Fragilaria micra Levkov & D.M.Williams, 2011

Levkov, Zlatko & Williams, David M., 2011, Fifteen new diatom (Bacillariophyta) species from Lake Ohrid, Macedonia, Phytotaxa 30, pp. 1-41 : 8-9

Fragilaria micra Levkov & D.M.Williams , sp. nov. ( Figs 46–63 )

Cellulae solitariae. Valvae lanceolatae quoad specimina maiora sed rhombico–lanceolatae quoad specimina minora, apicibus protractis, rostratis vel capitatis (quoad specimina maiora) ad cuneatis (quoad specimina minora). Longitudo valvae 7–18 µm, latitudo valvae 3.5–4.8 µm. Area axialis angusta, linearis, area centralis lata unlateralis. Striae coarse, uniseriatae, omnino radiantes 18–20 in 10 µm. Puncta striarum non aspectabilia microscopio photonico.

Type: — MACEDONIA: Lake Ohrid, Kaneo springs, macrophytes, collection date: 6 August 2004 (accession No. MKNDC 000589 ). Slide BM 101469 (holotype); Slide MKNDC 000589 (isotype) .

Cells solitary, valves lanceolate in larger specimens to rhombic-lanceolate in smaller specimens with protracted, rostrated or capitate (in larger specimens) to cuneate ends (in smaller specimens). Valve length 7– 18 µm, valve width 3.5–4.8 µ m. Axial area narrow, linear, central area large, unilateral. Striae coarse, uniseriate, radiate throughout, 18–20 in 10 µm; areolae not recognizable in LM ( LM).

External valve face ( Figs 59, 60, 62 ) flat without protrusions (marginal spines, spinules); transition from valve face to valve mantle gradual, with valve mantle being fairly low. Sternum narrow, gradually widening towards central area. Central area unilateral, wide, usually possessing depressions representing rudimentary striae. Striae uniseriate, composed of areolae with round to apically elongate foramina. Striae continuing onto valve mantle, not interrupted at the mantle/face junction. Single rimoportula present at one pole. External opening simple, oriented obliquely, slightly larger than areolae. Apical pore fields present on both poles; pore field composed of simple fine porelli arranged in regular rows parallel to the apical axis ( Fig. 62 ). Internally ( Figs 61, 63 ), costa well-developed, wider than striae. Areolae openings apically elongated; no closing plate observed ( SEM).

Distribution and ecology:—Specimens of F. micra were frequently observed in the epiphytic communities near the sublacustrine springs in Kaneo, Lake Ohrid. These sublacustrine springs are calcareous–rich. Here the diatom community is dominated by Rhoicosphenia macedonica Levkov & Krstic (in Levkov et al. 2007: figs 30: 1–17), Diatoma ochridana Lange-Bertalot & Rumrich (in Lange-Bertalot et al. 1991: figs 19–25, 45–48), several species of Gomphonema Ehrenberg , Cymbella Agardh and Encyonema Kützing.

Observations:—Two main features easily differentiate F. micra : its small valve size and valve shape (rhombic–lanceolate). Larger specimens of F. micra may be confused with F. capitellata (Grunow) J.B.Petersen. The valves of F. capitellata have narrowly lanceolate to linear–lanceolate outline (not inflated at the mid-valve) and strongly apiculate ends ( Tuji & Williams 2008a: figs 30–43). The valves of F. pectinalis (O.F.Müller) Lyngbye are linear with subcapitate to capitate ends and longer (L=28–37) with lower striae density, 14–15 in 10 µm ( Tuji & Williams 2008a: figs 14–27). Fragilaria perminuta (Grunow) Lange-Bertalot (in Krammer & Lange-Bertalot 1991) has a comparable valve size (L= 9–24 µm, B= 3–3.5 µm) to F. micra , but differs significantly by its valve shape (strictly lanceolate without inflation in the mid-valve). Fragilaria capucina is characterized by linear valves, bilateral central area and two small rimoportulae, one on each pole ( Tuji & Williams 2006a: figs 6: H–R).

The valves of F. vaucheriae (Kützing) J.B.Petersen are linear–lanceolate with coarse and distantly spaced striae (9–14 striae 10 µm) and possess a single rimoportula per valve ( Tuji & Williams 2006b: figs 21, 22). Linear–lanceolate valves are present in F. parva (Grunow) Tuji & Williams (2008b : figs 13–28). Additionally, F. parva has a wide, bilateral central area, whereas F. micra has a unilateral central area.

Order Naviculales Bessey (1907) sensu emend.

Family Naviculaceae Hendey (1937)

Genus Navicula Bory (1822)

Figures

FIGURES 46–63: LM and SEM micrographs of Fragilaria micra (type material). Figs 46–58. Diminution series of Fragilaria micra. Figs 59, 60. SEM external valve view. Fig. 61. SEM internal valve view. Internal foramina appear longer than external. Fig. 62. SEM external valve view. Detail of valve end showing small, indistinct opening of rimoportula. External opening simple, slightly larger than adjacent areolae. Fig. 63. SEM internal valve view. Detail of valve end showing internal opening of rimoportula. Scale bars: LM = 10 µm, SEM in Figs 59–61 = 5 µm, Figs 62, 63 = 1 µm.