Pleurosigma acus A. Mann (1925: 132

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 : 278-279

Pleurosigma acus A. Mann (1925: 132 , pl. 29, figs. 4 and 5) ( Figs. 12−19 , LM; 20−24, SEM)

= P. subrectum Cleve in Cleve & Grunow (1880: 53, pl. 3, fig. 72) syn. nov.

Type:— USNM! #3278 and #3279, (both selected specimens.) of which #3278 was designated the syntype in Stidolph (2002) and #3279 is the lectotype here designated.

Epitype material: BRAZIL. Sao Paulo: Toque-Toque Grande Beach, Sao Sebastiao, 23° 83’ S, 45° 51’ W, Dr. Ricardo Tsukamoto, 30 June 2012, leg. R.Y. Tsukamoto (in collections Sterrenburg #724 and Herbario Maria Eneyda P.K. Fidalgo, Instituto de Botanica de Sao Paulo #SP 428.775), here designated.

LM description: — Valves fusiform, non-sigmoid ( Figs. 12−15 ), ca. 170–250 μm long, ca. 17−22 μm wide, with acute apices. Raphe sternum non-sigmoid throughout but helictoglossae very slightly deflected ( Fig. 16 ). Striae transverse ca. 18−20 in 10 μm, oblique ca. 15−17 in 10 μm, stria angle 65−68° ( Figs. 18, 19 ). Central area small, round ( Fig. 17 ). Central raphe fissures long, almost overlapping and slightly undulating ( Fig. 17 ).

SEM description: — Externally, central raphe fissures long, deflected to the same side, almost overlapping ( Fig. 20 ). Apical raphe fissures sharply deflected and terminally strongly recurving forming a hook ( Fig. 21 ). A long accessory fissure terminates within this hook ( Fig. 21 , right arrow) and there is an accessory apical pore ( Fig. 21 , left arrow). Internally, areolar openings generally bisected by a small bar ( Fig. 22 —see also inset, Fig. 23 ) except around the elevated central raphe nodule, which is enclosed within two small central bars of approximately equal length ( Fig. 22 ). Helictoglossae prominent, slightly deflected, near the tip there is a small apical pore ( Fig. 23 , arrow). Basal and tegumental layers joined by columns ( Fig. 24 ), which are hollow (black arrow in Fig. 24 ). There are no septa dividing the valve interior into compartments ( Fig. 20 , arrow, and Fig. 24 ).

Habitat:— Littoral marine.

Distribution:— Type material from Philippines, for additional personal observations of specimens matching the type, see under P. subrectum .

Observations:— Figs. 18 (lectotype specimen) and 19 (epitype specimen) illustrate the perfect match as regards striation: identical stria densities and stria angles (ca. 68°). From the descriptions of these two taxa it becomes clear that no morphological differences can be demonstrated for P. subrectum and P. acus , including such fine details as the accessory apical fissures and the hollow columns shoring the two layers of the sandwich-structured valves (compare Figs. 11 and 24 ).

Figures

FIGURES 12−19. Pleurosigma acus, LM. 12−14, 18. Lectotype. 15−17, 19. Epitype specimen. All BF. 18, 19. Greatly enlarged detail of lectotype (Fig. 18) and epitype (Fig. 19) striation to facilitate verification of identical stria density and stria angle. Scale bars = 20 μm (Figs. 12−15), 5 μm (Figs. 16−19).

FIGURES 20−24. Pleurosigma acus, epitype material except for Fig. 24, SEM. 20. Valve centre, external. Valve morphogenesis not yet finished, tegumental layer in process of formation. Note absence of septa (arrow). Central raphe fissures almost overlap. 21. Valve apex, external. Left arrow marks apical pore, right arrow marks accessory fissure terminating within raphe “hook”. 22. Valve centre, internal. Note double areolar openings (see enlarged inset) and elevated central raphe nodule. 23. Valve apex, internal. Note double areolae. Prominent helictoglossa, arrow (see enlarged inset) marks apical pore. 24.Developing valve, tegumental layer not yet deposited. Specimen from California, USA, examined in Sterrenburg et al. (2005). Note non-loculate valve structure and hollow columns (black arrow at centre). Scale bars = 5 μm (Fig. 22), 2 μm (Figs 20, 21, 23), 1 μm (Fig. 24).

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).