Arcanodiscus crawfordianus Goeyers & Van de Vijver 2020, sp. nov.

Arcanodiscus crawfordianus Goeyers & Van de Vijver sp. nov. http://phycobank.org/ 102318 Figs 74–80, 116–124 Etymology

The species is named in honor of our colleague Dr Richard M. Crawford to acknowledge his work on melosiroid diatoms.

Holotype

CAMPBELL ISLAND • sub-Antarctic region; sample BAS303; 12 Jan. 1970; D. Vitt leg.; BR-4579.

Isotype

CAMPBELL ISLAND • same collection data as for holotype; slide at University of Antwerp, Belgium; PLP-370.

Light microscopy (Figs 74–80)

To date, frustules in girdle view not observed in LM. Valves strongly silicified, rounded with a thick mantle and clearly convex, weakly domed valve face. Valve dimensions (n = 10): valve diameter 9–18.5 μm. Central area formed by a large, smooth, hyaline zone, 6–11 μm, ca 60% of the total valve diameter, surrounded by a striated, irregularly bordered marginal zone. Thick spines, scattered in the marginal zone distinctly present. Marginal striae discernible in LM, 30–33 in 10 μm. Visible tube processes lacking.

Scanning electron microscopy (Figs 116–124)

Frustules discoid with very heavily silicified valves, visible as a thick mantle (Figs 116, 123–124). Frustule height approx. 11 μm. Cingulum comprising at least 4, rather narrow, very thin, non-perforated, open, ligulate copulae (Figs 116, 120, double arrow). Pars interior clearly fimbriate (Figs 116, 117, arrows) giving the impression of perforated copulae. Valve face with large, flat hyaline, almost rounded central area, free of areolae, but with a dense covering of small silica granules (Figs 119–121). Marginal zone distinctly sloping towards the mantle with a very narrow flat zone immediately next to the valve face/mantle junction. Regular striation pattern present on sloping marginal zone. Irregular marginal ring of large, acute, solid spines present (Figs 120–122) close to the hyaline central area, placed between the areolae. Irregular pattern of small granules (Fig. 122) extending between the spines, though never reaching the mantle. Striae composed of small, strictly rounded areolae. Areolae externally covered by perforated occlusions (Figs 118, 120). Openings of portulae not distinguishable from areolae even in SEM. Striae continuing over the valve face/mantle junction, interrupted by a thick marginal ridge running entirely around the mantle (Fig. 119), usually covered by girdle bands. Mantle edge rather narrow. Between marginal ridge and mantle edge, relatively deep groove present, bearing several rows of strictly rounded areolae.

Internally, areolae large, clearly rounded, each showing a distinct rota, organized in a regular striation pattern (Fig. 123). Mantle edge very broad, heavily silicified (Figs 123–124). Central area formed by a large hyaline, irregularly bordered, central zone (Fig. 123). Several perforated, thickened protuberances which we presume to be the inner openings of portulae are irregularly scattered between the areolae, never organized in a regular marginal ring (Fig. 123, arrows). Inner openings of these processes rounded, smaller than the areolae, weakly raised above the surface (Fig. 124).

Arcanodiscus crawfordianus sp. nov. was described from a Racopilum moss vegetation collected from a wet rock east of Moubray Hill. The sample, in which also the newly described Arcanodiscus indistinctus sp. nov. and Angusticopula cosmica sp. nov. were observed, was dominated by Frankophila dalevittii (Van de Vijver et al. 2020), Diatomella balfouriana and Diatomella colonialis. The presence in other samples needs to be confirmed with SEM observations.