Staurosira pottiezii Van de Vijver 2014, sp. nov.

Staurosira pottiezii Van de Vijver sp. nov. (Figs 1–25)

Cells linear to linear-lanceolate with weakly or markedly constricted margins and rostrate to subcapitate ends. Length 15–22 µm, width 3.2–3.6 µm, stria density 12–13 in 10 µm. Central sternum narrow, linear to lanceolate, narrow at apices. Striae alternate, parallel in central area and radiate at apices. Areolae slightly lineolate, running continuously from valve face to mantle, 50–60 in 10 µm and bearing intricate volae. Spines spatulate, lobed at ends and located on the virgae. Apical pore field at each apex, located on the valve mantle, comprising 3–5 rows of round poroids. Girdle bands open, non-perforated. Valvocopula wider than copulae, bearing well-developed fimbriae.

LM (Figs 1–18): Frustules are rectangular in girdle view, forming long ribbon-like colonies of up to several tens of individuals. Valves are linear to weakly linear-lanceolate with parallel, often constricted margins and rostrate to subcapitate apices. Valve dimensions (n=25): length 15–22 µm, width 3.2–3.6 µm. The central sternum is moderately narrow, linear to weakly lanceolate, gradually narrowing towards the apices. The striae are alternating, parallel and becoming slightly radiate near the apices; 12–13 in 10 µm. The areolae are weakly discernible in LM.

SEM (Figs 19–25): Frustules are linked by single, spatulate, lobed, solid linking spines, located on the costae at the valve face/mantle junction (Figs 19, 21). The valve surface is flat and smooth (Fig. 20). The costae are rarely raised above the striae (Fig. 20). Apical pore fields are present on each apex, located on the valve mantle and composed of several, usually 3–5, rows of small rounded poroids (Figs 20, 22). Internally, the apical pore fields are well-separated from the striae and formed within a depression at the valve apex (Fig. 25). Rimoportulae are absent. The striae are composed of narrow slit-like to rectangular areolae, running continuously from the valve face onto the mantle and decreasing in size toward the axial and mantle abvalvar areas (Figs 20, 23). Areolae ca. 50–60 in 10 µm along the striae (Fig. 23). There are usually 3–4 mantle areolae (Fig. 20). The mantle abvalvar edge has a ridge that is running continuously around the valve perimeter (Figs 19, 20). The ridge is sometime ornamented with blister-like depositions of silica located toward the very edge of the mantle (Fig. 19). The volae are clearly developed, forming a complex structure within the areolae (Fig. 23). Internally, the costae are raised and slightly thickened compared to the striae (Fig. 24). The girdle bands are open and lack perforations. (Figs 19–22). The valvocopula is fimbriate (Fig. 21) and wider than the other, quite narrow and usually broken, copulae (Fig. 19).

Type:— ANTARTICA. Pool near Domo Lake, Byers Peninsula, Livingston Island (South Shetland Islands), sample BY038 (62°38’56.6” S, 60°58’27.0” W), Leg. B. Van de Vijver, coll. date 12/01/2009 (BR-4325, holotype!, University of Antwerp, Belgium slide PLP-226, isotypes).

Etymology:—This species is dedicated to the senior author´s dear friend Jean Pottiez (Wilrijk, Belgium).

Observations:— Staurosira pottiezii resembles other species in the genus Staurosira, sharing with them the type of striae, position of spines, and characteristics of the apical pore fields and girdle bands. The species can be confused with several other Staurosira and Staurosirella species presenting a similar apically elongated valve outline with undulated margins. Staurosirella oldenburgiana (Hust. 1959: 29) E. Morales (2005: 118) has narrower valves (max. valve width <2.5µm) with narrower, more protracted apices. The costae bear only rudimentary spines that never show the spatulate structure that can be found in S. pottiezii. The apical pore fields are larger with larger poroids in S. oldenburgiana compared to the small poroids in S. pottiezii. Moreover, at the apices in valves of S. oldenburgiana, striae are absent isolating the apical pore field from the striae by a hyaline zone. Staurosirella confusa E. Morales (2005: 122) has a different valve outline, being much more lanceolate with acutely rounded, protracted apices. On average, valves are broader for any given length (width 3.5–4.5 µm) in S. confusa. The single apical pore field in this taxon is much larger with up to 10 rows of large, rounded poroids. Also in this taxon, between the striae, often two spines can be observed, but despite this, ribbon-like colonies have never been observed. Instead, S. confusa produces stellate colonies (Morales 2005). Staurosira construens var. binodis (Ehrenb. 1840: 212) P.B.Hamilton in Hamilton et al. (1992: 29) shows a similar valve outline, similar spine structure and similar ribbon-like colonies, but it differs from S. pottiezii in having larger valve dimensions (width 4.5–6.0 µm), smaller apical pore fields with only two rows of small poroids, and smaller areolae (Morales 2005). On the sub-Antarctic islands in the southern Indian Ocean, a similar taxon was found and identified by several authors as Fragilaria alpestris [also reported as Staurosirella alpestris (Krasske) Le Cohu (1999: 826) or Staurosira alpestris (Krasske) Van de Vijver & Beyens in Van de Vijver et al. (2002: 114)]. Le Cohu (1988) discusses the morphology of the F. alpestris populations in the Kerguelen Islands, one of the archipelagos in the southern Indian Ocean. The recorded populations on the other Indian Ocean islands also had always larger valve dimensions (valve length> 25 µm and valve width>4.0 µm) giving them a more robust appearance, contrary to S. pottiezii, where valves never exceeded 3.5 µm in breadth. Based on Le Cohu (1988), only one single apical pore field is observed in F. alpestris, whereas S. pottiezii always has two. In South Georgia, another similar taxon was found, most likely erroneously identified as Fragilaria neoproducta Lange-Bert. (1993: 48) in Van de Vijver & Beyens 1996. A preliminary LM analysis (Van de Vijver, unpubl. res.) showed that the valves are always larger (width>4.0 µm) with usually less subcapitate and more subrostrate apices. Conspecificity between S. pottiezii and the populations from the Southern Indian Ocean and South Georgia can most probably be excluded based on current data, but more detailed SEM analysis of the latter populations will be necessary for a final decision.

Ecology & Distribution:— Staurosira pottiezii was found on many islands of the South Shetland Island archipelago [Livingston Island, Deception Island, Nelson Island and Dart Island (R. Zidarova, pers. com.), King George Island]. However, its exact distribution is not well known due to confusion with other cosmopolitan taxa such as Fragilaria alpestris Krasske (in Hustedt 1931: 165), F. bidens Heiberg (1863: 60) and Staurosira construens Ehrenb. (1843: 424). Staurosira pottiezii seems to prefer small, shallow, usually temporary pools, most likely originating from meltwater streams or seepage areas. On average, these pools had an alkaline pH (7.5-9.0) and rather low conductivity (80–110 µS/cm). In one pool on Dart Island, a large population was found at pH 9.4 and a conductivity of 576 µS/cm (R. Zidarova, pers. com.). All major populations on Byers Peninsula were associated with large algal mats, developed in these small pools.