Orthoseira verleyenii Van de Vijver 2013, sp. nov.

Orthoseira verleyenii Van de Vijver sp. nov. (Figs 1–33)

Cells cylindrical forming long chains. Frustules in girdle view 45–95 µm long. Diameter 17–70 µm. Mantle height 15–25 µm. Central area forming a small hyaline zone (max. 1/3 of the total valve diameter). Marginal spines bifurcating, irregularly positioned on the valve face/mantle margin. Areolae on the valve face dispersed or arranged in radiate striae, 15–18 in 10 µm. On the mantle, striae parallel, distinctly areolated, 19–25 in 10 µm. Carinoportulae 1–6, usually 2.

Type: — Île Amsterdam, Grand Tunnel, sample AMS-W33 (37°48’47.1”S, 77°33’42.6” E, Leg. B. Van de Vijver, coll. date 04/12/2007), slide no. BR-4309 (holotype BR), slide PLP-224 (isotype University of Antwerp, Belgium).

Etymology: —The specific epithet verleyenii refers to my colleague and friend Dr. Elie Verleyen (University of Ghent, Belgium) in recognition of his important contribution to the (paleo-) ecology and biodiversity of Antarctic algae.

Observations:—LM (Figs 1–15): Cells cylindrical in girdle view (Figs 1–2), attached with linking spines (Figs 3–5), usually forming large chains, up to 8–10 valves. Cells 45–95 µm, mantle height ca. 15–25 µm. Valves disc-shaped (Figs 6–9), diameter 17–70 µm. Valve surface flat, forming a right angle with valve mantle. Striae on the mantle composed of uniseriate areolae, 19–25 in 10 µm. Valve face areolae small, radiate, irregularly scattered, rarely forming continuous series, occupying almost 2/3 of the total valve face surface, with remaining surface forming an irregular, small central hyaline area. Towards the valve centre, areolae more scattered, 15–18 areolae in 10 µm, irrespective of valve size. Central area with 1–6 carinoportulae (Figs 10–15) (for 100 valves, 48 % with only two, 23 % with three, 24 % with four and only 1–3% with one, five or six carinoportulae). No relationship noted between number of carinoportulae and valve diameter. Distinct ring of marginal linking spines at the valve face/mantle junction. No caverns or internal undulations present. Internal valves absent. Copulae number variable, between 5-7. (Figs 1–2). SEM (Figs 16–33): Valve face flat (Figs 16, 19) to weakly domed (Fig. 17). Hyaline central area smooth (Figs 16, 18) or even absent due to irregular scattering of areolae (Fig. 17). Areolae irregularly placed (Figs 17, 19) or in radiating series (Fig. 16), small and poroid, sometimes surrounded by a slightly raised rim (Fig. 18). No difference in areolae structure or size between valve face and mantle (Figs 16, 18). Areolae on the mantle always arranged in parallel striae. External marginal pore fields between spines absent (Figs 16–18). Carinoportulae well distinguishable in the central area, often surrounded by slightly raised, fine ridges (Figs 18–21), each with a double, pronounced (sometimes even conical) silica collar (Figs 20–22). Valve face/mantle junction abrupt with rounded edge (Fig. 18). Ring of large linking spines present (Figs 16–19, 30–33), regularly scattered along the entire valve edge. Spines either simple siliceous thickenings (Figs 18–19) or plate-like structures extending from the mantle (Figs 17, 30–31). Linked spines plate-like, bifurcated (Fig. 17) or with quite complex shape (Fig. 31). On the mantle small, irregular thickenings visible between the areolae (Figs 32–33). Some valves possessing a regular thickened line going around the entire valve (Fig. 32). Internally, valve face flat, smooth (Figs 23–24). Areolae appearing as small rounded poroids (Figs 23–24), without a velum (Fig. 24). Near the mantle, areolae organised in sometimes dichotomised rows with occasional short rows of areolae inserted between regular rows (Fig. 24). Lower part of the valve mantle near the cingulum without areolae, forming a hyaline zone (Fig. 24). Carinoportulae unoccluded, with small, simple rounded to slit-like external pits in comparison to the larger internal openings (Figs. 25–27). Inside small granules sometimes present (Fig. 25). Central to the carinoportulae, irregular slits usually present (Figs 25–26) though sometimes absent (Fig. 27).

Copulae open, with very small poroids scattered or organized into rows parallel to the pervalvar axis (Figs 28-29). Sometimes copulae unornamented (Fig. 30).

Ecology: — So far, Orthoseira verleyenii has only been found in lava tube samples on Ile Amsterdam (southern Indian Ocean). At the type locality, the species is not rare, occurring with a relative abundance of more than 5 %. Due to their large size, specimens were easily observed. The type locality is a small puddle in a cave that is part of a collapsed lava tunnel running from the lower Venus Crater towards the coast at an altitude of 220 m. The puddle is fed by water dripping from the ceiling of the cave. The bottom of the cave, surrounding the puddle is covered by mosses and liverworts. PH of the puddle was 5.8 with a specific conductance value of 239 µS/cm. No macro-vegetation was observed in the puddle.

Apart from Orthoseira verleyenii, the sample is dominated by Planothidium lanceolatum (Brébisson ex Kützing 1846: 247) Lange-Bertalot (1999: 287), Karayevia oblongella (Østrup 1902: 252) Aboal in Aboal, Alvarez-Cobelas, Cambra & Ector (2003: 159), Diadesmis crozetikerguelensis Le Cohu & Van de Vijver (2002: 124), Eunotia muscicola Krasske (1939: 366) var. muscicola and an unidentified Melosira species.