Achnanthidium sieminskae Witkowski, Kulikovskiy & Riaux-Gobin

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 : 9-11

Achnanthidium sieminskae Witkowski, Kulikovskiy & Riaux-Gobin ( Witkowski et al. 2012)

Figs 78-103

Morphological observations

Light microscopy ( Figs 78-99 )

Frustules in girdle view rectangular, clearly bent around the transapical axis and with clearly recurved apices ( Figs 78-80 ). Valves linear to narrowly linear-lanceolate with almost parallel to slightly convex

margins and clearly protracted, rostrate to even capitate apices ( Figs 81-99 ). Valve dimensions (n=25): length 9.5–18.0 µm, width 1.9–3.1 µm. Raphe valve ( Figs 81-91 ) concave with a very narrow linear axial area, almost not discernible near the valve apices. Central area elliptical to rounded, very small, almost indistinct, formed by 2–3 more widely spaced striae. Central striae often lacking extending the central area up to the valve margin. Raphe straight, filiform with simple, straight proximal raphe endings. Distal raphe hardly discernible in LM. Striae weakly radiate near the valve center, becoming parallel and even convergent near the apices, well visible in LM, ca. 32 in 10 µm, more densely spaced near the valve ends. Rapheless valve ( Figs 92-99 ) slightly convex with narrow, linear axial area, widening to form a weakly elliptically, never transapically elongated central area. Fascia never present. Central 2–3 striae more distantly spaced. Striae weakly radiate throughout, becoming more parallel to even convergent near the apices, ca. 35 in 10 µm.

Scanning electron microscopy ( Figs 100-103 )

Valve face of the raphe valve clearly concave. Valve face/mantle margin forming a weakly raised hyaline border separating valve face striae from mantle areolae. Striae, ca. 32–34 in 10 µm, composed of 2–3 small, rounded areolae ( Fig. 100 ) followed near the valve center by a thin, slit-like, marginal areola ( Fig. 100 ). Near the apices, striae more dense, ca. 40 in 10 µm, composed of only 3 rounded areolae. Mantle areolae slit-like ( Figs 100, 102 ). Raphe almost straight with simple, straight proximal raphe endings ( Fig. 100 ). Distal raphe fissures weakly deflected, continuing slightly beyond the last striae terminating on the valve face/mantle junction. Internally, proximal raphe endings shortly bent into opposite directions( Fig. 101 ). Distal raphe endings terminating in small helictoglossae ( Fig. 101 ). Internal areolae openings covered by hymenes ( Fig. 101 ). Striae of the rapheless valve almost equally spaced throughout the entire valve, 34–36 in 10 µm, composed of 2–5 rounded areolae sometimes terminating in one slit-like areola ( Fig. 102 ). Mantle areolae slit-like. Internal areolae openings covered by hymenes ( Fig. 103 ).

Ecology, distribution and associated diatom flora

Achnanthidium sieminskae was originally described as a brackish-littoral taxon from a tidal flat near the Kerguelen Islands ( Witkowski et al. 2012) but the authors already expressed some doubts about this observation. Most likely, their population was the result of a riverine deposition in the marine Bossière fjord on Kerguelen. As they stated themselves, the only way to find out would be the analysis of the living material which was unfortunately no longer available at the time of their publication. We reported this taxon (as A. minutissimum ) from a large number of purely freshwater habitats ranging from small pools to larger lakes and even streams from all sub-Antarctic islands in both the Indian and Atlantic Ocean ( Van de Vijver & Beyens 1996; Van de Vijver et al. 2001, 2002, 2004, 2008). In many cases, very large populations could be observed, occasionally up to 60–80% of all counted valves. These data indicate that this is typical limno-terrestrial taxon preferring wet moss vegetations in pools, bogponds and lakes with a pH between 5.2 and 7.4 and a low conductivity (<160 µS/cm). These data confirm the final conclusion in Witkowski et al. (2012) that the observed valves in their study are the result of the redeposition of a freshwater species in a marine environment. Given the large amount of available data and material, it is a pity the original authors did not verify their rather unusual observation more carefully.

Figures

Figs 78-103. Achnanthidium sieminskae Witkowski, Kulikovskiy & Riaux-Gobin. Light and scanning electron micrographs of a population on Iles Kerguelen. 78-80. LM views of some frustules in girdle view. 81-91. LM views of raphe valves. 92-99. LM views of rapheless valves. 100. SEM external view of an entire raphe valve. 101. SEM internal view of an entire raphe valve. 102. SEM external view of an entire rapheless valve. 103. SEM internal view of an entire rapheless valve. Scale bars represent 10 µm.