Achnanthidium lailae Van de Vijver in Zidarova et al.

Vijver, Bart Van De & Kopalová, Kateřina, 2014, Four Achnanthidium species (Bacillariophyta) formerly identified as Achnanthidium minutissimum from the Antarctic Region, European Journal of Taxonomy 79, pp. 1-19 : 8

Achnanthidium lailae Van de Vijver in Zidarova et al. ( Zidarova et al. 2009)

Figs 54-77

Morphological observations

Light microscopy ( Figs 54-74 )

Frustules in girdle view narrow, rectangular, bent around the transapical axis, apices weakly recurved ( Figs 54-56 ).Valves linear to very slightly linear-lanceolate with almost parallel margins and nonprotracted, broadly rounded, never rostrate or capitate apices ( Figs 57-74 ). Valve dimensions (n=30): length 10–14 µm, valve width 1.8–2.5 µm. Raphe valve ( Figs 57-64 ) concave with a rather narrow, linear to linear-lanceolate axial area, widening towards the central area. Central area forming a typical rectangular fascia. Shortened marginal striae occasionally present in the central area. Raphe straight to weakly undulating with inconspicuous straight proximal raphe endings. Distal raphe fissures not discernible in LM. Striae weakly but still distinctly radiate near the valve center, becoming more radiate near the apices, 30–33 in 10 µm. Rapheless valve ( Figs 65-74 ) slightly convex with moderately broad, clearly lanceolate axial area, widening near the valve centre. Central area elongated, rhombic lanceolate, never expanding into a fascia due to several longer marginal striae in the central area. Striae parallel to weakly radiate near the valve centre, more radiate near the apices, 28–30 in 10 µm.

Scanning electron microscopy ( Figs 75-77 )

Striae on the raphe valve composed of 2–3 small areolae ( Fig. 77 ). Areolae close to the axial area and at the apices rounded. Marginal areolae sometimes narrow, transapically elongated and hence slit-like, sometimes fused with the second areola ( Fig. 77 ). Striae of the rapheless valve composed of 2–3 rounded to slit-like external areola openings ( Fig. 75 ). Mantle areolae slit-like. Internal areolae openings covered by hymenes ( Figs 76, 77 ). When removed due to sample preparation, very narrow struts visible separating the areolae ( Fig. 76 ). Raphe slightly undulating becoming narrower towards the apices ( Fig. 77 ). Proximal raphe endings almost straight, inconspicuous. Distal raphe fissures weakly deflected, continuing slightly beyond the last striae, never onto the mantle ( Fig. 77 ). Internally, proximal raphe endings shortly bent into opposite directions, terminating in a thickened central pore ( Fig. 77 ). Distal raphe endings terminating on small helictoglossae.

Ecology, distribution and associated diatom flora

The type population was found in a large circumneutral lake (pH = 7.1) on Ulu Peninsula on James Ross Island ( Zidarova et al. 2009). Since then, several other large populations were discovered in lakes on Clearwater Mesa, a volcanic tableland on James Ross Island next to Ulu Peninsula. All populations were observed in alkaline lakes (pH 8.1–8.7) with relatively high conductivity (1000–2000 µS/cm). The samples were dominated by Halamphora sp., Pinnularia australomicrostauron Zidarova et al. (Zidarova et al. 2012), Nitzschia cf. commutata Grunow in Cleve & Grunow, and Achnanthes coarctata (Bréb.) Grunow in Cleve & Grunow ( Cleve & Grunow 1880) and Gomphonema sp. So far, no populations were found on other islands in the Maritime Antarctic Region ( Kopalová & Van de Vijver 2013, Van de Vijver et al. unpubl. res.).

Figures

Figs 54-77. Achnanthidium lailae Van de Vijver. Light and scanning electron micrographs of a population on Clearwater Mesa (James Ross Island). 54-56. LM views of some frustules in girdle view. 57-64. LM views of raphe valves. 65-74. LM views of rapheless valves. 75. SEM external view of an entire rapheless valve. 76. SEM internal view of an entire rapheless valve. 77. SEM external (right) and internal (left) view of an entire raphe valve. Scale bars represent 10 µm.