Eunotia elliotii J.C.Taylor & Cocquyt, 2025

Taylor, Jonathan C. & Cocquyt, Christine, 2025, Eunotia elliotii sp. nov. (Eunotiaceae, Bacillariophyta) from the Republic of the Congo, central Africa, Phytotaxa 705 (1), pp. 97-105 : 99-103

Eunotia elliotii J.C.Taylor & Cocquyt sp. nov. (LM: Figs 2–16 ; SEM: Figs 16–32 )

Description LM: — Valve with two prominent undulations on the dorsal side and slightly concave ventral margin. Poles isopolar, broadly rounded and protracted. Steep slope between the dorsal valve undulation and the pole. Undulations with rather acute peaks. Valve face with irregular surface. Length 26.5–74.5 µm, broadest near the top of the undulation 11.5–16.5 µm, narrowest at the mid-valve between the two undulations 8.5–11.5 µm. Striae coarse mid-valve, around 4–7 in 10 µm, exceptionally restricted to 3 in 10 µm, becoming denser near the poles up to 8–12 in 10 µm. Striae radiate throughout the valve face, except for the middle, where the striae are in most valves perpendicular to the ventral valve margin. Near the dorsal valve margin short striae are present intercalated between the striae that run from the ventral to the dorsal margin. These short striae can become somewhat longer on the top of the two dorsal undulations and extend onto the valve mantle like the regular striae. Striae composed of distinct areolae, about 26–28 in 10 µm. Terminal raphe endings curved onto the valve face, close to the poles and clearly visible in LM. Hyaline area around the terminal raphe endings very narrow and the striae extend almost to the raphe.

SEM:— Frustules with thick silica wall and very irregular surface on the outside of the valves ( Fig. 18 ) and a smooth inside ( Fig. 29 ). Striae located in shallow, irregular depressions or troughs between the raised silica thickening of the valve face ( Fig. 21 ). These depressions are characterized by many irregularly scattered round depressions ( Fig. 22 , arrow). Striae composed of round areolae ( Fig. 22 ). Areolae may be irregularly missing on the striae of both valve face and valve mantle. Externally, the areolae are covered by hymenes lying deep in the cell wall ( Fig. 22 ), which are usually attached at a single point. Terminal nodules narrow and close to the poles. Distal raphe fissures enlarged on the valve face ( Fig. 19 ) becoming narrower deeper in the thickened cell wall terminating in an enlarged end curved away from the poles ( Fig. 24 ). Internally the raphe terminates distally in small helictoglossae near the poles ( Fig. 29 ). Rimoportula absent. Several small round pores possibly penetrate the valve and occur most around the apices and dorsal valve margin ( Fig. 19 , arrow). Raphe fissures on the mantle laying deep in the thick cell wall; the endings are spathulate ( Fig. 28 ). Mantle striae denser beneath the raphe, up to 17 in 10 µm compared to the 12 in 10 µm between the raphe endings, the density of areolae remains the same as on the valve face, about 25 areolae in 10 µm. The surface of the valve mantle is ornamented with irregular silica plaques near the edge towards the valvocopula ( Figs 25–26 , arrow). Striae on the valvocopula about 40 in 10 µm. Beside the valvocopula, up to three other open girdle bands are present per valve, becoming narrower away from the valve. Beside the striation composed of irregularly placed and often lacking areolae, the girdle bands are ornamented with sutures near their edge orientated towards the valve and irregularly scattered silica granules.

Type:— REPUBLIC OF THE CONGO. Kouilou Province : unnamed first-order tributary of the Bondo River , 3.95171° S, 11.7663° E, epilithic sample 16-444, Gina Walsh, Valdie Boukaka and Jean de Dieu Mountou , 3rd August 2016 (holotype SANDC! 16-444, South African National Diatom Collection, South Africa, the valve representing the holotype is here illustrated in Fig. 13 ; isotype BR! 4786, BR! 4787, BR! 4788, BR! 4789, Meise Botanic Garden, Belgium) .

Registration:— http.//phyocbank.org/104058

Etymology:— We name this new Eunotia species for Elliot Walsh, son of Gina Walsh , who collected this species (University of Witwatersrand, South Africa) in recognition of her ongoing research on fishes throughout the region.

Ecology and distribution:— The species was sampled on cobbles in a very shallow clear water creek (<0.2 m deep, ~ 1 m wide) over diverse substrate types consisting of boulders, cobbles and pebbles with Anubias Schott , overhanging vegetation and high riparian forest cover (estimated 80% shade with 0.01 x 10 lux light value). It was collected in fast-flowing stream sections on steep slopes at 207 meters above sea level. The in situ water quality values collected indicate that the site is circum-neutral (pH = 7.85), highly oxygenated (dissolved oxygen = 84.2%) with low conductivity (electrical conductivity = 34.9 µs/cm). Despite sampling similar environments in neighbouring headwater tributaries of the Bondo River, the species was not found, suggesting that Eunotia elliotii is distributed over a small area, and possibly restricted to headwater tributaries of the Noumbi River, which mostly occur outside of the Conkouati-Douli National Park.

Figures

FIGURES 2–16. Eunotia elliotii sp. nov., LM. Valve views showing size diminution series. Valves and frustules from the holotype slide SANDC 16-444, unnamed tributary of the Bondo River, Kouilou Province, Republic of the Congo. 13. Valve representing the holotype. Scale bar: 10 µm.

FIGURES 17–22. Eunotia elliotii sp. nov. from type material SANDC 16-444, SEM external views. 17–18. Overview of an entire valve. 19. Details of the pole showing isolated perforations on the valve face (arrow). 20. Enlarged terminal endings of the raphe. 21–22. Detail of the valve face showing uneven thickened silica ridges between the striae. Note the striae sunken between ridges and surrounded by numerous round depressions in the valve surface (arrow). Note the recessed hymenes anchoured at a single point. Scale bar: 10 µm (Figs 17, 18), 1 µm (Figs 19–22).

FIGURES 23–28. Eunotia elliotii sp. nov. from type material SANDC 16-444, SEM external views. 23. Overview of an entire frustule, dorsal side. The mantle at the peak of the undulations has a concave step (arrow) which is separated by a narrow hyaline area from the vertical abvalvar section of the valve. 24. Overview of an entire frustule, ventral side, showing proximal raphe endings in spathulate grooves. 25–28. Detail of the mantle and copulae showing the raphe with enlarged spathulate proximal endings. 25. Silica plaques (arrow) at the edge of the mantle adjoining the valvocopula. Note also the scattered silica nodules on the copulae. Scale bar: 10 µm (Fig. 24), 5 µm (Fig. 23), 2 µm (Figs 25–28).

FIGURES 29–32. Eunotia elliotii sp. nov. from type material SANDC 16-444, SEM internal views. 29. Overview of an entire valve. 30–32. Detail of the interior of the valve showing the unoccluded areolae. 30. Note the thickened terminal nodule (arrow). 31. Illustrates the thickened cell wall. Scale bar: 5 µm (Fig. 29), 2 µm (Figs 30, 32), 1 µm (Fig. 31).