Gomphonema subaequale Levkov, 2011

Levkov, Zlatko & Williams, David M., 2011, Fifteen new diatom (Bacillariophyta) species from Lake Ohrid, Macedonia, Phytotaxa 30, pp. 1-41 : 25

Gomphonema subaequale Levkov , sp. nov. ( Figs 149–160 , 178–184 )

Frustula aspectu cingulorum cuneata singulis punctis crassis in limbis valve claviformes. Vertices obtuse rotundati vel late-rotundati, poli basales distincte protracti vel subcapitati. Longitudo valvae 14–21 µm, latitudo valvae 4.5–6.0 µm. Area axialis angustissima, linearis. Area centralis parva variabiliter dilatata ad instar, lanceolata vel transverse elliptica ad rhombico-ellipticam. Stigma solitaria distincte adest. Raphe modice lateralis fissura externa undulata poris centralibus parum incrassatis punctiformibus. Striae transapicales radiantes in media parte subparallelae ad leviter convergentes sub apices, 17–20 in 10 µm. Puncta striarum facile discernanda microscopio photonico.

Type:— MACEDONIA. Lake Ohrid, Kaneo springs, macrophytes, collection date: 6 August 2004 (accession No. MKNDC 000589 ). Slide BM 101477 (holotype). Slide MKNDC 000589 (isotype) .

Frustules in girdle view distinctly cuneate with single row of punctae on girdle bands. Valves clavate with obtusely rounded head pole, distinctly protracted and subcapitate foot pole. Valve length 14–21 µm, valve width 4.5–6.0 µm. Axial area very narrow linear, central area small, variable in shape, lanceolate to rhombiclanceolate or transversally widened. Single isolated stigma present in central area. Raphe moderately lateral and undulate, central pores small, barely expanded. Striae radiate in valve middle, becoming subparallel or slightly convergent towards valve ends, 17–20 in 10 µm. Areolae hardly visible with LM. (LM).

Valvocopulae bear a single row of poroids, coarser around pole ( Fig. 179 ). Apical pore field situated mainly on mantle, bisected by raphe fissure, composed of small porelli arranged in oblique rows ( Figs 178, 183 ). Porelli smaller than striae areolae, separated from them by narrow hyaline area ( Fig. 183 ). Raphe lateral and undulate ( Figs 178, 180 ). Externally, at both poles, raphe expands and continues as terminal fissures deflected opposite from stigma ( Fig. 181 ). Internally, distal raphe endings terminate with helictoglossae, while proximal endings terminate on elevated central nodule ( Figs 182, 184 ). Proximal raphe fissures recurved ( Fig. 182 ). External opening of stigma small and round, surrounded by rim. Internally, stigma with tear-dropshaped opening. Striae uniseriate, composed of C-shaped areolae ( Fig. 184 ). Internally, areolae openings round with small finger-like occlusions ( Fig. 182 ) ( SEM).

Observations:—The most conspicuous features of G. subaequale are the shape of the foot pole and the densely spaced striae. It may be confused with smaller specimens of G. balcanicum Levkov & Krstic (in Levkov et al. 2007: figs 169: 11–23) but both taxa can be differentiated by valve shape (narrowly clavate valves with slightly protracted foot pole in G. balcanicum ) and striae density (13–16 in 10 µ m in G. balcanicum ). Representatives of the G. minutum complex have biseriate striae, a feature not present in G. subaequale . Gomphonema irroratum Hustedt (1945 : figs 42: 47–49) has larger valves (L= 30–55 µm) with distinctly biseriate striae. G. subaequale can be easily distinguished from the G. pumilum complex by valve shape (linear to linear–lanceolate in G. pumilum ) and striae density (11–15 in 10 µm).

Figures

FIGURES 131–160: LM micrographs. Figs 131–139. Gomphosphenia tenuis. Figs 140–148. Gomphonema mihoi. Figs 149–160. Gomphonema subaequale. Scale bar = 10 µm.

FIGURES 178–184: SEM micrographs of Gomphonema subaequale. Figs 178–180. External valve view. Fig. 179. Tilted specimen showing girdle elements. Each girdle band bears a single row of small, round poroids. Fig. 181. Detail of mid-valve, showing proximal raphe endings, stigma and external foramina. Proximal raphe endings widened and slightly deflected towards stigma. External opening of stigma simple and round. Striae uniseriate, composed of C-like areolae. Fig. 182. Internal view of mid-valve showing proximal raphe endings, stigma and internal openings of areolae. Internal fissures crochet-shaped. Fig. 183. Detail of base pole showing APF and distal raphe ending. Fig. 184. Internal valve view. Raphe terminates with small helictoglossae. Scale bars in Figs 178–180, 184 = 5 µm; Figs 181–183 = 2 µm.