Achnanthidium neocryptocephalum

Novais, Maria Helena, Jüttner, Ingrid, Vijver, Bart Van De, Morais, Maria Manuela, Hoffmann, Lucien & Ector, Luc, 2015, Morphological variability within the Achnanthidium minutissimum species complex (Bacillariophyta): comparison between the type material of Achnanthes minutissima and related taxa, and new freshwater Achnanthidium species from Portugal, Phytotaxa 224 (2), pp. 101-139 : 116-121

Achnanthidium neocryptocephalum (Grunow in Van Heurck) Novais & Van de Vijver, stat., comb. et nom. nov.

Basionym: Achnanthes minutissima var. cryptocephala Grunow in Van Heurck, Synopsis des Diatomées de Belgique. Atlas ,

pl. 27, figs 41–44, 1880

A new specific epithet “ neocryptocephalum ” is required due to the existence of Achnanthidium cryptocephalum Naegeli ex Kützing 1849 . Grunow in Van Heurck (1880) presents this taxon as ‘ A. minutissima var. cryptocephala Grun. ( A. cryptocephalum Naegeli ?)’, which suggests that he had doubts whether these two taxa are synonyms. The latter was first mentioned in Kützing (1849: 890) as ‘ A. cryptocephalum Naegeli in litt.’ (suggesting that it was mentioned in their correspondence, but not in a publication), with no illustration. Later it was mentioned by Rabenhorst (1853: 25) as ‘ Achnanthidium cryptocephalum Naeg. ’, who provided no illustration but stated that it was originally from Switzerland. In view of the absence of information to prove that these are (or not) synonyms, we propose the new combination for Grunow’s taxon.

Comparison with similar species:— Krammer & Lange-Bertalot (1991) considered Achnanthidium lineare W.Smith as a synonym of A. neocryptocephalum . As in A. neocryptocephalum , A. lineare has a central area forming a rectangular fascia. Potapova & Hamilton (2007) also showed that these taxa are grouped together. Nevertheless, both taxa differ morphologically. The valve outline of A. neocryptocephalum is linear-elliptic to linear-lanceolate with slightly to moderately convex margins and protracted rostrate to subcapitate apices. The valves of A. lineare are linear to narrowly lanceolate with non-protracted broadly rounded to rostrate apices. The striae in the middle part of the raphe valve of A. neocryptocephalum are composed of 3–4 (rarely one or two) rounded, near the valve margin transapically elongated areolae, whilst in A. lineare the striae consist of 1–3 rounded areolae, which are sometimes slit-like near the valve margin. Achnanthidium neocryptocephalum is also wider (2.9–4.4 μm) than A. lineare (2.2–2.8 μm). Hence, A. lineare and A. neocryptocephalum constitute two morphologically different species and both are morphologically different from A. minutissimum ( Van de Vijver et al. 2011a) .

Achnanthidium jackii is also similar to A. neocryptocephalum , in terms of valve outline and ultrastructure. It also has a central area forming a rectangular fascia. The comparison with the type material illustrated by Potapova & Hamilton (2007) revealed that A. neocryptocephalum is slightly wider (2.9–4.4 μm vs. 2.2–3.4 μm) but there is an overlap. The cluster analysis in Potapova & Hamilton (2007) hardly separates both taxa. A re-investigation of the type material of A. jackii would be very useful and it might reveal that A. neocryptocephalum is a synonym of A. jackii .

Achnanthidium sehuencoense E.Morales in Morales et al. (2009: 273) also resembles A. neocryptocephalum . Both taxa have a rectangular fascia. However, the valve outlines and striation pattern differ. The stria density is higher in A. neocryptocephalum (32 and 30 in 10 μm for raphe valve and rapheless valve, respectively) than in A. sehuencoense (20–24 stria in 10 μm for raphe valve and 21–24 in 10 μm for rapheless valve). In A. sehuencoense the valve margins taper more gradually from the valve centre to the poles, and the striae are composed of 4–5, rarely 2 or 3, round areolae. In A. neocryptocephalum the valves taper closer to the poles and the striae consist of 3–4 areolae, rarely 1, 2, and rarely 5 on the rapheless valve, which can be slit-like near the valve margins.

Achnanthes minutissima var. inconspicua Østrup (1910: 129 , fig. 79) ( Figs 126–139, 171–174)

Østrup (1910: 129, fig. 79) described this new variety from rock fissures in the fortification of Hammershus (Bornholm Island, Denmark). The valves are described as linear with inconspicuous striae, a narrow axial area and rostrate apices (14–20 μm long, 2.7 μm wide). It is further mentioned that it is a freshwater taxon ( Østrup 1910).

References and illustrations:— Apart from the original drawing ( Østrup 1910, pl. 3, fig. 79), two photos of the type material were shown by Lange-Bertalot & Krammer (1989, type slide, Østrup collection in the Botanical Museum Copenhagen 3873), p. 269, pl. 51, figs 46, 47 (LM). Lange-Bertalot & Krammer (1989: 104) and Krammer & Lange-Bertalot (2004: 58) regarded Achnanthes strenzkei (Hust.) Simonsen (1987: 476 , pl. 729, figs 1–8) (≡ Navicula strenzkei Hust. 1962: 246 ) as a synonym of Achnanthes minutissima var. inconspicua .

Type material analysis:— Specimens on Østrup’s slide 3873 were rare, and few were suitable for photography ( Figs 126–139, 171–174). Frustules rectangular and slightly arched in girdle view ( Figs 138, 139). Valves linear to narrowly lanceolate with almost parallel margins ( Figs 126–137). Apices broadly rounded, mostly non-protracted, but in larger specimens they can be subrostrate. Valve dimensions (n=14): length 7.5–14.9 μm, width 1.9–2.6 μm. Raphe valve concave with a narrow linear axial area slightly widening towards the centre and a central area forming a rectangular fascia. Raphe straight, filiform and with slightly expanded proximal fissures, which are straight or very slightly deflected in the same direction ( Fig. 171). Internally proximal fissures slightly deflected in opposite directions and distal fissures terminating in small helictoglossae ( Fig. 172). Transapical striae weakly radiate throughout the entire valve, becoming slightly denser and more strongly radiate towards the apices (28–32 in 10 μm near the middle of the valve). Striae composed of three, rarely two, rounded areolae ( Figs 171–173), near the valve margin sometimes slit-like ( Figs 171, 172). Rapheless valve slightly convex with narrow, linear axial area slightly widening towards the central area. Central area weakly elliptical and almost absent, not transapically elongated. Transapical striae weakly radiate throughout the entire valve, becoming more strongly radiate and slightly denser near the apices (ca. 32 in 10 μm near the middle of the valve). Striae composed of three, rarely two, rounded to transapically elongated areolae near the valve margin, sometimes slit-like ( Fig. 174). A single row of narrow, elongated areolae along the mantle ( Fig. 173), separated from the striae on the valve face by a hyaline area; mantle areolae not discernable under LM in girdle view ( Figs 138, 139).

Our analysis conforms to the original description, although the size range found was smaller (length 7.5–14.9 μm vs. 14–20 μm, width 1.9–2.6 μm vs. 2.7 μm). The characters including shallow, V-shaped cells in girdle view, valve outline, striation and raphe confirm its placement in the genus Achnanthidium by Jüttner et al. (2010: 309), as Achnanthidium minutissimum var. inconspicuum (Østrup) Jüttner & E.J.Cox in Jüttner et al. (2010: 309). Compère & Riaux-Gobin (2009) proposed Achnanthidium minutissimum f. inconspicuum (Østrup) Compère & Riaux-Gobin (2009: 40) as new combination for this taxon, recommending that these small linear morphs with widely rounded apices should be ranked as forma at most.

The synonymy with Achnanthes strenzkei seems doubtful, since the type material of Achnanthes minutissima var. inconspicua depicted here differs from the type material of Navicula strenzkei (see Simonsen 1987, pl. 729, figs 1–4: LM, 5–8: SEM) with respect to valve outline and striation. The valves of Achnanthes strenzkei are wider, broadly elliptical, and the frustules in girdle view are not or only slightly bent. The central area is not a broad fascia, but either absent or very narrow. The striae of Achnanthes strenzkei are also composed of 4–5, rarely 3 areolae.

In fact, Achnanthes minutissima var. inconspicua closely resembles Achnanthidium lineare in valve dimensions, valve outline, the presence of a rectangular fascia on the raphe valve and striation. Because these species cannot be distinguished morphologically we propose the synonymy between Achnanthidium minutissimum var. inconspicuum and A. lineare . The latter name established by Smith in 1855 has priority.

Achnanthes nana F. Meister (1935: 88 , fig. 2) ( Figs 175–258, 259–269)

Achnanthes nana was described by Meister from the Riffelsee in Switzerland and documented by one LM photograph ( Meister 1935, p. 88, pl. 1, fig. 2). No other illustrations were found in the literature. The reinvestigation of the type material from the Riffelsee was difficult and only light micrographs could be taken because the taxon was rare in the sample ( Figs 249–258). Specimens from Portugal, Scotland and Nepal, which were distinct from Achnanthidium minutissimum but had not been previously identified, are morphologically identical or very similar to Achnanthes nana . They are illustrated in LM ( Figs 175–248) and SEM ( Figs 259–269) to document this species in more detail.

The size range of the type material was larger than suggested by Meister (length 6.4–10.9 μm vs. 8–9 μm, width 1.9–2.8 μm vs. 2.5–3 μm, n=10). The size ranges in other regions were larger than in the type material: Portugal (n=50) length 5.0–15.7 μm, width 2.3–3.3 μm ; Scotland (n=24) length 4.9–11.5, width 1.7–2.4 μm; Himalaya (n=10) length 9.7–12.3 μm, width 2.4–2.9 μm.

Frustules in girdle view rectangular and slightly arched ( Figs 236–238, 248). Valves linear to linear-elliptic ( Figs 175– 235, 239–247, 249–258). Apices broadly rounded, non-protracted or slightly subrostrate. Raphe valve with a narrow linear axial area, slightly widening towards the centre ( Figs 259, 260, 263, 265). Central area inconspicuous, bordered by one more widely spaced stria on both sides, sometimes one of the central stria is absent. Raphe straight and filiform. Externally, proximal raphe pores slightly expanded and straight ( Figs 259, 263, 265), Internally slightly deflected in opposite directions ( Figs 260, 268). Distal endings straight externally ( Figs 259, 263, 265) and internally terminating in a small helictoglossa ( Figs 260, 268). Transapical striae slightly radiate, more strongly radiate towards the apices. Striae often almost equidistant throughout the valve or slightly denser towards the apices, 30–35 in 10 μm ( Portugal), 28–36 in 10 μm ( Scotland, Himalaya). Striae composed of 4–5, rarely three, rounded to transapically elongated areolae in the Portuguese specimens ( Figs 259, 260) and of 3–4, rarely two, areolae in the Scottish and Himalayan specimens ( Figs 263, 265). Rapheless valve with narrow, linear axial area slightly widening towards the centre ( Figs 261, 262, 264). Central area indistinct sometimes bordered by one more widely spaced stria on both sides. Transapical striae almost equidistant, parallel to slightly radiate, more strongly radiate towards the apices, 30 in 10 μm ( Portugal), 28–36 in 10 μm ( Scotland, Himalaya). Striae mainly composed of 3–4, rarely 5 rounded to transapically elongated areolae ( Figs 261, 262, 264, 266, 267). On both valves, areolae internally covered by hymenes ( Figs 266, 268). A single row of elongated or slit-like areolae on the mantle ( Fig. 269), separated from the striae on the valve face by a hyaline area ( Fig. 269), and not discernable in LM ( Figs 236–238, 248).

The valve characters of Achnanthes nana including outline, striation and raphe suggest that the species belongs to the genus Achnanthidium and we propose the following combination: