Staurosirella frigida Van de Vijver & E.Morales, 2014

Vijver, Bart Van De, Morales, Eduardo A. & Kopalová, Kateřina, 2014, Three new araphid diatoms (Bacillariophyta) from the Maritime Antarctic Region, Phytotaxa 167 (3), pp. 256-266 : 261-263

Staurosirella frigida Van de Vijver & E.Morales sp. nov. ( Figs 52–76 )

Cells ovoid to elliptic, openly convex margins and rounded apices. Length 10–14 µm, width 3.1–4.2 µm, stria density 9–10 in 10 µm. Central sternum narrow, linear to weakly lanceolate, raised and forming a continuum with the virgae. Striae alternate, parallel in central area and radiate at apices. Areolae lineolate, running continuously from valve face to mantle, 60 in 10 µm, and bearing delicate volae. Spines spatulate and located on the virgae. One apical pore field at each apex, located on the valve mantle and composed of 4–5 rows of round poroids. Girdle bands open and non-perforated. Valvocopula wider than rest of girdle elements and bearing well-developed fimbriae.

LM ( Figs 52–69 ): Frustules are rectangular in girdle view ( Fig. 69 ). Valves are ovoid to elliptic with convex sides and rounded apices ( Figs 52–68 ). Valve dimensions (n=25): length 10–14 µm, width 3.1–4.2 µm. The central sternum is narrow and linear to weakly lanceolate. The striae are alternating, parallel and becoming slightly radiate near the apices, 9–10 in 10 µm. The areolae are not discernible in LM.

SEM ( Figs 70–75 ): Frustules are linked by single, spatulate and solid linking spines, that are located on the costae at the valve face/mantle junction ( Figs 70–74 ). The valve surface is wavy due to the raised costae and the axial area, which are smooth ( Figs 70, 74 ). Apical pore fields are present on each apex, located on the valve mantle and composed of several, usually 4–5, short rows of small rounded poroids ( Figs 72, 73 ). Internally, the apical pore fields are well- separated from the striae and formed within a depression at the valve apex ( Fig. 75 ). Rimoportulae are absent. The striae are composed of narrow slit-like to rectangular areolae running continuously from the valve face onto the mantle and become smaller toward the axial and mantle abvalvar areas ( Figs 70, 74 ). Areolae ca. 60 in 10 µm along the striae ( Fig. 74 ). There are usually 3–5 mantle areolae. The volae are delicate and form a complex structure within the areolae ( Figs 72–74 ). Internally, the costae are raised and slightly thickened compared to the striae ( Fig. 75 ). The girdle bands are open and lack perforations. The valvocopula are fimbriate and wider than the other copulae.

Type:— lake next to Limnopolar Lake , Byers Peninsula , Livingston Island ( South Shetland Islands ), sample BY047 (62°38’54.9”S, 61°06’39.6”W), Leg. B. Van de Vijver, coll. date 14/01/2009 (BR-4327, holotype!, University of Antwerp, Belgium slide PLP-228, isotypes!) .

Etymology:—The species epithet makes reference to the cold ecological conditions in which this taxon thrives.

Ecology & Distribution:— Staurosirella frigida is a rather common species on Livingston Island. It was found in several mostly larger lakes with a rather alkaline pH varying between 7.2 and 7.9 and a specific conductance value between 60 and 130 µS/cm. Most lakes are fed by meltwater coming from nearby snowfields. Other dominant taxa in the samples include Fragilaria capucina s.l. Desm. (1825: fasc. 10, No. 453) and several Nitzschia [ N. homburgiensis Lange-Bert. (1978: 650) , N. paleacea Grunow in Van Heurck (1881: pl. 68, fig. 9-10)] and Navicula [ N. cremeri Van de Vijver & Zidarova in Van de Vijver et al. (2011: 289), N. australoshetlandica Van de Vijver in Van de Vijver et al. (2011: 287)] species.

Observations on Staurosirella antarctica and Staurosirella frigida :—Both new species of Staurosirella correspond entirely with the morphological description of the genus Staurosirella (as outlined in Morales & Manoylov 2006a). They both show slit-like areolae, volae forming complex subareolar structures and apical pore fields developing on both valve apices. Staurosirella antarctica and Staurosirella frigida can be separated from each other by several features. The apical pore fields are larger and better developed in S. antarctica with 6 rows of small poroids, whereas S. frigida has smaller apical pore fields with a maximum of 4–5 rows of poroids. In LM, the two species can be separated by their valve outline and valve width, with S. antarctica being slightly narrower (valve width 2.9–3.9 vs. 3.1–4.2 µm) than S. frigida , and has a typical heteropolar, more slender valve outline contrary to the isopolar and rather sturdy valves with convex margins in S. frigida .

Only a few Staurosirella species from the former S. pinnata complex can be confused with the new species. Staurosirella oldenburgiana looks similar to S. antarctica , but has typical isopolar valves, smaller -even only rudimentary- spines, more developed apical pore fields and Staurosirella dubia ( Grunow 1862: 54) E.Morales & Manoylov (in Morales et al. 2010a: 43) is much larger with valves up to 38 µm in length and a valve width of 3.5–6.5 µm. Likewise, Staurosirella ovata E.Morales in Morales & Manoylov (2006a: 357) and S. martyi ( Héribaud 1902: 43) E. Morales & Manoylov (2006a: 254) have large valve widths (up to 7 and 9 µm, respectively) and lack marginal spines ( Morales & Manoylov 2006a). Staurosirella incognita E. Morales & Manoylov (2006b: 331) has heteropolar valves, but has larger valves (width 4–6 µm) than S. antarctica , lacks marginal spines and has larger apical pore fields with up to 10 rows of poroids on the foot pole ( Morales & Manoylov 2006b). Also similar is S. confusa showing comparable valve dimensions, but presenting a more elongate valve outline with clearly protracted apices and typical stellate colonies, which clearly separates this taxon from the two Staurosirella species described here.

Figures

FIGURES 52–76: Staurosirella frigida Van de Vijver & E.Morales sp. nov. All pictures taken from the holotype population (sample BY047). Figs 52–69. LM images. Fig. 69 shows a girdle view whereas Figs 52–68 show valve views. Figs 70–76: SEM images. Fig. 70: External view of an entire valve showing the linking spines, the striae and part of the girdle. Fig. 71: Entire frustule linked to another valve showing the linking spines, parts of the girdle bands and the mantle. Fig. 72: Detail of the apical pore field at the foot pole, separated from the striae. Fig. 73: Detail of the apical pore field at the head pole showing 2 rows of small, rounded poroids. Fig. 74: Detail of the slit-like areolae. Fig. 75: Internal view of an entire valve. Fig. 76: Valvocopula with the fimbriate edges. Scale bar represents 10 µm except for Figs 72–74 where scale bar = 1µm.