Valves are weakly asymmetric, elliptical-lanceolate with convex margins and round apices (Figs 258–288). Valve length is 15–27 μm and width is 8.5–11.5 μm. The axial area is narrow, linear to lanceolate, slightly widening into a longitudinally elongate and weakly asymmetric central area (Figs 263, 288, 289), 1.3–2.7 μm wide. Externally, the canal is linear to lanceolate, slightly expanded in the middle of the valve with two rows of cribrate (6–10 poroids) areolae narrowing into one at the valve apices (Figs 263, 288, 293). Internally, a thick non-porous slightly raised silica plate encloses the longitudinal canal (Figs 294, 297). Externally, the raphe is filiform, curved with strongly deflected and expanded proximal ends. The proximal raphe ends are in linear expanded teardrop depressions (Figs 288, 289). The distal raphe ends are unilaterally bent to the same side and terminate on the upper mantle (Figs 288, 290, 291). Internally, the raphe is curved with simple proximal and distal ends that are slightly elevated in a depression formed by the longitudinal canal (Figs 294, 295, 298). The striae are parallel at mid-valve becoming radiate towards the apices, 10–13(14) in 10 μm. Striae are biseriate throughout (white arrow in Figs 292, 296). The striae are composed of round areolae covered externally with cribra (>15 poroids), 20 in 10 μm. The inter-areolar thickenings have fin-like silica ridges serrated with ca. 5–10 notched edges. The areolae increase in size towards the valve margins (Fig. 291). Internally, the alveoli open via a single elongated opening covered with a thin silica layer (Figs 295, 298). The valvocopula has serrated advalvar edges (Figs 294, 298).

Type:— REPUBLIC OF ZAMBIA, Lake Tanganyika, Kalambo Falls Lodge, at 770 m elevation; mud, 18 m water depth, collected SCUBA diving, 8°37’25.6” S 31°11’59.7” E, H. Büscher, 1 st September 2018 (holotype designated here, circled specimen BM-108987! = Fig. 269, isotypes ANSP-GC17216!, CANA-129337!). Type material CANA-129315. Registration: http://phycobank.org/103706

Pictures of the isolated specimen:— LM micrograph on 1000× magnification (Fig. S3 aa).

Sequence data:— Plastid gene rbc L sequence (GenBank accession: OQ 660302).

Etymology:— The specific epithet ‘ duplex’ refers to the extended biseriates formation of the striae.

Ecology and distribution:— Diploneis duplex sp. nov. has only been observed in the southern and central sub-basins of Lake Tanganyika in Tanzania and Zambia (including Burundi, see Cocquyt 1998, fig. 14: 5). In the alkaline, moderately mineral-rich and highly transparent waters, this rather small and common species lives on sand and sandfish nests (rarely on mud) or on submerged rocks between 15 and 33 m water depth. Well-established populations were found at Kalambo Falls Lodge and Mahale National Park, with some smaller populations in Isanga Bay and Chituta Bay (Fig. 1c, 3, f). The species typically occurs together with D. major sp. nov., D. salzburgeri sp. nov., D. serrulata sp. nov., D. gigantea sp. nov., D. fossa sp. nov., D. disjuncta sp. nov., and D. kilhamiana sp. nov.

Main differential characters:— Valve size, striae pattern, narrow canals, and external fin-like ornamentations across the valve.

Similar species:— Diploneis disjuncta sp. nov., Diploneis navahoarum Lange-Bertalot, Fuhrmann & Werum (2020: 88), and Diploneis puellafallax Lange-Bertalot & Fuhrmann (2016: 174).