Staurosirella minutissima Van de Vijver 2022, sp. nov.

Staurosirella minutissima Van de Vijver sp. nov. (LM Figs 31–51, SEM 74–79)

Description:— LM (Figs 31–51): Frustules in girdle view rectangular, in pairs (Fig. 31) or solitary (Fig. 32). Long colonies not observed. Valves isopolar to occasionally very slightly heteropolar (e.g. Figs 35, 36), elliptic to ellipticlanceolate. Apices broadly rounded, not protracted. Valve dimensions (n=30): valve length 5–12 µm, width 3.0–3.5 µm. Sternum very narrow, linear to occasionally very weakly lanceolate. Striae alternating at both sides of the sternum, weakly radiate near the valve middle, becoming distinctly more radiate towards the apices, 15–16 in 10 µm. Areolae not discernible in LM. Valvocopula with fimbriae (Fig. 51).

SEM (Figs 74–79): Valve face surface weakly undulating with slightly raised virgae and striae slightly sunken in ‘punch hole-like’ depressions (Figs 74, 76). Virgae broader that the striae. Striae extending without interruption from the valve face onto the mantle narrowing on the valve mantle (Fig. 76). Large hyaline zone present at the abvalvar mantle edge (Figs 75, 76). Striae uniseriate, composed of short, slit-like, linear areolae, running parallel to the apical axis (Figs 74–78). Vimines narrow, not raised. Marginal spines located on the virgae, always in pairs flanking the striae. Spines rounded, thin at the base (Fig. 76), becoming broader, more spatulate at the top (Fig. 78). Mantle plaques absent (Fig. 76). Apical pore fields present at both apices, usually similar in size and shape (Figs 74, 75–70), located at the valve face/mantle junction, extending more onto the valve mantle, isolated from neighboring striae. Pore fields composed of three rows of small, rimmed pores. Girdle composed of an open valvocopula and several open, plain copulae (Fig. 75). Internally, striae distinctly sunken between the raised virgae and the sternum (Figs 77, 79). Areolae internally occluded by irregularly shaped volae, extending from the sides of the areolae projected towards the valve interior (Figs 77, 79).

Type:— SWITZERLAND, Casaccio, south side of the Lukmanier (border between the cantons of Graubünden and Ticino), prepared from Rabenhorst exsiccata sample 1441 (exsiccata set Algen Sachen’s & Europa’s) (holotype BR-4712!, isotype Slide 403 (University of Antwerp, Belgium)). The holotype is represented here by Fig. 11. Duplicates of the Rabenhorst material No. 1441 and Wartmann & Schenk as sample 233 can be found in numerous herbaria worldwide (e.g. Stafleu & Cowan 1983: 465; 1988: 93).

Etymology:— The specific epithet ‘ minutissima ’ refers to the small valve dimensions.

Associated diatom flora in Rabenhorst 1441:— The sample is populated by a highly diverse diatom flora. Dominant species (>5 % of the total diatom count) include Achnanthidium polonicum Van de Vijver et al. (in Wojtal et al. 2011: 223), Cymbella excisiformis Krammer (2002: 31), Denticula tenuis Kützing (1844: 43), Delicatophycus delicatulus (Kützing 1849: 59) M.J. Wynne (2019: 1), Pseudostaurosira robusta (Fusey) D.M. Williams & Round (1987: 278), Staurosirella coutelasiana, S. minutissima and S. neopinnata. Less frequent taxa include Brachysira neoexilis Lange-Bertalot in Lange-Bertalot & Moser (1994: 51), Caloneis latiuscula (Kützing 1844: 93) Cleve (1894: 61), Cymbella helvetica Kützing (1844: 79), Nitzschia bryophila (Hustedt 1937: 204) Hustedt (1943: 232), Pseudostaurosira parasitica (W. Smith 1856: 19) E.Morales (in Morales & Edlund 2003: 287) and Staurosira inflata (Heiden 1900: 14) A.Rusanov et al. (in Rusanov et al. 2018: 341). This diatom flora represents an unpolluted, alpine environment. Most species are typically found in circumneutral to alkaline (calcium-carbonate rich), oligotrophic, oligosaprobic lakes and rivers with low to moderate electrolyte contents (Lange-Bertalot et al. 2017). As both Staurosirella coutelasiana and S. minutissima are rather abundantly present in the sample, it is highly likely that both are typically found in these conditions.