Fragilaria pectinalis (O.F.Müller) Lyngbye, epitype population (Figs 1–24) and additional population from Rabenhorst sample 2143—Gohlis (Leipzig) (Figs 25–62)

Basionym:— Conferva pectinalis O.F. Müller (1788: Nova Acta Academiae Scientiarum Imperialis Petropolitanae 3: 91, figs 4–7).

Synonyms:—homotypic: Diatoma pectinalis (O.F.Müller)C. Agardh (1812: 34), Bacillaria pectinalis (O.F.Müller) Nitzsch (1816: 107), Candollella pectinalis (O.F.Müller) Gaillon (1833: 10), Diatoma pectinalis (O.F.Müller) Kützing (1844: 47).

Lectotype & Epitype (designated in Tuji & Williams 2006, p. 198):— Lectotype: figure 4 in O. F. Müller (1788) Nov. Acta Acad. Sci. Imp. Petropol. 3: 91; Epitype: an individual from slide BM 101152 in BM! (located at England finder L33-3: Fig. 4B in Tuji & Williams 2006), “New river near Hornsey, March 5, 1802 ”.

Type locality:— DENMARK, In palustribus Hauniensibus [in swamps near Copenhagen].

Material examined: — UNITED KINGDOM, Dillwyn’s original packet labelled “New river near.

Hornsey, Mar 5 1802 ” (BM 101152-3, slide BR-4891); GERMANY, Gohlis (Leipzig), aus ein Gräben in Gohlis, Rabenhorst exsiccata sample n°2143 (BR-4892).

Light microscopy (Figs 1–18):—Frustules solitary or in pairs, rectangular in girdle view (Fig. 1). Valves linear to rarely narrowly lanceolate with almost straight to rarely weakly convex margins. Apices distinctly protracted, subcapitate to rostrate. Valve dimensions (n=20): valve length 25–40 µm, width 4.0 µm. Sternum narrow throughout its entire length, only slightly widening near the central area. Central area large, occasionally buttressed, either unilateral with large hyaline zone on one side and slightly shortened striae on the opposite side, or forming a transapical fascia. Ghost striae weakly discernible in LM (Figs 3, 5, 14). Striae almost entirely parallel throughout the entire valve, alternating, 15–17 in 10 µm. Areolae not discernible in LM.

Scanning electron microscopy (Figs 19–24):—Girdle composed of several open, perforated bands (Fig. 19). Striae on the mantle composed of only a few small areolae continuing with a shallow slit (Fig. 19). Marginal spines absent to very rudimentary (Figs 21 & 22), usually reduced to small papilla-like siliceous outgrowths (Fig. 22, small arrows). Striae uniseriate, composed of very broad virgae (Figs 20 & 21) and series of areolae in depressed grooves (Figs 21 & 22). Ghost striae in the central area visible as shallow grooves (Fig. 22). Raised ridges present on the virgae. Areolae small, rounded (Fig. 21), covered by individual cribra. Large rimmed rimoportula present at one apex on the sternum. Rimoportula located on the last stria, replacing some of the areolae (Figs 20 & 21). Apical porefield of the ocellulimbus type, very large, composed of five long rows of small pores, located on the mantle covering the entire apex mantle (Fig. 21). Internally, rimoportula large, transverse (Figs 23 & 24). Virgae broad, raised (Fig. 23).

Ecology and associated diatom flora:—The Dillwyn sample is dominated by a large Eunotia species forming long, ribbon-like colonies that was originally identified as Eunotia pectinalis (Kützing) Rabenhorst. Tuji & Williams (2005) examined original Kützing material from Jever and reported that since the type of Himantidium pectinale did not produce colonies, it is unclear how the Eunotia species in Dillwyn’s material should be identified. Other species in the sample include Fragilaria radians (Kützing 1844: 64) D.M. Williams & Round (1988: 269), Melosira varians C. Agardh (1827: 628), Tabularia cf. fasciculata (C. Agardh 1812: 35) D.M. Williams & Round (1986: 326) and an at present unidentified Ulnaria species. Based on this species composition it is, however, not possible to determine the ecological conditions of the type sample.

The Rabenhorst sample 2143 from Leipzig is almost entirely dominated by F. pectinalis. Several other species occur in much lower numbers: Gomphonema parvulum (Kützing 1844: 83) Kützing (1849: 65), Fragilaria campyla (Hilse in Rabenhorst 1861: no. 1024) Van de Vijver et al. (in Van de Vijver et al. 2022: 97), Surirella terricola Lange-Bertalot & E.Alles (in Lange-Bertalot & Metzeltin 1996: 107), Planothidium lanceolatum (Brébisson ex Kützing 1846: 247) Lange-Bertalot (1999: 287) and Nitzschia palea (Kützing 1844: 63) W. Smith (1856: 89). These species point to more eutrophic, neutral to weakly alkaline conditions (Lange-Bertalot et al. 2017).

Taxonomic remarks:—The second investigated population in Rabenhorst sample 2143 (Gohlis near Leipzig), was initially identified as Synedra vaucheriae (Kützing) Kützing (1844: 65). The population spans a larger length range with the smallest observed valves being on 12 µm long. The apices are a little bit more elongated than in the epitype population. Nevertheless, we consider these differences insufficient to split both taxa as separate species.