Arcanodiscus desmetianus Van de Vijver, 2019

Vijver, Bart Van De & Houk, Václav, 2019, Two new centric diatoms (Bacillariophyta) from the sub-Antarctic region, Phytotaxa 394 (1), pp. 50-58 : 52-55

Arcanodiscus desmetianus Van de Vijver sp. nov. Figs 26–48

LM ( Figs 26–41 ): Frustules discoid, almost square to rectangular with broadly rounded edges, always solitary. Girdle bands not discernible in LM. Valves rather strongly silicified, rounded with a thick mantle and flat valve face. Valve dimensions (n=25): valve diameter 10–16 μm, frustule height 7–9 μm. Central area formed by a large, smooth, hyaline zone, almost 50 % of the total valve diameter, surrounded by a more rugose, irregularly shaped, marginal zone. Visible processes lacking. SEM ( Figs 42–48 ): Frustules discoid with very heavily silicified valves, visible as a thick mantle ( Figs 42, 43 ). Valve face with large, flat hyaline, almost rounded central area, free of areolae and ornamentation ( Figs 45, 46 ). Marginal zone gently sloping towards the mantle, entirely covered with short slit-like to almost rounded areolae, vaguely organized in irregular striae, the latter ca. 35 in 10 μm. Areolae externally not covered by occlusions

( Figs 44, 45, 46 ). Very small, rounded, external openings of portulae visible, scattered irregularly among the striae ( Fig. 45 , see arrows), difficult to discern even in SEM. Irregular silica thickenings, often joined forming short, undulating ridges present at the valve face/mantle junction, entirely surrounding the valve face ( Fig. 45 ). Striae continuing over the valve face/mantle junction, interrupted by a series of up to 5 parallel ridges running entirely around the mantle ( Fig. 44 ). Parallel ridges usually covered by girdle bands ( Fig. 42 ). Areolae on the mantle gradually changing from being slit-like to small and rounded.

Internally, areolae rather large, each showing a clear rota, covered by very thin hymenes, organized in irregularly running striae ( Fig. 48 ). Central area formed by a large hyaline, irregularly bordered, central zone ( Fig. 47 ). Large number of perforated, thickened protuberances visible, irregularly scattered between the areolae, never organized in a regular marginal ring ( Fig. 48 ). Inner openings of these protuberances rounded, smaller than the areolae, located in a shallow pit ( Fig. 48 ). Cingulum composed of at least 4, rather narrow, very thin, perforated, open copulae ( Fig. 43 ). Perforations on the copulae transapically elongated, slitlike ( Figs 42, 43 ).

Type:— Pointe Basse, Ile de la Possession, Iles Crozet, Subantarctica, sample BA-200 (46°21’33.7”S / 51°42’40.9”E, leg. B. Van de Vijver), coll. date 16/02/2002 (holotype BR!, slide no. XXXX, isotype PLP! Slide no. XXXX, University of Antwerp, Belgium).

Etymology:— The new species is named after my dear friend and colleague Prof. Dr. Willem De Smet (University of Antwerp, Belgium), a rotifer specialist with whom I spent several summers in both the Arctic and sub-Antarctic Regions, influencing greatly my entire research career.

Ecology, distribution and associated diatom flora:— Arcanodiscus desmetianus was observed on all four islands and archipelagos in the southern Indian Ocean (Iles Crozet, Heard Island, Prince Edward Islands and Iles Kerguelen (always under the name of Melosira aff. guillauminii ). The largest populations were found living on wet, shaded soils, occasionally covered by mosses, almost always influenced by seaspray or marine mammals and birds (penguin rookeries) ( Van de Vijver et al. 2002a, 2004, 2008). Several populations were found co-occurring with Ferocia subantarctica on wet, terrestrial, shaded mosses. Other taxa present in the samples include diverse Planothidium species such as P. subantarcticum Van de Vijver & C.E.Wetzel (2013: 111) and P. cyclophorum ( Heiden & Kolbe 1928: 580) Van de Vijver in Van de Vijver et al. (2002: 99) and various Humidophila taxa (e.g., H. vidalii , H. ingeae ). In aquatic conditions, the species was never observed (Van de Vijver, unpubl. res.).

Figures

FIGURES 26–48. Arcanodiscus desmetianus Van de Vijver sp. nov. Light (LM) and Scanning electron (SEM) micrographs of the holotype population (sample BA200) from Pointe Basse, Ile de la Possession (Iles Crozet), Subantarctica. Figs 26–33 LM photographs of several frustules in girdle view. Figs 34–48. LM photographs in valve face view showing the large hyaline central area. Figs 42 & 43. SEM view of two frustules in girdle view showing the the girdle bands, several portulae and the parallel ridges on the mantle. Fig. 44. SEM view of a valve in girdle view showing the areolae and the parallel ridges on the mantle. Fig. 45. SEM view of a valve face showing different portulae (arrows) and irregular thickenings on the valve face/mantle junction. Fig. 46. SEM valve face view showing clearly the hyaline central area and the gently sloping valve face/mantle junction. Fig. 48. Internal SEM view of entire valve. Fig. 49. Internal SEM view of the areolae with rota and the openings of the portulae. Scale bars represent 10 μm except for Fig. 48 where scale bar = 1 μm.