Sellaphora vincentiana Alibert & Hamilton & Pienitz & Antoniades 2023, sp. nov.

Sellaphora vincentiana sp. nov. (Figs 10 AU-BJ; 14 A-F)

HOLOTYPE. — Canada. Québec, Lake 22AA, 56°33’24”N, 76°28’47”W, 128 m a.s.l., 22.VIII.2015, D. Antoniades (microscope slide designated as the holotype, holo-, CANA [CANA 129481]).

ISOTYPE. — Canada. Québec, Lake 22AA, 56°33’24”N, 76°28’47”W, 128 m a.s.l., 22.VIII.2015, D. Antoniades (iso-, ANSP [ANSP-GC68070]).

TYPE LOCALITY. — Canada. Québec, Lake 22AA, 56°33’24”N, 76°28’47”W, 128 m a.s.l.

ETYMOLOGY. — The epithet ‘vincentiana’ is named in honour of our dear colleague Warwick Vincent, whose contributions to diatom research are substantial and cannot be measured.

ECOLOGY AND DISTRIBUTION. — This species was found in two lakes of the study: 22-Z and 22-AA. It reached 4.6% relative abundance in lake 22-AA and only 0.6% in 22-Z. The low number of occurrences prevented us from discerning clear relationships with lake chemistry, but both sites where it occurred had circumneutral conditions, and Lake 22-AA had the highest TP concentration in the dataset.

REGISTRATION. — http://phycobank.org/103921.

Frustules rectangular and narrow in girdle view. Valves linear-elliptic to elliptic with restricted rostrate ends. Valve dimensions (n =25): length 8-18.5 µm, width 2.5-4.5 µm. External valve face flat, margin rounded. The stria density is 49-51 in 10 µm. Axial area curved and narrow; external sternum raised. Central area weakly expanded, with 4-6 marginal striae between proximal raphe fissures, 1-2 striae shorter than the others (Fig. 14 A-C). Raphe curved, with no kink-like irregularity halfway between mid-valve and apex; externally, central raphe fissures linearly expanded with rounded ends sometimes bent to primary side (Fig. 14B, C).Terminal raphe fissures bent down onto upper region of mantle, opening into a small circular depression (Fig. 14D). Internally, raphe on sternum, straight, with no kink-like irregularity. Proximal raphe fissures teardrop-shaped and positioned on raised central nodule (Fig. 14E); terminal fissures end on small helictoglossae, isolated from apex mantle (Fig. 14F). Striae more widely spaced around central area, and parallel throughout. Striae continuous, straight to weakly arched from valve face to mantle. Voigt faults indistinct. Striae continuous around apices. Areolae round to rectangular, 8-10 per stria, 3-4 at apices (Fig. 14D). Areolae covered with a recessed fine poroid velum with circular rows of small and almost indistinct pores around outside edge and random distribution on inner area. Areolae along axial sternum more evidently depressed. Internally, areolae between virgae weakly recessed with volate pore cover.

The small linear to linear-elliptic shape with subrostrate to rostrate apices is a common shape of many species and genera which creates confusion in identifications.Furthermore, the high striae counts may create confusion between Adlafia, Eolimna and Sellaphora. As a result, taxonomic confusion persists today when trying to distinguish taxa within these genera. After SEM study, the morphology of these valves is clearly aligned with the genus Sellaphora, with recessed volate occlusion of the areolae, raised external axial and terminal areas, distinctly larger areolae along the axial area, and bent terminal raphe fissures down onto the mantle. A distinct asymmetric internal central nodule also helps to distinguish this taxon from Adlafia and Eolimna species. Sellaphora stauroneioides (Lange-Bertalot) Vesela & Johansen is comparable to Sellaphora vincentiana sp. nov., but is larger, with clear hyaline areas at the apices and is more linear than linear-elliptic. Electron microscopic comparisons are needed to distinguish these small Sellaphora species. In addition, Navicula arvensoides Hustedt and Sellaphora pseudoarvensis (Hustedt) Wetzel & Ector are small taxa with similar valve outlines, linear valve margins and distinct rostrate or subcapitate apices (Simonsen 1987; Table 3). With no SEM examination of these taxa, detailed comparisons are limited to LM which shows a triundulate valve margin with broad rostrate apices, and visible striae in N. arvensoides; in contrast, S. pseudoarvensis has linear valve margins, curved raphe branches, no visible striae and subcapitate apices.