Valves are weakly asymmetric, broadly lanceolate to rhombic-elliptic becoming elliptic-circular with smaller cell size (Figs 108–126). The length of the valve is 25.5–43 μm; and the width of the valve is 16–23.5 μm. The axial area is narrow and slightly expanding into the lanceolate and slightly asymmetric central area (Fig. 110), 3.5–5 μm wide. Externally, the longitudinal canal is lanceolate, slightly expanded in the middle of the valve with two-three rows of cribrate (<16 poroids) areolae narrowing into one at the valve apices (Figs 108–125, 126). Internally, a thick non-porous slightly raised silica plate encloses the longitudinal canal (Fig. 130). Externally, the raphe is filiform, curved and positioned within an apically expanded depression; proximal ends simple or weakly bent to the same side of the valve (Figs 126, 128). The distal raphe ends are unilaterally bent to the same side and terminate at the valve face mantle junction (Figs 126, 127). Internally, the raphe is curved with simple proximal and distal ends that are slightly elevated in a depression formed by the longitudinal canal (Figs 129, 131). The striae are parallel at mid-valve becoming radiate towards the apices, 8–10 in 10 μm. Striae are uniseriate throughout (Fig. 126). The striae are composed of large round to rectangular areolae covered with cribra (>45 poroids), 10–15 in 10 μm. Externally, each areola forms a deep pit (Fig. 126). The inter-areolar thickenings have fin-like crest silica ornamentations, which are serrated with ca. 3–5 notched edges (white arrow in Fig. 127). The fin-like ridge crests over the canal are slightly bent into semi-circular shapes, positioned towards the striae whereas those associated with the striae are only slightly bent towards the canal (Figs 126, 128). The areolae increase in size towards the valve margins (Fig. 126). Internally, the alveoli open via a single elongated opening covered with a thin silica layer (Figs 129, 131).

Type:— UNITED REPUBLIC OF TANZANIA, Lake Tanganyika, Mahale National Park, at 782 m elevation; sand, 25 m water depth, collected SCUBA diving, 6°10’25.1” S 29°44’25.2” E, W. Salzburger, 28 th June 2019 (holotype designated here, circled specimen BM-108989! = Fig. 109, isotypes ANSP-GC17218!, CANA-129318!). Type material CANA-129318. Registration: http://phycobank.org/103721

Pictures of the isolated specimen:— LM micrograph on 1000× magnification (Fig. S3x).

Sequence data:— Plastid gene rbc L sequence (GenBank accession: OQ 660278).

Etymology:— The specific epithet ‘ tumida ’ refers the relatively broad width of this species.

Ecology and distribution:— Diploneis tumida sp. nov has only been observed along the Tanzanian and Zambian coasts of Lake Tanganyika. In the alkaline, moderately mineral-rich and highly transparent waters, the species normally inhabits muddy and sandy (sometimes with mollusk shells) substrates between 9 and 33 m water depth in the southern, central, and northern sub-basins. The population size is moderate in Mahale National Park and Ndole Bay, while it is smaller on Mutondwe Island, Isanga Bay, Kalambo Falls Lodge, Kalya Bay, and Kiganza Bay near Gombe National Park (see Fig. 1c–f). The species usually occurs together with D. salzburgeri sp. nov., D. cocquytiana sp. nov., D. serrulata sp. nov., D. gigantea sp. nov., D. kilhamiana sp. nov., D. angusta sp. nov., and D. cristata sp. nov.

Main differential characters:— Valve shape, striae pattern, striae density, and external fin-like ornamentations across the valve.

Similar species:— Diploneis latiuscula sp. nov., D. cristata sp. nov., and D. elliptica.