Placoneis subelegans Levkov, 2011

Levkov, Zlatko & Williams, David M., 2011, Fifteen new diatom (Bacillariophyta) species from Lake Ohrid, Macedonia, Phytotaxa 30, pp. 1-41 : 16

Placoneis subelegans Levkov , sp. nov. ( Figs 97–113 )

Valvae lanceolatae ad parum elliptico–lanceolatas apicibus parum protractis et acute rotundatis. Longitudo valvae 26– 44 µm, latitudo valvae 12–15 µm. Raphe filiformis, recta vel parum lateralis. Area axialis angusta linearis, area centralis angusta variabilis, aliquuid rhomboidea ad instar. Striae omnino radiantes 8–10 in 10 µm, puncta striarum non facile discernanda microscopio photonico.

Type:— MACEDONIA. Lake Ohrid, Biological Station Transect, mud 24 m depth, collection date: 3 April 2007 (accession No. MKNDC 001233 ). Slide BM 101473 (holotype). Slide MKNDC 001233 (isotype) .

Valves lanceolate to elliptic–lanceolate with weakly protracted and narrowly rounded ends. Valve length 26–44 µm, valve width 12–15 µm. Raphe filiform to weakly lateral towards centre. Axial area narrow linear, central area narrow, variable in shape, approximately rhombic. Striae radiate throughout 8–10 in 10 µm (LM, areolae hardly visible in LM).

Valve face flat without projections ( Figs 108–111 ). Raphe weakly undulate with dilated proximal endings ( Figs 108, 110 ). Distal fissures recurved in same direction—symmetrical ( Figs 112, 113 ). With post-initial cells ( Fig. 108 ), terminal fissures do not continue onto the valve mantle, but are bordered with short striae at valve apex. striae uniseriate, composed of small elliptical to round areolae ( SEM).

Observations:— Placoneis subelegans is similar to P. elegans Metzeltin et al. (2005 : figs 75: 1–10); the latter has more elliptical valves with obtusely rounded ends and a broader central area. The valves of Navicula dicephala var. cuneata (M.Møller) Foged (1977 : fig. 29: 6) appear similar to P. subelegans but differ in the shape of the valve ends (shortly protracted and broadly rounded), valve width (B= 9 µm) and striae density (S= 11–12 in 10 µ m). The smaller specimens of P. ignorata (Schimanski) Lange-Bertalot (= Navicula ignorata Schimanski 1978 : figs 6: 1–9) also have a similar valve shape to P. subelegans but the valves are narrower (around 8 µm), the central area is much wider, and the striae are coarsely punctate. Placoneis explanata (Hustedt) Mayama (= Navicula explanata Hustedt 1948 : figs 7, 8) has abruptly protracted and broadly rounded ends, a broad transapically widened central area and distinctly punctate striae, features that clearly differentiate it from P. subelegans . Navicula elginensis var. cuneata Kobayasi (in Kobayasi & Ando 1975: figs 4: 54, 55) has smaller valves (L= 19.5–20.5 µm; B= 8 µm) with a large, rhombic central area in contrast to P. subelegans , which is characterized with longer and wider valves (L= 26–44 µm; B= 12–15 µm) with a small central area that is variably shaped.

Figures

FIGURES 90–107: LM micrographs. Figs 90, 91. Placoneis clementis. Figs 92–95. Placoneis pseudabundans. Fig. 96. Placoneis juriljii. Figs 97–107. Placoneis subelegans. Scale bar = 10 µm. Figs 90, 91, fossil deposit Köpecz, Romania, Figs 92–107. Lake Ohrid, recent.

FIGURES 108–113: SEM micrographs of Placoneis subelegans. Fig. 108. External view of initial cell; distal raphe endings not passing on valve mantle. Fig. 109. Detail of mid-valve showing proximal raphe ends, small central area and uniseriate striae. Stigma not present. Figs 110, 111. SEM external view of whole valves. Figs 112, 113. Detail of valve ends showing distal raphe endings. Both raphe fissures symmetrical and doubly curved. Scale bars in Figs 108, 111 = 20 µm; Fig. 109 = 5 µm; Fig. 110 = 10 µm; Figs 112, 113 = 2 µm.