Brachysira praegeri Kennedy & Allott, 2017

Kennedy, Bryan & Allott, Norman, 2017, A review of the genus Brachysira in Ireland with the description of Brachysira praegeri and Brachysira conamarae, new raphid diatoms (Bacillariophyceae) from high status waterbodies, Phytotaxa 326 (1), pp. 1-27 : 6-10

Brachysira praegeri Kennedy & Allott , sp. nov. ( Figs 3–24 )

Type:— IRELAND. Co. Galway: Lough Nambrackmore , 53.4450215 N, 9.93394597 W, elev. 21.9 m, coll. B. Kennedy, coll. date 15 August 2013. (Holotype: specimen here designated Finder: P41 (The England FinderTM Graticule) TCD0018137 , illustrated in Fig. 8 ; type, slide ZU 10/78, Hustedt Collection, Alfred Wegener Institute) .

LM ( Figs 3–18 ):—Cells solitary. Valves lanceolate with convex margins tapering evenly to non-protracted, rounded to occasionally moderately rounded blunt apices. Valve dimensions (n=50): length 16–22.5 μm, width 4–5.5 μm. Axial area narrow, linear. Central area very narrow due to slightly shortened striae, symmetrical to asymmetrical, typically rounded-elliptical to barely discernible from axial area. Raphe straight, filiform with simple, straight, closely located proximal endings. Distal raphe endings not visible in LM. Striae lineate, uniseriate, radiate at center changing to parallel and slightly convergent at apices; predominately 34 striae ranging from 32–36 in 10 μm, often denser at apices. Each stria composed of 3–4 areolae, discernible with LM.

SEM ( Figs 19–24 ):—Valves surrounded by an elevated ridge, marginally thicker near valve apices ( Fig. 19 ). Striae uniseriate, composed of 3–4 transapically elongated areolae, decreasing in number at valve centre and apices. Areolae linear of similar length, or sometimes longer in the outer row, with small gaps between individual areolae producing narrow apically running hyaline undulations on valve surface. Small papillae present on interstriae, usually 4–5 per interstriae at valve centre, reducing to single papillae at apices. Irregular papillae often well dispersed over or around central area ( Figs 19, 22–24 ). Voigt discontinuity not observed. Mantle slopes away at a shallow angle from valve face to reveal striae composed of single elongated bacilliform areola with 1–2 papillae present between them. Areolae foramina with small indentations. Around apices, mantle striae closer together. Raphe branches bordered by an elevated ridge running from central area to apices, never fusing with marginal ridge. Raphe straight, with weakly expanded straight proximal endings ( Fig. 19 ). T-shaped distal raphe endings present with curved branches. Internally, central area symmetrical to asymmetrical, distal raphe endings terminate on weakly raised helictoglossae ( Fig. 20 ). Striae internally covered by subdivided hymenes.

Etymology:—The specific epithet “praegeri ” commemorates the Irish naturalist Robert Lloyd Praeger for his diverse investigations on the natural history of Ireland. His search for biological endemism, a characteristic apparent in the genus, culminated in the publication of the Clare Island Survey which was undertaken near the region where this species was discovered.

Similar species:—The combination of morphological characteristics including, the overall valve size and symmetric outline, shape of the apices, structure and density of the striae, and narrow central area with closely positioned proximal raphe ends allow differentiation from similar taxa using LM alone ( Figs 3–18 ).

Brachysira praegeri has morphological features in common mostly with Brachysira brebissonii or associates of the brebissonii -complex (sensu Van de Vijver 2014) having lanceolate valves with rounded apices. However, B. praegeri has a stria density approaching the upper end of the published range for this taxonomic aggregate. Conspecificity with taxa having obviously widened rhombic outlines, coarser striae or broadly lanceolate valves tapering to acutely rounded apices can easily be ruled out including, Brachysira manfredii Lange-Bertalot (in Lange-Bertalot & Moser 1994: 44), Brachysira metzeltinii Lange-Bertalot (ibid: 45), and Brachysira sandrae (Van de Vijver 2014: 141) .

In common with B. brebissonii and B. sp. (cf. B. brebissonii, Alfinito & Lange-Bertalot 2013: 140 ) from Sierra Leone, the apices of B. praegeri are never protracted but tend towards more obviously rounded poles. There is some overlap in valve dimensions with B. brebissonii , but B. praegeri is smaller in dimension ( Table 2). Stria density is also higher (32–36 in 10 μm) and the structure of the striae and areolae differs to that found in the type material of B. brebissonii (29–31 in 10 μm) which also possesses much more easily discernible apically running hyaline undulations ( Figs 81–91 ). The central area is also less well defined and the proximal raphe ends are more narrowly disposed in B. praegeri . Stria density is higher in the B. sp. (cf. B. brebissonii morph) from Sierra Leone (35–40 in 10 μm) and valve widths do not overlap. Ultrastructural differences, including the papillae shape and their higher number on the mantle differ considerably. Separation from B. sp. (aff. B. brebissonii ( Vouilloud et al. 2014: 148) which has a similar size range can also be determined from its coarser striae (27–30 in 10 μm), presence of well delimited wider hyaline lines and the presence of a clearly rhombic central area in this species. Brachysira subtile ( Potapova et al. 2014: 103) can also be separated easily based on the longer more linear-lanceolate valve outline and its apically elongated central area. B. praegeri overlaps in size and also has a narrow reduced central area similar to Brachysira ontonageniana Kociolek & Lowe (in Anderson et al. 2013: 15) and Brachysira gatesii Kociolek & Lowe (ibid). However, B. praegeri never displays their transapical asymmetry and also differs from the valve outline found in these taxa. Their coarser stria density (28–31 & 29–32 in 10 μm, respectively) and the protracted apices in B. gatesii are also distinctly different.

Although stria density sometimes overlap, the valve outline of Brachysira microcephala often has clearly protracted and elongated ends while the apices of B. praegeri are less acutely rounded to moderately rounded and never protracted. Comparison with non-protracted forms of B. microcephala is also warranted (see Brachysira neoexilis Lange-Bertalot Morphotype I in Lange-Bertalot & Metzeltin 1996 TFL 36 figs 27–28; B. sp. (cf. B. neoexilis Lange-Bertalot & Moser 1994 TFL 46 figs 19–22)) but conspecificity with this morphotype can also be excluded based on the narrower lanceolate outline and more acutely rounded elongate apices of B. microcephala . Ultrastructurally, the sloping angle of the mantle visible with SEM in B. praegeri contrasts with that found for B. microcephala ( Figs 168–172 ) which also lack papillae between the mantle aerolae. Based on these clearly different combination of features used in the characterisation of the genus ( Round & Mann 1981), separation of B. praegeri from other taxa is warranted.

Distribution & Ecology:— Brachysira praegeri was observed in five lakes and three rivers in west and northwest Ireland ( Fig. 2 ) reaching a maximum relative abundance of 17.4% in L. Maumwee. The diatom assemblage there is dominated by Brachysira microcephala , with Achnanthidium caledonicum (Lange-Bertalot in Lange-Bertalot & Moser) H. Lange-Bertalot (1999: 277) and Encyonopsis neerlandica Van de Vijver et al. (2012: 44) of secondary importance. It is a species of low alkalinity (4.0–5.0 mg/l CaCO 3), slightly acidic waters (pH: 6.0–6.8) and relatively low colour (29–83 Hazen) with nutrients at low concentration below the limit of detection ( Table 3).

during this study. Mean values are given in parenthesis under the range. *Dist.PRE indicates the distance between the proximal raphe endings.

for some parameters for common Brachysira taxa (for samples>1.0% abundance) determined in this study.

Figures

FIGURE 2. The distribution of the more abundant Brachysira taxa in Irish lakes and rivers.

FIGURES 3–24. Brachysira praegeri sp. nov. Figs 3–18. LM. Fig. 8. Holotype specimen. Figs 7–10. Range of valves from the holotype population(TCD0018137) L.Nambrackmore.Figs 3–6, 11–17.Valves showing morphological range andsize variation in other populations. Fig. 18. A teratological valve giving the abnormal impression of rostrate ends. Figs 19–24. SEM. Fig. 19. External view. Fig. 20. Internal view. Figs 21–24. Variation in valve outline and occurrence of papillae in the central area. Scale bar in Fig. 18 = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.

FIGURES 81–98. Brachysira brebisssonii and Brachysira intermedia. Figs 81–91. B. brebissonii, LM. Figs 92–94. B. intermedia, LM. Fig. 95. SEM, B. intermedia. Figs 96–98. SEM, B. brebissonii. Scale bar = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.

FIGURES 136–172. Brachysira microcephala morphotype BMIC-1. Figs 136–165. LM. Figs 166–172. SEM. Scale bar = 10 μm for all LM figures. Scale bar in all SEM micrographs = 2 μm.