Afrocymbella symmetrica, Stone & Wilson & Jovanovska, 2022

Stone, Jeffery R., Wilson, Mallory C. & Jovanovska, Elena, 2022, The genus Afrocymbella (Bacillariophyceae) from lakes Malawi and Tanganyika, with description of new fossil and extant species, Phytotaxa 556 (1), pp. 21-48 : 25

Afrocymbella symmetrica sp. nov. (LM Figs 2–23 , SEM Figs 24–31 )

Description:— Valve length 34–49 μm, valve breadth 7.5–9.5 μm. Length to breadth ratio: 5.8. Valves distinctly dorsiventral, slightly heteropolar, semirhomboid-lanceolate ( Figs 2–31 ). Dorsal margin distinctly arched; ventral margin with wide gibbous center portion, typically 1/3 length of valve, otherwise weakly convex ( Figs 24, 26 ). Valve apices acutely rounded and slightly protracted, with headpole and footpole nearly symmetrical, often only differentiated by APF and slightly greater breadth in footpole ( Figs 25, 26 ). Internally, each column of the APF poroids partially surrounded by knobby jointed ridge of silica ( Fig. 29 ). From inside, valve apices with small to indistinguishable pseudosepta ( Figs 29, 31 ). Axial area narrow, curved, ventrally displaced ( Fig. 24 ). Central area small, asymmetric and diamond-shaped, notably wider on dorsal side ( Figs 2–24 ). Distinct single stigmoid present on dorsal side of central nodule; externally slightly elongated ( Fig. 27 ), internally forming antler-like intermissio ( Fig. 30 ). Raphe slightly sinusoidal ( Figs 24, 25 ). External proximal raphe ends slightly expanded into small pores, deflected towards the dorsal (primary) margin ( Fig. 27 ). External distal raphe terminal fissures ventrally deflected ( Figs 25, 26 ). Internal proximal raphe ends covered by nodular or flap-like development from primary side of valve ( Figs 28, 30 ). Internal distal raphe terminal fissures bisect the APF and terminate into small lobed helictoglossa at some distance from valve apices (white arrow in Fig. 29 ). Striae almost parallel in the middle, strongly radiate towards the apices, and continue uninterrupted onto valve mantle. Stria density in middle portion 15–17 in 10 μm dorsally, often slightly coarser ventrally (15–16 in 10 μm) and increasing slightly (17–18 in 10 μm) towards valve apices. Areolae elongate slits, becoming smaller and nearly circular at edge of valve mantle, 26–30 in 10 μm. Internally, areolae ellipsoid without velum ( Figs 28, 30, 31 ).

Type:— UNITED REPUBLIC OF TANZANIA. Lake Tanganyika central basin, core NPO4-KH1-1K, Drive 4; mud, sampled at 43–44 cm (estimated to be ~ 50 ka), 6°35’27.0” S 30°06’00.0” E, J. R.Stone, 30th March 2019 (holotype designated here, circled specimen ANS-GC17192! = Fig. 9 , isotypes circled specimens ANS-GC17193!, BM-101977!). Cleaned type materials: ANS-GCM 33025 .

Registration:https://zoobank.org/NomenclaturalActs/2d7a7068-a78b-425f-a9e6-5d41e121a43b

Etymology:— The specific epithet ‘ symmetrica ’ refers to the symmetrical outline that characterizes this species.

Distribution:— Afrocymbella symmetrica sp. nov. was first observed in sediment assemblages from Lake Tanganyika, dating back to about 60–50 ka from the core NPO4-KH1-1K (4K, Fig. 1 ) and was not observed in the more recent sediments from this core. Initially, we thought it may have been putatively extinct, but have subsequently observed living specimen from modern samples in Tanzania collected in shallow water near Kigoma.

Figures

FIGURE 1. Map of the lakes Tanganyika and Malawi, showing the sampling locations of modern (red circles) and fossil (cores MAL05- 1C and NPO4-KH1-1K; white circles) specimens used in this study.

FIGURES 2–23. Afrocymbella symmetrica sp. nov., LM valve views. 2–19, 23.Valves from the type material, core NPO4-KH1-1K, Drive 4, 43–44 cm sample (~50 ka). Fig 9. Holotype specimen. 20–22. Valves from modern sample. Scale bar = 10 μm.

FIGURES 24–27. Afrocymbella symmetrica sp. nov., SEM external valve views. 24, 26. View of the whole valve showing raphe structure, striae on valve mantle, APF on footpole, and dorsally positioned stigmoid. 25. Detail of valve apex showing the striae structure and APF. 27. Detail of mid-valve showing dorsally deflected proximal raphe ends, small diamond-shaped central area, and slightly elongated stigmoid. Scale bars = 5 μm (Figs 24, 26), 2 μm (Fig. 27), 1 μm (Fig. 25).

FIGURES 28–31. Afrocymbella symmetrica sp. nov., SEM internal valve views. 28. View of the whole valve. 29. Detail of valve ends showing APF with knobby jointed ridge of silica. 30. Detail of mid-valve showing internal openings of areolae without velum, proximal raphe ends covered by flap-like silica development, and antler-like intermissio stigmoid. 31. Close view of the bisected distal raphe terminal fissures, terminating into small lobed helictoglossa (black arrow). Scale bars = 10 μm (Fig. 28), 2 μm (Figs 29–31).