Navicula subviridula Levkov 2011, sp. nov.

Navicula subviridula Levkov, sp. nov. (Figs 18–24, 64–68)

Valvae lineari–lanceolatae apicibus plus minusve longe protractis et paene acute rotundatis, longae 39–57 µm, latae 9– 10 µm. Fissurae raphis paullo laterales poris centralibus distincte declinatis ad latus primum valvae (opposite inordinationem Voigtii). Area axialis angusta, linearis, area centralis distincte dilatata irregulariter elliptica ad instar, vix asymmetrica. Striae transapicales radiantes valde convergentes ad apices, 8–10 in 10 µm. Lineolae striarum fere crassae 24–28 in 10 µm.

Type:— MACEDONIA. Lake Ohrid, Bay of St. Naum, sand near the shore, collection date: 17 March 2007 (accession No. MKNDC 001123). Slide BM 101470 (holotype). Slide MKNDC 001123 (isotype).

Valves linear-lanceolate with more or less long protracted to acute ends, length 39–57 µm, width 9–10 µm. Raphe slightly lateral, central pores clearly deflected to primary side. Axial area narrow, linear, central area large, irregularly elliptical to asymmetric. Striae radiate at middle, becoming strongly convergent towards apices, 8–10 in 10 µm. Lineolae distinct, coarse, 24–28 in 10 µm (LM).

Valve face flat, valve mantle narrow; valve face/mantle transition gradual (Figs 64, 65). Axial area (sternum) very narrow, linear; central area slightly expanded laterally and apically, round. Raphe system central, proximally raphe terminates with expanded central pores oriented towards primary valve side (Fig. 65). Internally, raphe sternum strongly silicified, prominent; helictoglossae prominent (Figs 66, 67). Terminal fissures hooked, finishing at valve face/mantle transition. Striae radiate to slightly arcuate at middle, becoming convergent near apices. Areolae openings apically elongated (lineolae), smaller towards valve midline. Virgae often wider than striae (SEM).

Observations:—The main differential character for N. subviridula is the long protracted and acutely rounded valve ends. Navicula subviridula resembles species in the N. viridula group (section Alinea sensu Lange-Bertalot 2001). Navicula viridula (Kützing) Kützing (1844: 91) is a large-celled species of Navicula with linear–lanceolate valves, shortly protracted to obtusely rounded ends and coarsely punctate striae. Navicula viridula var. linearis Hustedt (1937: figs 19: 1, 2) also has larger valves (L= 65–100 µm, B= 15 µm) with cuneate ends, and coarsely punctate and distantly spaced striae (8–9 in 10 µm). Navicula viridula var. rostrata Skvortzow (1938: fig. 1: 17) has a similar valve shape (linear–lanceolate) to N. subviridula, but differences can be noted in the shape of the valve ends: rostrate to subcapitate vs. acute. The valves of Navicula viridula var. rostrata f. jeniseyensis Skvortzow (1971: figs 10: 1, 2) are broader (B= 14.1 µm) than N. subviridula and have rostrate to subrostrate poles.

Navicula viridulacalcis Lange-Bertalot (2001: figs 36: 4–7) has similar valve outline (linear–lanceolate with slightly convex valve sides) to N. subviridula but the valve ends are shortly protracted and broadly rounded. Navicula viridulacalcis ssp. neomundana Lange-Bertalot & Rumrich (in Rumrich et al. 2000: figs 38: 1–4) is also similar to Navicula subviridula having parallel valve sides. However, the differences can be observed in the shape of the valve ends: abruptly cuneate vs. long protracted to acute ends in N. subviridula.

Navicula novaesiberica Lange-Bertalot (1993: figs 61: 1–8) is probably the most similar taxon to N. subviridula. It is characterized by elliptic–lanceolate to linear–lanceolate valves with abruptly wedge-shaped and obtusely rounded ends. The smaller specimens of N. subviridula might be confused with larger specimens of N. novaesiberica (e.g. Lange-Bertalot 2001: fig. 36: 14), but can be differentiated by the shape of valve and valve ends (elliptic lanceolate valves with wedge-shaped and obtusely rounded ends in N. novaesiberica) and valve size (L= 39–57 µm, B= 9–10 µm in N. novaesiberica). The valves of N. novaesiberica from Danube River (Szabó et al. 2007: figs 1–14) differ significantly from N. subviridula in the shape of the valve ends (subprotracted and obtusely rounded). Levkov et al. (2007: figs 63: 6–10) depicted several valves from Lake Prespa, most probably of N. novaesiberica. However, this species significantly differs from N. subviridula with respect to its valve shape and valve apices.

Family Stauroneidaceae Greville (1833)

Genus Prestauroneis K.Bruder in Bruder & Medlin (2008)