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

Arcanodiscus indistinctus Goeyers & Van de Vijver sp. nov.

http://phycobank.org/ 102319 Figs 81–96, 125–131

The specific epithet ‘ indistinctus ’ refers to the lack of any specific peculiarity of this species as a distinguishing feature.

Holotype

CAMPBELL ISLAND • sub-Antarctic region; sample BAS303; 12 Jan. 1970; D. Vitt leg.; BR-4580.

Isotype

CAMPBELL ISLAND • same collection data as for holotype; slide at University of Antwerp, Belgium; PLP-371.

Light microscopy (Figs 81–96)

Frustules discoid, almost square to rectangular with broadly rounded edges and convex valve faces. Frustules never seen attached to each other, always solitary. Multiple discoid chloroplasts present. Girdle bands not discernible in LM. Valves rather strongly silicified, rounded with a thick mantle and convex, weakly domed valve face. Valve dimensions (n = 25): valve diameter 9–17 μm, frustule height (n = 2): 7–9 μm. Central area comprising a large, smooth, hyaline zone, almost 75% of the total valve diameter, surrounded by a clearly striated marginal zone, 27–35 striae in 10 μm. Visible processes lacking.

Scanning electron microscopy (Figs 125–131)

Frustules discoid with very heavily silicified valves, visible as a thick mantle (Figs 125, 130).

Girdle composed of several, narrow, non-perforated copulae (Fig. 126). Pars interior of the copulae clearly fimbriate giving the impression of perforate bands (Fig. 126). Valve face with large, flat hyaline, almost rounded central area, lacking areolae or any other ornamentation (Figs 127–128). Marginal zone gently sloping towards the mantle, entirely covered with short slit-like to almost rounded areolae (the latter when clearly eroded), ca 45 in 10 μm, roughly organized in irregular striae. Areolae externally covered by small, weakly raised, individual occlusions (Fig. 127). No obvious tube process openings apparent. Striae continuing over the valve face/mantle junction, interrupted by a series of up to 3 thin, parallel ridges running entirely around the mantle (Fig. 129). Irregular silica thickenings, often connected to the first marginal ridge, present at the valve face/mantle junction (Fig. 128, arrows). Mantle areolae small, clearly rounded. Mantle edge very narrow, irregularly bordered (Fig. 129).

Internally, areolae large, each showing a clear rota, covered by very thin hymenes, organized in irregularly running striae (Fig. 130–131). Central area consisting of a large hyaline, irregularly bordered, central zone (Fig. 130). No thickened protuberances (called ‘portulae’) observed (Fig. 130). Mantle edge very broad, strongly silicified (Figs 130–131).

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