Achnanthidium indistinctum Van de Vijver & Kopalová, 2014

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 : 4-5

Achnanthidium indistinctum Van de Vijver & Kopalová sp. nov.

Figs 2-28

Morphological observations

Light microscopy ( Figs 2-24 )

Frustules short, rectangular in girdle view, bent around the transapical axis ( Figs 2-4 ). Mantle striae often apparently thickened in LM ( Fig. 4 ). Valves narrowly lanceolate with slightly to clearly convex margins and distinctly protracted, rostrate apices ( Figs 2-24 ). Valve dimensions (n=30): length 8.5–13.0 µm, width 1.8–2.2 µm. Raphe valve ( Figs 5-15 ) concave with a very narrow linear axial area, almost invisible towards the valve apices, almost not widening towards the central part of the valve. Central area very small, indistinct, bordered by several (usually two) more widely spaced striae. Raphe straight, filiform, almost invisible, with simple proximal raphe endings. Distal raphe fissures not discernible in LM. Striae moderately radiate throughout the entire valve, becoming indiscernible (due to higher density) near the apices. Rapheless valve ( Figs 16-24 ) slightly convex with narrow, linear axial area, not or very slightly widening towards the valve centre. Central area almost non-existing, never forming a fascia or subfascia. Central striae very weakly more distantly spaced. Striae weakly radiate throughout, only very slightly more densely spaced near the apices, 30–35 in 10 µm.

Scanning electron microscopy ( Figs 25-28 )

Striae in the rapheless valve composed of a series of 3–4 distinct, rounded to square areolae ( Fig. 25 ). Slit-like areolae apparently never present. Central striae less dense than in the rest of the valve ( Fig. 25 ). Near the apices, striae only composed of 1–3 rounded areolae ( Fig. 25 ). Mantle areolae elongated, slitlike, often enlarged ( Fig. 25 ). Striae on the raphe valve, ca. 36 in 10 µm, composed of 2–3 irregularly shaped (mostly rounded or elongated rectangular) areolae ( Fig. 26 ). Near the apices, stria density higher (up to 38 in 10 µm), composed of fewer areolae ( Fig. 26 ). Internal areolae openings on both valves covered by hymenes ( Figs 27, 28 ). External raphe branches straight terminating in simple, short proximal raphe endings ( Fig. 26 ). Distal raphe fissures straight, terminating just beyond the last apical stria ( Fig. 26 ). Internally, proximal raphe endings shortly bent into opposite directions ( Fig. 28 ). Distal raphe endings terminating in small helictoglossae ( Fig. 28 ).

Etymology

The specific epithet was given because the species is rather small and can be easily overlooked.

Type material

Holotype

S lide BR-4361 (National Botanic Garden, Meise, Belgium).

Type locality

Byers Peninsula , Livingston Island , South Shetland Islands, sample BYM051 (62°38’20.1”S 61°06’44.2”W) (Leg. B. Van de Vijver; coll. date 15 Jan. 2009).

Isotypes

Slides PLP-251 ( UA, University of Antwerp, Belgium).

Ecology, distribution and associated diatom flora

So far, A. indistinctum has only been found on Livingston Island (South Shetland Islands). The type population was observed living on submerged mosses collected in a large lake on the central plateau, located between Midge Lake and Limnopolar lake. The lake had an alkaline pH (7.35) with a low specific conductance value (97 µS/cm) and a high phosphate level (5.2 mg /l). The flora in the sample was dominated by several Psammothidium species [ P. abundans (Manguin) Bukht. & Round ( Bukhtiyarova & Round 1996), P. papilio (D.E. Kellogg, M. Stuiver, T.B. Kellogg & G.H. Denton) Van de Vijver & Kopalová ( Kopalová et al. 2012), P. subatomoides (Hust.) Bukht. & Round ( Bukhtiyarova & Round 1996)], Nitzschia cf. perminuta (Grunow) M.Peragallo ( Peragallo 1903), Brachysira minor (Krasske) Lange-Bert. in Lange-Bertalot & Moser ( Lange-Bertalot & Moser 1994), Sellaphora seminulum (Grunow) D.G.Mann ( Mann 1989) and Amphora sp. It is possible that the species is also present on Signy Island (South Orkney Islands) based on the illustrations in Oppenheim (1994). The distribution in the Antarctic region and other geographical areas is unclear due to the insufficient discrimination between species in the Achnanthidium minutissimum -complex. All published records should be verified in order to know the exact biogeography of this and all other taxa.

Figures

Figs 2-28. Achnanthidium indistinctum Van de Vijver & Kopalová sp. nov. Light and scanning electron micrographs of the type population on Byers Peninsula (Livingston Island). 2-4. LM views of some frustules in girdle view. 5-15. LM views of raphe valves. 16-24. LM views of rapheless valves. 25. SEM external view of an entire rapheless valve. 26. SEM external view of an entire raphe valve. 27. SEM internal view of an entire rapheless valve. 28. SEM internal view of an entire raphe valve. Scale bars represent 10 µm.