Fragilaria striis-distantioribus (Grunow) Van de Vijver, Kusber & D.M.Williams comb. nov. stat. nov. epitype population (Figs 63–104)

Basionym: — Synedra capitellata f. striis-distantioribus Grunow in Van Heurck (1881): Synopsis des diatomées de Belgique, Atlas pl. XL [40]: fig. 26B, C)

Synonym: — heterotypic: Synedra capitellata var. cymbelloides Grunow in Van Heurck (1881)

Registration: http://phycobank.org/105483

Lectotype & Epitype (designated in Tuji & Williams 2008, p. 139):— Lectotype: Grunow in Van Heurck (1881): PI. 40, Fig. 26b; Epitype: An individual from slide 2657a with the description of Arnott 507 in Grunow collection in W.

Material examined: — UNITED KINGDOM, Haverfordwest, Wales, Grunow sample 2657, leg; Okeden, W 2011-0002610, new slide made at Meise Botanic Garden based on original Grunow material (BR-4893)

Light microscopy (Figs 63–101):—Frustules solitary or in pairs, rectangular in girdle view (Fig. 63). Valves narrowly lanceolate with weakly (longer valves) to clearly (smaller valves) convex margins.Apices distinctly protracted, clearly capitate. Valve dimensions (n=40): valve length 14–30 µm, width 4.0–4.5 µm. Sternum very narrow throughout its entire length, only slightly widening near the central area. Central area very large, buttressed, shallowly depressed, clearly unilateral, with large hyaline zone on one side and almost not shortened striae on the opposite side. Ghost striae not discernible in LM. Striae parallel to very weakly radiate throughout the entire valve, alternating, 14–17 in 10 µm. Areolae not discernible in LM.

Scanning electron microscopy (Figs 102–104):—Marginal spines absent (Fig. 102). Striae uniseriate, composed of very broad, raised virgae compared to the small vimines (Figs 102 & 103). Very low raised ridges present on the virgae. Areolae small, rounded (Fig. 103). One large, rimmed rimoportula at one apex, located on the last stria, replacing some of the areolae, close to the sternum (Figs 102 & 103, arrows). Apical porefield of the ocellulimbus type, relatively large, almost quadratic, composed of five rows of small pores, entirely located on the mantle (Fig. 103). Internally, rimoportula large, transverse (Fig. 104). Virgae broad, raised (Fig. 104).

Ecology and associated diatom flora:—Sample 2657 from Haverfordwest is almost entirely dominated by two species: Fragilaria striis-distantioribus and Meridion circulare (Greville 1822: 213) C. Agardh (1831: 40). Gomphonema parvulum and Planothidium lanceolatum were also found in the sample but in very low numbers. The dominance of M. circulare points to running waters with moderate electrolyte contents and usually lower nutrient levels (Lange-Bertalot et al. 2017). The presence of G. parvulum and P. lanceolatum may point to a slightly higher nutrient level.

Nomenclatural note:—The taxon under discussion was described by Grunow in Van Heurck (1881, plate XL [40], figs 26B.C.) and published as “ S. CAPITELLATA GRUN. FORMA STRIIS DISTANTIORIBUS. | 15 à 17 stries en 0,01 mm.” The question, if Grunow described an unnamed forma with image and measurements and a short diagnosis “striis distantioribus” or a forma named ‘stiis distantioribus’ with image and measurements was answered by Tuji & Williams (2008: 139) by using ‘striis distantioribus’ as the epithet, hyphenated according to the ICN (see Turland et al. 2025 60.12 a2 and b1). This procedure is accepted here and the name used as the basionym.