Pleurosigma subrectum Cleve, 1880

Sterrenburg, Frithjof A. S., Stidolph, Stuart R., Sar, Eugenia A. & Sunesen, Inés, 2015, A comparative study of Pleurosigma subrectum and P. acus (Bacillariophyta), Phytotaxa 207 (3), pp. 273-280 : 275-277

Pleurosigma subrectum Cleve in Cleve & Grunow (1880: 53, pl. 3, fig. 72) ( Figs. 1−6 : LM, 7−11: SEM)

Lectotype:— “E251, Finnmark, Maasö, a, P. T. Cleve, 12.1881, von E. Weissflog ” here designated, geographic coordinates 71° 00’ N, 24° 00’ E. BRM! ZU 9 /91, slide coordinates 3.2 E, 6.0 S. This material is a subsample of Cleve & Möller (1877 −1882) #312, “Finnmark, Maasö”.

Isolectotypes:— In slide BM 13071 at slide coordinates 6.4 E, 5.7 S ; 11.8 E, 8.4S ; 11.4 E, 7.8 S; 4.6 E, 8.0 S and 10.1 E, 12.0 S; also marked on slides S 123001-01 and S 123001-02.

LM description:— Valves fusiform, non-sigmoid ( Figs. 1−4 ), ca. 160−280 μm long, 18−22 μm wide, with acute apices. Raphe sternum non-sigmoid throughout but helictoglossae very slightly deflected ( Fig. 5 ). Striae transverse ca. 19−21 in 10 μm, oblique ca. 15−18 in 10 μm, stria angle 63−68° ( Fig. 6 ). Central area small, round ( Figs. 3, 6 ). Central raphe fissures long, almost overlapping and slightly undulating ( Fig. 6 ).

SEM description: —E xternally, central raphe fissures long, deflected to the same side, almost overlapping ( Fig. 7 ). Apical raphe fissures sharply deflected and terminally strongly recurving forming a hook ( Fig. 8 ). A long accessory fissure terminates within this hook ( Fig. 8 , bottom arrow) and there is an accessory apical pore ( Fig. 8 , top arrow). Internally, areolar openings generally bisected by a small bar ( Fig. 9 — see also inset, Fig. 10 ) except around the elevated central raphe nodule, which is enclosed within two small central bars of approximately equal length ( Fig. 9 ). Helictoglossa prominent, slightly deflected, near the tip there is a small apical pore ( Fig. 10 , arrow). Basal and tegumental layers joined by columns ( Fig. 11 ), which are hollow (white arrow in Fig. 11 ) and showing tiny additional cavities (black arrows in Fig. 11 ).

Habitat:— Littoral marine.

Distribution:— Type material North Scandinavian coast, specimens matching the type were personally observed in materials from the UK, Spain, W. coast of the USA and Brazil.

Observations:— The species is fairly common in the type material, 20 intact specimens were seen in the 4 slides examined. The hollow columns joining the basal and tegumental layers are a particularly noteworthy finding—see under P. acus .

Figures

FIGURES 1−6. Pleurosigma subrectum, type material, LM. 1, 2. Survey of small and large specimens. 3−6. Lectotype specimen. All BF except Fig. 5, DIC, stack of 6 images. Scale bars = 50 μm (Figs. 1, 2), 10 μm (Figs. 3, 4), 5 μm (Figs. 5, 6).

FIGURES 7−11. Pleurosigma subrectum, type material, SEM. Fig. 7. Valve centre, external. Central raphe fissures almost overlap. Fig. 8. Valve apex, external. Top arrow marks apical pore, bottom arrow marks accessory fissure terminating within raphe “hook”. Fig. 9. Valve centre, internal. Note double areolar openings (see enlarged inset) and elevated central raphe nodule. Fig. 10. Valve apex, internal. Prominent helictoglossa, arrow marks apical pore (see enlarged inset). Fig. 11. Broken valve shows hollow columns (white arrow) with additional minute cavities (black arrows). Scale bars = 10 μm (Figs 7, 9), 5 μm (Figs 8, 10), 1 μm (Fig. 11).