Afrocymbella gibba, 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 : 35-36

Afrocymbella gibba sp. nov. (LM Figs 116–135 , SEM Figs 136–143 )

Description:— Valve length 52.5–93.0 μm, valve breadth 16.5–22.5 μm. Length to breadth ratio: 2.8–4.2. Valves distinctly dorsiventral and heteropolar, semirhomboid-lanceolate ( Figs 116–143 ). Dorsal margin widely arched ( Figs 116–135 ). Ventral margin ranging from slightly to strongly tumescent, typically broad and spanning ½ – 2/3 length of valve, otherwise weakly convex ( Figs 116–135 ). APF of round poroids present at footpole ( Figs 136, 138 ), covered internally by knobby jointed ridge of silica ( Fig. 143 ). Valve apices narrowly rounded, with headpole becoming slightly rostrate in smaller specimens ( Figs 116–135 ); internally with small to indistinguishable pseudosepta ( Figs 142, 143 ). Axial area narrow, curved, slightly ventrally displaced ( Fig. 136 ). Central area large, oval shaped, notably wider and more diamond-shaped on dorsal side ( Fig. 139 ). Prominent single (rarely 2) stigmoid present on dorsal side of central nodule ( Fig. 136 ), externally round to transapically elongated ( Fig. 139 ), while internally elongated slit ( Fig. 141 ). Raphe curved and slightly sinusoidal ( Fig. 136 ). External proximal raphe ends slightly expanded into small pores, dorsally deflected ( Fig. 139 ). External distal raphe terminal fissures ventrally deflected ( Figs 136–138 ). Internal proximal raphe ends covered by nodular or flap-like silica development formed by primary side of valve ( Figs 140, 141 ). Internal distal raphe terminal fissures slightly elevated into small lobed helictoglossa at some distance from valve apices ( Figs 140, 142, 143 ). Striae radiate or radiate-curved, uninterruptedly continuing onto valve mantle ( Fig. 136 ), 13–15 in 10 μm (counted dorsally at central area). External areolae slit-like poroids, appearing blocky in LM, 16–18 in 10 μm. Internal areolae ellipsoid without velum ( Figs 140, 142 ).

Type:— REPUBLIC OF MALAWI. Lake Malawi , Chembe Bay, at 469 m elevation, rock scrape, 14°00’43.4” S 34°51’00.1” E, E.Jovanovska, 22 nd September 2018 (holotype designated here, circled specimen ANS-GC17200! = Fig. 120 , isotypes circled specimens ANS-GC17201!, BM-101981! Cleaned type materials: ANS-GCM 33029.

Registration: https://zoobank.org/NomenclaturalActs/429ab0b2-a77b-4be4-8101-0c5b99bb6b27

Etymology:— The species name ‘ gibba ’ refers to the wide, tumescent shape of the ventral margin that distinguishes it from other similar large Afrocymbella species.

Distribution:— Afrocymbella gibba sp. nov. has been observed in both Lake Malawi and Lake Tanganyika. It was common in samples from Chembe (Cm) and Mangochi (Ma) in the southern sub-basin of Lake Malawi ( Fig. 1 ). We also observed many specimens with similar characteristics to this taxon from the sediments of the core MAL05-1C, although rare in occurrence.

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 116–135. Afrocymbella gibba sp. nov., LM valve views. Figs 116–120, 122, 124–127. Valves from the type material. Fig. 120. Holotype specimen. Scale bar = 10 μm.

FIGURES 136–139. Afrocymbella gibba sp. nov., SEM external valve views. 136. View of the whole valve showing raphe structure, slit-like areolae, and APF on footpole. 137. Detail footpole showing striae structure and distal raphe terminal fissure. 138. Detail of valve apex showing the slit-like areolae and APF of round poroids. 139. Detail of mid-valve showing dorsally deflected proximal raphe ends and slightly elongated stigmoid on the dorsal side of the large diamond-shaped central nodule. Scale bars = 10 μm (Fig. 136), 5 μm (Fig. 139), 2 μm (Figs 137–138).

FIGURES 140–143. Afrocymbella gibba sp. nov., SEM internal valve views. 140. View of the whole valve. 141. Detail of mid-valve showing proximal raphe ends covered by flap-like silica development and antler-like intermissio stigmoid. 142. Distal raphe fissures slightly raised into small helictoglossae. 143. Close view of distal raphe terminal fissure andAPF with knobby jointed ridge of silica. Scale bars = 20 μm (Fig. 140), 2 μm (Figs 141–143).