Hyalosynedra laevigata Grunow

Sabir, J. S. M., Theriot, E. C., Lobban, C. S., Alhebshi, A. M., Al-Malki, A. L., Hajrah, N. H., Khiyami, M. A., Obaid, A. Y., Jansen, R. K. & Ashworth, M. A., 2018, Systematics of araphid diatoms with asymmetric rimoportulae or densely packed virgae, with particular attention to Hyalosynedra (Ulnariaceae, Bacillariophyta), Phytotaxa 347 (1), pp. 1-49 : 7-9

Hyalosynedra laevigata Grunow from Grunow 839 ( Figs 2–9 )

D escription: —Outlines observed were broadly to narrowly lanceolate ( Figs 2–4 ). Margins were convex across the center and then converging nearly linearly towards the apical region (usually within 3–6 μm of the end) where the curvature reversed and the sides either became nearly parallel or weakly subcapitate. Fully intact specimens ranged from 80.5–227.6 μm long by 3.0–6.6 μm in width. Virgae were visible on most specimens in the LM ( Fig. 5 ).

In SEM, the two terminal pore fields were weakly sunken forming ocellulimbi. Sometimes the depression of the pore field was barely perceptible ( Fig. 6 ). Pores were linearly arranged longitudinally and latitudinally. There always appeared to be three rows of pores from the margin of the valve towards the valve face, with the top and bottom row composed of elongate pores and the middle of circular. Density was 5–9 pores per 1 μm along the margin. There were 4–5 short spines over each ocellulimbus, extending from a roughly triangular hyaline area, at the apex of which was the external opening of the rimoportula. The hyaline shelf was penetrated by simple pores at the edges at its edge.

Internally, each ocellulimbus appeared to be 4 or more pores tall, and all pores appeared to be round ( Fig 7 ). However, externally there were always but three rows of pores, with the top and bottom row pores being elongated, suggesting that the top and bottom rows of external pores were formed by merger of the top two and bottom two rows of internal pores. The ocellulimbus was 8—22 pores wide. The hyaline area of the face near the apex above the ocellulimbus was marked by pores scattered near the edge. Internally, it was often sunken relative to the rest of the internal valve surface, with the rimoportula at or near the edge of the depression. The rimoportula coverings were asymmetric, with the lip towards the valve terminus relatively small and partially encircled by the much larger opposite lip, resembling a parrot’s beak. Henceforth, we refer to this simply as an asymmetric rimoportula.

Stria structure is best understood through observations on partially formed or broken valves ( Figs 8, 9 ). The interior silica membrane was solid except for pores at the junction of the face and mantle ( Figs 7, 8 ). The external membrane consists of small vimines (silica bars connecting the virgae), with a secondary transapical bar crossing the resulting pore. The result is biseriate striae running over a continuous chamber or alveolus ( Figs 6 , 9 ). Virga densities (measured only in SEM) were 52–69 in 10 μm.

Remarks: —We did not observe any girdle bands attached to valves in either rinsed or oxidized material, and it appeared that the samples had been thoroughly cleaned before drying on the original Grunow mica. Grunow (1877) reported 38 striae or more in 10 μm. His lower minimal value is likely due to inability to resolve all virgae in the LM on a mica slide.

Along with the nominate variety, also from Honduras, was described S. laevigata var. obtiuscula Grunow (1877 , 166) which he wrote had more linear sides and more obtuse apices. Our observations lead us to conclude that a continuum in variation between these shapes exists and we consider them synonymous.

Williams and Round (1988) did not illustrate an entire specimen of this species from their Philippine material. However, the apices illustrated in their figures 17, 18 and 20 appear identical to larger Grunow specimens in shape, rimoportula placement and width of the ocellulimbus. The smaller specimens, however, represented in their figures 19 and 21 may be of another species of Hyalosynedra .

Figures

FIGURES 2–4. Hyalosynedra laevigata from Grunow 839. 2. Bright field LM of large specimen. 3. Differential interference contrast LM of large specimen. 4. External view of entire valve in SEM.

FIGURES 5, 6. Hyalosynedra laevigata from Grunow 839. 5. Detail of central area in LM. 6. Detail of apex in SEM.

FIGURES 7–9. Hyalosynedra laevigata from Grunow 839. 7. Interior of apex showing rimoportula and terminal pore field. 8. Interior of entire valve. Note break near center. 9. Details of bilayered valve structure.