Gomphonema beaugerianum Van de Vijver

Vijver, Bart Van De, 2024, Gomphonema beaugerianum, a new Gomphonema species (Bacillariophyta) found in a historic William Smith sample from the Auvergne (France), Phytotaxa 653 (3), pp. 290-296 : 291-292

Gomphonema beaugerianum Van de Vijver sp. nov. ( Figs 1–15 LM, 16–20 SEM)

Frustules solitary, clearly wedge-shaped ( Fig. 1 ). Valve typically clavate, in valve outline gradually tapering from the broadly rounded headpole towards the acute footpole. Weak but still distinct constriction present right below the headpole, separating the inflated central part from the headpole. Valve dimensions (n=25): length 35–70 µm, width (headpole) 10–14 µm, width (mid-valve) 9.5–12.0 µm. Axial area moderately broad, almost ¼ of the total valve width, linear with almost parallel margins. Occasionally, irregularly scattered, shallow depressions present in the axial area ( Fig. 18 ). Central area wedge-shaped due to radiate striae bordering the area. Central striae irregularly shortened, usually with one longer and two shorter striae. Longest central stria on the primary side terminating by the weakly isolated stigmoid. Stigmoid small, rounded, located close to the central raphe endings ( Figs 16 , 18 ). Raphe clearly lateral, undulating. Central raphe endings only weakly expanded ( Figs. 16 , 18 ), clearly deflected. Terminal raphe fissures deflected, bisecting the apical pore field on the footpole ( Fig. 16 ). Apical pore field large, located on the valve mantle and partly on the valve face ( Figs 16 , 17 ). Striae radiate near the central area, becoming parallel to weakly radiate near the apices, 9–11 in 10 µm, more densely packed at the headpole (up to 16 in 10 µm). Striae uniseriate, composed of large areolae, occluded by semi-lunar, reniform siliceous flaps ( Figs 16 , 18 ). Areolae quite discernible in LM ( Figs 2–15 ). Marginal ridge absent ( Fig. 17 ). Valve mantle broad. Advalvar part with distinct mantle striae, continuing from the valve face striae.Abvalvar part usually hyaline with a few irregularly shattered small areolae ( Fig. 17 ). Pseudosepta present on both apices ( Fig. 19 ). Internally, striae sunken in long depressions ( Fig. 19 ). Areolae large, rounded ( Fig. 20 ). Siliceous struts separating the areolae absent ( Fig. 20 ). Internal opening of the stigmoid slit-like. Central raphe endings long, right-angled and hook-shaped ( Fig. 20 ).

Type:— FRANCE. Lac de Guéry , Auvergne, William Smith sample Lac de Guéry-River Mortes, leg. W. Smith, coll. date 26.vi.1854., elev. 4066 ft (holotype slide BR-4847 = Fig. 5 , isotype slide 447 in Collection University of Antwerp, Belgium).

Registration:— http://phycobank.org/104621

Etymology:— The species is named after my colleague and friend Dr Aude Beauger (GEOLAB UMR 6042, Maison des Sciences de l’Homme, France) in honour of her extensive work on the diatoms of the Auvergne and Central Massif region.

Ecology & associated diatom flora:— The sample was taken from Lac de Guéry, a mountain lake in the Monts Dore area of the French Massif Central. The lake with a total surface of 25 ha, has a volcanic origin. Rioual (2002) analysed in 2002 more than 20 lakes in the Auvergne region including Lac de Guéry and measured a pH of 6.9, a conductivity of 33 µS/cm, and low levels of total phosphorus (12 mg /l) and nitrate (196 µg/l). These are of course recent physico-chemical and most likely do not entirely reflect the situation in 1854 when William Smith collected the sample. The diatom flora is quite diverse and dominated by Staurosira neoproducta ( Lange-Bertalot 1993: 48) Chudaev & Gololobova (2012: 74) , Navicula radiosa Kützing (1844: 91) , Fragilariforma undata (W. Smith 1855: 7) Heudre et al. (2017: 264) , F. bicapitata (A. Mayer 1917: 21) D.M. Williams & Round (1988: 265) , several Aulacoseira taxa, Pseudostaurosira cf. brevistriata (Grunow in Van Heurck 1885: 157) D.M. Williams & Round (1988: 276), Gomphonema coronatum Ehrenberg (1841: 211) , Cymbella cymbiformis C. Agardh (1830: 10) , Cymbopleura naviculiformis (Auerswald ex Heiberg 1863: 108) Krammer (2003: 56) and Tabellaria cf. flocculosa ( Roth 1797: 192) Kützing (1844: 127) . This diatom flora points to oligo- to weakly mesotrophic, occasionally weakly acid, conditions with a low to moderate electrolyte content on siliceous substrates ( Lange-Bertalot et al. 2017).

Figures

FIGURES 1–16. Gomphonema beaugerianum Van de Vijver, sp. nov. LM and SEM images taken from the holotype sample (BR-4847, Lac de Guéry, Auvergne, France, sample W. Smith). 1. LM view of a frustule in girdle view. 2–15. LM views of the population arranged in decreasing length. 16. SEM external view of an entire valve. Scale bars = 10 µm.

FIGURES 17–20. Gomphonema beaugerianum Van de Vijver, sp. nov. LM and SEM images taken from the holotype sample (BR-4847, Lac de Guéry, Auvergne, France, sample W. Smith). 17. SEM external view of an entire valve in oblique view showing the valve mantle. 18. SEM external detail of the central area showing the isolated pore and the depressed areolae with the reniform siliceous flaps. Note also the shallow depressions in the axial area. 19. SEM internal view of an entire valve. 26. SEM internal detail of the central area showing the slit-like isolated pore and the hooked central raphe endings. Note the absence of silica struts separating the areolae. Scale bars = 10 µm (Figs 17–19) = 1 µm (Fig. 20).