed primary side of the raphe sternum ( Figs 15, 16 , 26, 28 , wavy arrows). Externally, the primary side of the raphe sternum is much more developed than the secondary side which seems nearly absent ( Fig. 27 , double-headed arrow indicates the more developed primary side). Internally, the primary side of the raphe sternum is also much more developed than the secondary side ( Figs 25, 26 , #'s; Fig. 28 , arrows). A groove is formed between the axial costa and the primary side of the raphe sternum ( Figs 25, 26 , double-headed arrow, respectively).

The central areas of the valve exterior exhibit a more or less quadrangle shape ( Figs 17–19 ), rotate a number of degrees about the valve apical axis ( Fig. 17 , double-headed arrow). Within the inner central region, a central fusiform nodule ( Fig. 22 , double-headed arrow) borders the two shallow grooves ( Fig. 20 , two double-head arrows) which are, in turn, enclosed by a pair of crescent siliceous bars ( Fig. 20 , two arrows). For the epivalve or hypovalve consisting of a frustule, the two external proximal raphe fissures on the epivalve and the ones on the hypovalve both deflect towards the convex side of each half-valve and terminate in a slightly expanded arrangement ( Figs 17–19 , two arrows respectively). Inner proximal raphe ends of the raphe T-shaped ( Fig. 21 , two arrows).

The outer distal raphe fissures are curved towards the concave side of each half-valve and terminate on the mantle ( Figs 23, 24 ). The inner distal raphe fissure ends at a knob-like helictoglossa ( Figs 25, 26 ). The areola outside openings have at least two different shapes: one is slit-like and the other is tiny, rounded. The openings of the first shape are present on the whole valve surface except at the margins, where the outside openings of areolae (which comprise the two outermost, longitudinal striae) belong to the second shape ( Figs 23–26 , arrowheads). Areola inside openings can be oblong, round, or elliptic. Most of the inside openings are covered by hymens ( Figs 25, 26 ) whereas those of the outermost areolae seem unoccluded ( Figs 25, 26 ). Apical pores are present, consisting of a row of long, isolated crescent pores with very small rounded outside openings ( Figs 23, 24 , arrows) and much larger inside openings ( Fig. 26 , arrows).

Type:— CHINA. Hunan Province: East Dongting Lake, sampling point in the littoral zone (29°19′7″ N, 112°47′32″ E, 39 m asl.), collected by Bing Liu, March 5 th, 2023. Holotype: Slide DIA2024001 , specimen circled on slide = Fig. 1 , deposited in the Herbarium of Jishou University ( JIU), China. Registration: http://phycobank.org/ 104492.

Etymology:— Named after Dongting Lake where the species was found.

Ecology:— Gyrosigma dongtingense sp. nov. is a rare species as it was found only in one of the 30 diatom samples collected from the surface sediments of East Dongting Lake.Associated species comprised Craticula ambigua ( Ehrenberg 1843: 417) D.G. Mann (in Round et al. 1990: 666), C. cuspidata ( Kützing 1834: 549) D.G. Mann (in Round et al. 1990: 666), Dorofeyukea kotschyi ( Grunow 1860: 538) Kulikovskiy, Kociolek, Tusset & T. Ludwig (2019: 178) , Gyrosigma kuetzingii ( Grunow 1860: 561) Cleve (1894: 115) , Hantzschia amphioxys ( Ehrenberg 1843: 413) Grunow (in Cleve & Grunow 1880: 103), Surirella angusta Kützing (1844: 61) , S. suecica Grunow (in Van Heurck 1881: pl. LXXXIII, fig. 19), Ulnaria rhombus D.M. Williams (in Liu et al. 2019a: 54) and some species of Navicula Bory , Nitzschia Hassall , Sellaphora Mereschkowsky , Tryblionella Smith. Such an assemblage suggests alkaline and meso- to eutrophic conditions. The following environmental parameters were measured in the field. Conductivity was 494 ± 0.9 μS∙ cm-1, pH was 8.4 ± 0.1 and water temperature was 27.8 ± 0.3 °C.

Figures

FIGURES 1–4. Gyrosigma dongtingense, sp. nov., LM, ×630. 1–4. Four valves showing two different types: S-shaped valve (Figs 1, 2) and Ƨ-shaped (reverse S-shaped) valve (Figs 3, 4). 1. Illustration of holotype specimen. Note the convex and concave sides of each halfvalve (labelled in Fig. 3) and the two curvatures of each half of the raphe sternum (labelled in Fig. 4). Scale bar = 20 μm.

FIGURES 5–10. Gyrosigma dongtingense, sp. nov., LM, ×1000. 5–7 and 8–10. Magnifications from Fig. 1 and Fig.2, respectively, showing details of the valve middle part and apices. Scale bar = 10 μm.

FIGURES 11–16. Gyrosigma dongtingense, sp. nov., SEM. 11, 13. Two valve types, either S-shaped or Ƨ-shaped, in external view. 12, 14. Middle part details from Fig. 11 and Fig. 13, respectively, note that both external proximal raphe fissures are deflected towards the convex side of each half-valve. 15, 16. Two valve types, either S-shaped or Ƨ-shaped, in internal view. Note the axial costa (four wavy arrows) along the raphe sternum. Scale bar = 10 μm (Figs 11–16).

FIGURES 17–22. Gyrosigma dongtingense, sp. nov., SEM. 17–19. Three valve middle parts in external view showing the rotated central areas (e.g., Fig. 17, double-headed arrow), the outer proximal raphe fissures deflected in opposite directions (two arrows in Figs 17–19, respectively). 20–22. Three valve middle parts in internal view showing the two central bars (e.g., Fig. 18, two arrows), the T-shaped inner proximal endings (e.g., Fig. 19, two arrows), and the fusiform central nodule (e.g., Fig. 20, double-headed arrow). Scale bar = 1 μm (Figs 15–20).

FIGURES 23–28. Gyrosigma dongtingense, sp. nov., SEM. 23, 24. Two apices in external view showing the apical field pores, i.e., an isolated row of pores (arrows), a row of rounded areolae (arrowheads), and the triangle terminal area (asterisks). 25, 26. Two apices in internal view showing the apical field pores (e.g., arrows, Fig. 24), a row of rounded areolae (arrowheads), the triangle-shaped terminal area (asterisks), the axial costa (wavy arrows), and a groove (double-headed arrows) between the primary side (#'s) of the raphe sternum and the axial costa (wavy arrows). 27, 28. Two apices at lower magnification showing the more developed primary side of the sternum (Fig. 27, double-headed arrow, external view; Fig. 28, arrows, internal view), the internal axial costa (Fig. 28, wavy arrows), and the groove between the primary side of the raphe sternum and the axial costa. Scale bar = 1 μm (Figs 21–24), 10 μm (Figs 25, 26).