Placoneis cocquytiae, Fofana & Sow & Taylor & Ector & Vijver, 2014

Fofana, Cheikh Abdoul Kader, Sow, El Hadji, Taylor, Jonathan, Ector, Luc & Vijver, Bart Van De, 2014, Placoneis cocquytiae, a new raphid diatom (Bacillariophyceae) from the Senegal River (Senegal, West Africa), Phytotaxa 161 (2), pp. 139-147 : 140-143

Placoneis cocquytiae sp. nov. ( Figs 1–10 )

Type:— SENEGAL. Kidira, sample SEN–4, leg. B. Van de Vijver , coll. date 06/03/2007 (holotype, slide no. BR –4356; isotypes, slide PLP –250, University of Antwerp, Belgium) .

Valves broadly lanceolate with clearly convex, never parallel, margins and broadly protracted, bluntly rounded, rostrate apices. Valve dimensions (n=20): length 35–58 µm, width 17.0– 20.6 µm. Axial area narrow, linear but slightly curved following the curved raphe branches, gradually but only slightly widening towards the central area. Central area variable in shape due to several irregularly shortened striae, usually transapically enlarged. Isolated pore absent. Raphe branches curved terminating in straight, weakly expanded proximal endings. Distal raphe fissures unilaterally deflected at the apices. Striae clearly radiate throughout the valve, 9–11 in 10 µm. Areolae discernible in LM, ca. 35 in 10 µm.

Ecology and Distribution:— Placoneis cocquytiae has so far been found only in small quantities in a sample from the Senegal survey collected near Kidira. Single valves have also been observed in samples collected near Ndioum and Ngouy-Doué. In Senegal, P. cocquytiae was found in a small pool next to the main river bed that is used as drinking pool for cattle and sheep. The pool had a pH of 7.5 and a specific conductance value of almost 100 µS/cm. Nutrient values, sulphate and chloride were low to moderate (NO 3 - = 1.2 mg /l, SO 4 - = 9.8 mg /l, Cl - = 4.1 mg /l). Dominating taxa in the sample include Nitzschia amphibia Grunow (1862: 574) and several Gomphonema taxa.

Remarkably, P. cocquytiae was also found at four localities in the southern part of Zambia; two sites in the Zambezi River (main channel) and at two sites in one of its tributaries, the Kabompo River. Both rivers had an alkaline pH (7.7–8.4). The samples from the Zambezi River had a very low specific conductance (<100 µS/cm) whereas the Kabompo River had a specific conductance between 300 and 400 µS/cm.

It is likely that the species has a larger distribution in Africa, but confusion with taxa such as Placoneis gastrum (Ehrenb.) Mereschk. (1903: 13; basionym: Pinnularia gastrum Ehrenb. 1843: 421 ), obscures at the moment our actual knowledge of the species’ distribution.

Etymology:—The species is named after our colleague, Dr. Christine Cocquyt (Botanic Garden Meise) in recognition of her research on African diatoms.

Observations:— LM ( Figs 1–10 ): The valves are broadly lanceolate with clearly convex margins giving the species a somewhat elliptical valve outline. The apices are clearly protracted, bluntly rounded, rostrate, but never (sub-)capitate: dimensions are (n=20): length 35–58 µm, width 17.0– 20.6 µm, length/width ratio 2.4–2.8. The valve width range in specimens from the Zambian populations is larger than the type: 15.7–23.2 µm. The axial area is narrow, about 1/10 of the total valve width; it is linear but clearly curved and follows the curved raphe branches. The central area is irregularly shaped and formed by alternating shortening of the central striae. The isolated pore is absent from the central area. The raphe branches are clearly curved. The proximal raphe endings are straight and weakly but distinctly expanded. The distal raphe fissures are unilaterally deflected. The striae are radiate throughout the entire valve and are sometimes weakly geniculate near the apices ( Figs 2, 5 & 9 ), 9–11 in 10 µm. The areolae are quite visible in LM, appearing as large, distant pores.

SEM ( Figs 11–17 ): The striae are uniseriate, composed of small, rounded areolae ( Fig. 11 ), 32–38 in 10 µm. The valve mantle striae are composed of small series of 3–4 rounded areolae ( Figs 12, 14 ). Near the apices, there is visible an irregular doubling of the final striae ( Figs 11, 14 ). The valve mantle is stepped and shallower towards the apices, with a large hyaline edge ( Fig. 12 ). The raphe branches are straight near the proximal endings and show a distinct undulation towards the apices ( Fig. 11 , arrows). The proximal raphe endings are straight terminating in weakly spathulate pores ( Fig. 13 ). The distal raphe fissures are hooked, continuing onto the valve mantle ( Figs 12, 14 ). The area around the distal fissures is enlarged and lacks striation or individual areolae ( Fig. 14 ). Internally, the raphe sternum is clearly raised above the valve plane and transapically expanded in the central area to form a weakly raised stauros which does not reach the valve margins ( Figs 15, 16 ). The virgae are weakly raised between the slightly sunken striae, the latter, internally, composed of small, square areolae, covered by volate occlusions ( Figs 16, 17 ). The proximal raphe endings are very short but markedly unilaterally and bent towards the primary side ( Figs 15, 16 ). The distal raphe endings terminate in small helictoglossae ( Figs 15, 17 ).

Figures

FIGURES 1–10. LM: Placoneis cocquytiae, images to depict variation; all from the type population in Kidira, Senegal. Scale bar = 10 µm.

FIGURES 11–17. SEM: Placoneis cocquytiae, all images from type population, Kidira, Senegal. Fig. 11. External overview of an entire valve; arrows indicate presence of an undulation in raphe branches. Note also irregular final striae near apex. Fig. 12. External overview of an entire valve in oblique view showing the series of areolae on mantle and stepped mantle edge. Fig. 13. External detail of central area allowing a view of the proximal raphe pores. Fig. 14. External detail of valve apex showing mantle areolae, distal raphe ending and irregular last stria on valve face. Fig. 15. Internal overview of entire valve. Fig. 16. Internal detail of central area showing raised central part and short deflected proximal raphe endings; note volate occlusions. Fig. 17. Internal detail of valve apex with helictoglossa. Scale bars represent10 µm.