Cymbopleura amicula Udovič, Kulaš, Šušnjara, Arapov, Blanco & Levkov, 2022, stat. nov. et nom. nov.

Cymbopleura amicula stat. nov. et nom. nov.

Replaced synonym: Cymbopleura rupicola var. korana Krammer 2003, Diatoms of Europe, Diatoms of the European Inland waters and comparable habitats, 4: 156; pl. 67, fig. 9–16

Heterotypic synonym: Cymbopleura rupicola var. minor Krammer 2003: 47, 156, pl. 67: figs 1–8

LM (Figs 2–20):—Valves slightly dorsi-ventral, lanceolate, dorsal margin moderately convex, ventral margin slightly convex to almost linear (n=68). Valve apices slightly protracted, broadly rounded and weakly dorsally bent. Valve length 22–35 µm (initial cells up to 41 µm), valve width 6.0–8.0 (initial cell up to 9.0 µm). Axial areal narrow arched, central area indistinct to weakly expanded ventrally. Raphe slightly lateral, raphe branches biarcuate, convex to dorsal margin, proximal ends weakly inflated and deflected ventrally, distal fissures comma-shaped and deflected dorsally. Striae radiate throughout, more densely spaced towards apices. Central striae in some specimens more distantly spaced, 13–16 in 10 µm. Areolae not visible under LM.

SEM (Figs 21–25):—Frustule in girdle view almost rectangular (Fig. 21). Girdle bands open, bearing single row of small, round to elongate poroids (Figs 21, 23). Transition of valve face to mantle gradual (Figs 23–25). Valve face flat, in some specimens central nodule slightly thickened (Fig. 22). Raphe reverse lateral, proximal raphe ends expanded into drop-shaped central pores (Figs 22, 24, 25). Central pores slightly ventrally bent. Distal raphe fissures comma-shaped, deflected to dorsal side and passing on valve mantle (Figs 24, 25). Striae uniseriate, composed of small, transversally elongated to round areolae. Areolae number 25–30 in 10 µm. Internally, virgae broad, separating the narrow striae subdivided by narrow vimines (Figs 26–28). Areola openings not occluded, transversally elongated (Figs 29, 30). Small struts present across the vimines (black arrows on Fig. 30). Raphe “lacking an intermissio” or proximal raphe ends are covered by overgrowth of silica that hides the deflected endings (Figs 29, 30). Distal raphe ends terminate in small, knob–like helictoglossae (Fig. 31).

Ethymology:—Having separated from C. rupicola and raising its rank to species, the epithet “ korana ” is no longer available as it is a later homonym of Cymbopleura korana Krammer (Krammer 2003: 156). Therefore, the new name Cymbopleura amicula is proposed. The specific epithet amicula is used to describe the friendship with the Krka River. From the beginning of our research of the Krka River we fell in love with this habitat. This river has drawn us to many explorations throughout the year and called us to explore its uniqueness. We remain connected to the river and in friendship with all our colleagues who have worked with us on a number of projects over the years.

Ecology:—The locality where C. amicula was observed represent a large karstic spring with high flow. The water is slightly alkaline (pH=7.39), with more or less constant temperature during the year (T=11.1 °C during collection), high oxygen concentration and saturation (O 2 =10.64 mgL-1, O 2 =99 %), and moderate conductivity (380 µScm-1) due to the presence of high calcium bicarbonate concentration. Diatom assemblages are dominated by Achnanthidium pyrenaicum (Hustedt) H. Kobayasi (1997: 148), Navicula cryptotenella Lange-Bertalot (in Krammer & Lange-Bertalot 1985: 62), Cymbella excisiformis Krammer (2002: 160) and Encyonopsis subminuta Krammer & E.Reichardt (in Krammer 1997: 195).

Cymbopleura amicula resembles C. rupicola (Grunow) Krammer (2003: 47) with respect to the valve size and general appearance (Table 1). Cymbopleura rupicola is characterized by rhomboid-lanceolate valves and narrowly rounded to pointed and slightly set-off from valve body, valve length 20–60 µm (initial cell up to 60 µm) and valve width 6.0–11.4 µm. Differences between C. amicula and C. rupicola might be noticed in the valve shape, valve apices and coarser striae (areolae 21–24 in 10 µm). Cymbopleura rupicola sensu Bahls (2019, figs 56: 1–26) has similar valve outline as C. amicula, but it can be differentiated by the distinct central area (up to 1 / 3 of valve width in C. rupicola sensu Bahls), narrower apices and coarsely punctate striae. Cymbopleura maggieae Bahls (2014: 65) has similar valve outline as C. amicula, but it can be easily differentiated by the shape of central area (1 / 2 to 2 / 3 of valve width) and broader axial area (lanceolate, widening towards central area). Cymbopleura subaequalis (Grunow) Krammer (2003: 101) has similar valve outline and size as C. amicula, but it can be easily differentiated by shape of central (wide, asymmetrical 1 / 4 – 1 / 3 of the valve width) and axial (gradually tapering towards apices) areas. Additionally, C. subaequalis has intersmissio that is lacking in C. amicula. Cymbopleura margalefii Delgado, Novais, Blanco & Ector in (Delgado et al. 2013: 98) is described from a similar habitat (Deià torrentis, Maiorica Insula, Spain) and it is characterized by distinctly dorsiventral, broad, asymmetrically elliptical–lanceolate valves without protracted apices. Both species can be easily differentiated by the valve shape and areola density (40–45 in 10 µm). Cymbopleura korana Krammer (2003: 156) was described from Korana River, Croatia (Plitvice Lakes) and was frequently observed in Krka River, Croatia, belongs to species complex around C. austriaca (Grunow) Krammer (2003: 50) and possesses different valve outline (distinctly dorsiventral, subelliptical to elliptical-lanceolate with not protracted and obtusely rounded valve ends). Cymbopleura gokyoensis Jüttner & Van de Vijver (in Van de Vijver et al. 2011: 251) has similar valve size (L= 27–49 µm, W= 6.5–8.5 µm) and stria density (15–17 in 10 µm) as C. amicula, but both species can be easily differentiated by shape of central area (rhombic to almost circular area extending 1 / 2 – 2 / 3 of the valve width in the former species).