Eolimna eurofarta Tahri & Van de Vijver sp. nov. (Figs 1 –22 LM, 23–29 SEM)

Valves elliptical to weakly linear-elliptical with clearly convex margins and broadly (only very rarely acutely) rounded apices (Figs 1 –22). Valve mantle plain, unperforated (Fig. 23). Girdle composed of several open, plain bands (Fig. 23). Occasionally small areolae present at the valve face/mantle junction at the apices. Valve dimensions (n=30): length 9–22 µm, width 5.5–10.0 µm, L/W ratio> 2.2. Axial area rather narrow formed by narrow raphe sternum, bordered by two longtitudinal shallow grooves running from apex to central area (Figs 23–25, 27–28). Grooves occasionally interrupted (Fig. 26). Central area enlarged, bordered by 1–3 shortened striae. Raphe filiform, with straight raphe branches. Central raphe endings indistinct to very weakly gradually enlarged, straight. Terminal raphe fissures unilaterally bent, shortly continuing onto the valve mantle. Valve face flat but covered by numerous shallow depressions, each bearing a very small, point-like areola (Fig. 28). Striae uniseriate, straight to weakly radiate composed of up to 6 rounded, in LM highly discernible areolae. Virgae rather broad. Largest areolae at the valve margin (Fig. 28). Irregular pattern of small groupings of small silica granules present scattered over the valve face, mostly bordering the areolae in the depressions. Areolae closed by hymenes positioned in the middle of the areolar canal (Figs 28, 29). Internally, raphe branches straight with indistinct central endings. Central nodule not developed. Terminal raphe endings terminating onto very small helictoglossae (Fig. 29). Areola foramina very small in the valve interior. Row of areolae near the axial area separated from the rest of the areolae (Fig. 29).

Type:— FRANCE. Lac d’Issarlès, Département Ardèche, sample LI-44, coll. date 13.02.2025, leg. A.Tahri (holotype slide BR-4901= Fig. 6, isotype slide 455, University of Antwerp, Belgium). PhycoBank registration: http://phycobank. org/105557

Etymology:— The specific epithet refers to the similarity with Navicula farta and its geographical distribution in Europe.

Ecology & associated diatom flora:— The French populations were found in a sediment core collected in the 108 m deep volcanic Lac d’Issarlès, a natural lake of 90 hectares, situated at 1 000 m altitude (Defive et al, 2023). It has been used as a reservoir for the Montpezat hydroelectric complex since 1954 and receives water from three dams (Auroy, 1956). The sample was taken at a depth of 44.8 cm in the sediment core, corresponding to the 1950s. The sample is therefore contemporary with the work carried out to use the lake in the Montpezat hydroelectric complex. The current lake water was analysed and show an circumneutral pH, oligo-mesotrophic lake with a low conductivity (46 to 51µS/cm), a low nutrient levels (nitrates 0.12 to 1.9 mg /l, phosphates 0.02 to 0.04 mg /l) (Tahri et al., unpubl. res.).

The samples were dominated by several centric taxa such as Pantocsekiella delicatula (Hustedt 1952: 376) K.T.Kiss & E.Ács (in Ács et al. 2016: 66) and P. kuetzingiana (Thwaites 1848: 169) K.T.Kiss & E.Ács (in Ács et al. 2016: 67), several unidentified Staurosirella and Pseudostaurosira taxa, Cavinula jaernefeltii (Hustedt 1942: 111) D.G.Mann & Stickle (in Round et al. 1990: 665), and Karayevia clevei (Grunow in Cleve & Grunow 1880: 21) Bukhtiyarova (1999: 94).

Taxonomic comments -2015931426:—The new species is placed in the small freshwater genus Eolimna, originally described in 1997 from Germany as a Lower Oligocene fossil genus (Schiller et al. 1997) but validated in 2024 (Lange-Bertalot et al. 2024). Eolimna species are characterised in having small valve sizes, a more or less elliptical valve outline, short, bent terminal raphe fissures, occasionally shallow, longitudinal grooves running parallel with the raphesterum, usually uniseriate (rarely biseriate) striae, areolae with hymenes countersunk in a medium position in the areolar canal, and a cingulum composed of 2–4 narrow, open bands bearing a row of small perforations (Schiller et al. 1997, p. 166). Since its description in 1997, a large number of species was transferred to the genus, only to be removed again in 2015 when Wetzel et al. (2015) reinvestigated several of them, placing them in the genus Sellaphora. Currently, AlgaeBase lists 9 accepted species names and 31 species names in synonymy (Guiry & Guiry 2025). Most of the features observed in the genus Eolimna are also found in E. eurofarta, apart from the girdle bands that appear to be plain, unperforated (Figs 23, 26). The most morphological feature is the closing of the areolae positioned in the middle of the areolar canal, distinctly discernible in the French population (see for instance Figs 28 & 29).

Eolimna eurofarta closely resembles E. metafarta but shows small, but constant, morphological differences. The external valve surface of E. eurofarta, and especially around the areolae, is covered with an irregular pattern of concentrations of very small silica granules, a feature not visible on all images of E. metafarta, although this could be the result of erosion of the latter valves. Secondly, E. eurofarta shows more elliptical valves with broadly rounded, almost never acutely rounded apices, contrary to E. metafarta where the apices are usually acutely, almost never broadly rounded. The valve outline in E. metafarta is more linear-elliptica with often parallel valve margins (see for instance figs 1–6 in Kulikovskiy et al. 2015) whereas in E. eurofarta, valves are strictly elliptical with distinctly convex margins. This also leads to a usually higher length/width ratio in E. metafarta up to almost 3. In E. eurofarta, the maximum observed L/W ratio is 2.2 with most valves having a L/W ratio <2. Another difference is the number of areolae per stria in E. eurofarta that is constantly much higher (usually up to 6 areolae per stria) than in E. metafarta where the maximum number did not exceed 4 areolae per stria. The combination of all these small differences, justifies the description of E. eurofarta as a new species.

For typification of the species, we chose to use the entire slide as the type, following article 8.2 of the International Code of Nomenclature for algae, fungi, and plants (Turland et al. 2025). Diatoms show a broad variability along their cell cycle making the choice for the entire population on the slide more obvious, but because of admixtures, one valve was indicated to best illustrate the taxon (see Figures). All novelties are registered proactively according to Art. 42.3 (Turland et al. 2025).