Actinocyclus cuneiformis (Wallich) F. Gómez, Lu Wang & Senjie Lin, 1860

Lobban, Christopher S. & Witkowski, Andrzej, 2023, Marine benthic diatoms of Guam: new records, Dictyoneis apapae sp. nov., and updates to the checklist, Micronesica 2023 (2), pp. 1-75 : 2

Actinocyclus cuneiformis (Wallich) F. Gómez, Lu Wang & Senjie Lin Figs 1, 2

Basionym.: Hemidiscus cuneiformis Wallich

Ref. illus.: Hustedt 1927 –1930, p. 904, fig. 542d; Ricard 1987, Figs 30–34 ; the images in Round et al. 1990, pp. 192–193, are also evidently this species; Gómez et al. 2017, figs 1–23

Samples: GU66F-4

Dimensions: long axis 71–74 µm, short axis 41–45 µm

Diagnostics: Distinctively shaped, lens-shaped in valve view but the frustules wedge-shaped (cuneiform) because the valve faces are at an angle to one another. Areolae with a series of radiating patterns around the margin, especially on the more strongly curved side. Large rimoportulae visible in the oblique view. A pseudonodulus is present on the flatter side.

Comments: Two valves observed in one sample. The ventral curvatures of A. cuneiformis vary from straight to sinuous and Hustedt (1927 – 1931) combined several older Eudotia species as varieties. Although Round et al. (1990) say that the valves are cuneiform, it seems to us that Wallich’s (1860) epithet refers to the shape of the frustule (described by Round et al. 1990 as “shaped like orange segments”), rather than to the shape of the valve. The genus Hemidiscus was subsumed into Actinocyclus by Gómez et al. (2017); they did not discuss any varieties of H. cuneiformis but their images, like ours, conform to var. ventricosa (Castracane) Hustedt. We have therefore omitted any varietal designation.

Figures

Figures 1–7. Figs 1, 2. Actinocyclus cuneiformis, LM, arrows showing rimoportulae. Figs 3, 4. Amphora spectabilis: Fig. 3. LM at two focal planes showing different striae densities on dorsal and ventral surfaces. Fig. 4. Palau specimen in SEM showing character of striae. Figs. 5–7. Amphora subhyalina. Fig. 5 LM, showing the hyaline character. Fig. 6. SEM detail of valves with conopea at central raphe endings (arrow), also showing striae on dorsal (double arrowhead) and ventral (single arrowhead) part of girdle band. Fig. 7. Frustule showing broad dorsal part of some girdle bands. Scale bars: Figs 1–5, 7 = 10 µm, Fig. 6 = 5 µm. Unless indicated otherwise, all light microscopy (LM) was with differential interference contrast (DIC).

Figures 26–31. Biddulphiella and Biremis. Figs. 23–26. Biddulphiella tridens. Fig. 26. Frustule in valve view at two focal planes. Figs 27, 28. Valves in girdle and valve view, respectively, SEM. Fig. 29. Valve in oblique view, emphasizing the deep grooves (sulci) between the elevations. Figs. 30, 31. Biremis ambigua frustules in SEM. Fig. 30. Oblique view showing raphe and paired external alveolus openings. Fig. 31. Broken frustule showing internal side of alveoli (arrow) and exterior of girdle bands. Scale bars = 10 µm.

Figures 32–38. Biremis and Campylodiscus. Figs. 32–37. Biremis ridicula. Fig. 32. Frustule in “ventral” girdle view, LM. Fig. 33. Valve and girdle bands in girdle view at two focal planes, LM. Fig. 34. Valve, interior view, SEM. Fig. 35. Frustule in “ventral” girdle view. Fig. 36. Valve in valve view showing asymmetry. Fig. 37. Detail of Fig. 34 showing alveoli and asymmetry. Fig. 38. Campylodiscus robertsianus valve in LM. Scale bars: Fig. 38 = 25 µm, Figs 32–36 = 10 µm, Fig. 37 = 5 µm.