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Planothidium africanum Van de Vijver, Gogne, Hoogsteyns, Van de Velde, Vlaminck, Kabota, Teunen & C.E.Wetzel sp. nov. (Figs 1–28 LM, 29–35 SEM) Valves lanceolate (in the largest specimens) to elliptic-lanceolate with distinctly convex margins and protracted, rostrate but truncated apices. Valve dimensions (n=40): length 11–25 μm, width 5.5–6.5 μm. Rapheless valve (Figs 1–14, 29, 30, 33 & 35): Axial area usually narrow, straight, linear, occasionally weakly lanceolate and broadening near the central area. Irregularly shaped, shallow depressions present in both central and axial area (Figs 29, 33). Central area with a unilateral large horseshoe-shaped hyaline area. Cavum distinctly visible, internally passing over the neighboring striae (Figs 30, 35). Striae on the opposite site only weakly to almost shortened. Striae parallel to weakly radiate near the central area, becoming more radiate near the apices, 15–16 in 10 µm. Striae multiseriate, composed of three rows of small, rounded areolae at the valve margins, becoming biseriate towards the axial area (Fig. 29, 33). On the mantle, irregular groupings of small, rounded areolae present, continuing the valve face striae on the mantle (Fig. 33). Internally, striae located in deep grooves (Fig. 30). Raphe valve (Figs 15–28, 31, 32 & 34): Axial area very narrow, linear, not widening towards the central area. Central area asymmetrical, rectangular to slightly wedgeshaped, bordered on each side by 2–3 shortened striae, whereas on the opposite side, striae either lacking or extremely shortened. Raphe branches straight with expanded, drop-like central raphe pores (Fig. 31). Terminal raphe fissures unilaterally deflected (Fig. 31). Striae weakly radiate throughout the entire valve, 14–16 in 10 μm. Internally, central nodule clearly raised (Fig. 32). Internal central raphe endings weakly deflected into opposite directions, terminating onto the central nodule (Fig. 32). Internally, striae located in broad, deep grooves (Fig. 32). Cingulum composed of multiple open girdle bands (Fig. 34). Type:— TANZANIA. Morogoro, sample L10 stone, coll. date VII.2022, leg. Van de Velde (holotype slide BR-4778= Fig. 6, isotype slide 422, University of Antwerp, Belgium). PhycoBank registration: — www.PhycoBank.org/103609 Ecology & associated diatom flora:— Sample L10 Stone (S 06° 49’ 00.1” / E 037° 42’ 56.8”) was collected from some submerged stones in a small, shallow (Navicula species [N. rostellata Kützing (1844: 95), N. schroeteri Meister (1932: 38), N. symmetrica R.M. Patrick (1944: 5)], Achnanthidium exiguum (Grunow in Cleve & Grunow 1880: 21) Czarnecki (1994: 157), Cocconeis cf. euglypta, Planothidium africanum, P. rostratum (Østrup 1903: 35) Lange-Bertalot (1999: 279), several (non-identified) Gomphonema species, and Geissleria cf. decussis with lower numbers of Seminavis strigosa (Hustedt 1949: 44) Danielidis & Economou-Amilli (in Danieldis & Mann 2003: 30), Halamphora sp. and Pleurosigma salinarum (Grunow 1878: 116) Grunow (in Cleve & Grunow 1880: 54). Based on Lange-Bertalot et al. (2017) and Taylor et al. (2007), this diatom flora is typical for eutrophic, freshwater with higher electrolyte content to even brackish, and β- to α-mesosaprobic conditions, pointing to a high degree of pollution. Taxonomic comments:— Table 1 lists the morphometric features of several similar Planothidium species. Planothidium africanum has a distinct cavum, excluding several species bearing a comparable valve outline but with a sinus from being conspecific. Planothidium californicum Stancheva & N.Kristan (in Stancheva et al. 2020: 8) has a similar valve outline and similar valve dimensions, but lacks the typical cavum and hence can be relatively easily separated from the new species. Planothidium dubium (Grunow in Cleve & Grunow 1880: 23) Round & Bukhtiyarova (1996: 352) and P. reichardtii Lange-Bertalot & Werum (in Werum & Lange-Bertalot 2004: 172) have a similar elliptic-lanceolate valve outline with truncated, rostrate apices and an asymmetrical central area in the raphe-bearing valve. But both taxa also lack the distinct cavum structure excluding hence all conspecificity. The African species discussed in N’Guessan et al. (2014) sufficiently differ not to be confused with P. africanum. Planothidium comperei shows the highest resemblance based on its valve outline, although most valves have more convex valve margins. The stria structure of the rapheless valve also shows some similarity with the multiseriate nature of the striae at the valve margins becoming biseriate near the axial area. But the species can be separated from P. africanum in having an almost symmetrical central area in the raphe-bearing valve with a more radiate striation pattern, contrary to P. africanum having more parallel striae and a distinctly asymmetrical central area with on one side an almost complete lack of (or very shortened) striae. The most similar species is P. incuriatum C.E.Wetzel, Van de Vijver & Ector, split off from P. biporomum (M.H. Hohn & Hellerman 1963: 273) Lange-Bertalot (1999: 275) in 2013. Most likely, P. africanum, P. incuriatum and P. biporomum are close to each other, given a similar shape in cavum structure, a similar stria structure with a multiseriate base at the valve margin and a biseriate part at the axial area, and similar shallow markings in the axial and central area. However, Planothidium incuriatum has a more elongated, slender valve outline with more protracted apices, a symmetrical central area in the raphe-bearing valves (contrary to the distinctly asymmetrical central area in P. africanum) and slightly wider valves (6.5–7.0 µm versus 5.5–6.5 µm in P. africanum). The striae in the rapheless valve of P. africanum have a distinct grouping of 3 series of areolae at the valve margin, a feature not observed in P. incuriatum that have a very faint third series of areolae at the margin (see for instance Wetzel et al. 2003, fig. 51). This grouping is also visible in the mantle areolae (Fig. 33). Finally, other rostrate, cavum-bearing taxa such as P. rostratum (present in the same sample as P. africanum) and P. rostratoholarcticum Lange-Bertalot & Bąk (in Bąk & LangeBertalot 2014: 354) differ in having multiple slit-like grooves in their axial and central area, so far not observed in P. africanum. Contrary, P. africanum possesses an irregular pattern of small, shallow depressions in the axial area, not seen in P. rostratum and P. rostratoholarcticum. Wetzel et al. (2019) discuss several small-celled cavum-bearing Planothidium species such as P. frequentissimum but they can all be separated based on their valve dimensions (usually shorter and/or narrower), shape of the apices (typically broadly rounded, hardly protracted in P. frequentissimum) and valve outline, most of them being more strictly lanceolate instead of the elliptic-lanceolate valves observed in P. africanum. |
