Diploneis angusta Jovanovska & Wilson & Hamilton & Stone 2023, sp. nov.

Diploneis angusta sp. nov. (LM Figs 323–343, SEM Figs 344–348) Valves are weakly asymmetric, linear-elliptic with parallel to weakly convex margins and round apices (Figs 323– 345). Valve length is 28–48 μm and width is 11–14.5 μm. The axial area is linear to lanceolate, widening at the center to form a longitudinally elongate and weakly asymmetric central area (Figs 326, 345, 348), 2.3–4 μm wide. Externally, the canal is covered with two rows of cribrate (ca. 10 poroids) areolae narrowing into one at the valve apices (Fig. 345). Internally, a thick non-porous slightly raised silica plate encloses the longitudinal canal (Fig. 346). Externally, the raphe is filiform, curved with expanded proximal ends deflected to one side; the proximal ends are positioned within an elongated depression (Figs 344, 345, 348). The distal raphe ends are unilaterally bent to the same side and terminate on the valve face at the mantle (Fig. 345). Internally, the raphe is curved with simple proximal and distal ends that are slightly elevated in a deep depression formed by the longitudinal canal (Fig. 346). The striae are parallel at mid-valve becoming radiate towards the apices, 10–12 in 10 μm. Striae are uniseriate throughout (white arrow in Fig. 347). The striae are composed of round areolae covered externally with fine pored cribra (>35 poroids), 15–20 in 10 μm. Inter-areolar thickenings at mid-valve (not ends) have scattered fin-like silica ridges serrated with ca. 4–7 notched edges (Fig. 347). The areolae increase in size towards the valve margins (Figs 345, 347). Internally, the alveoli open through a single elongated opening, typically closed by silica, which is corroded in the illustrated specimen (Fig. 346). The valvocopula has serrated advalvar edges (Fig. 346). Type:— REPUBLIC OF ZAMBIA, Lake Tanganyika, Cape Nandu, Kasaba Bay, at 768 m elevation; sand and mud, 20 m water depth, collected with Van Veen grab sampler, 8°30’18.9” S 30°38’32.3” E, E. Jovanovska, 2 nd October 2021 (holotype designated here, circled specimen BM-108993! = Fig. 331, isotypes ANSP-GC17222!, CANA-130010!). Type material CANA-129326. Registration: http://phycobank.org/103698 Pictures of the isolated specimen:— LM micrograph on 1000× magnification (Fig. S2k). Sequence data:— Plastid gene rbc L sequence (GenBank accession: OQ 660295). Etymology:— The specific epithet ‘ angusta ’ refers to the narrow shape of the valve. Ecology and distribution:— This species has only been observed in Lake Tanganyika, which is an alkaline, moderately mineral-rich and very transparent system. The species is widespread in the southern, central, and northern sub-basins on the Tanzanian and Zambian sides, especially in Ndole Bay and Cape Nangu in Kasaba Bay, as well as in Isanga Bay, Kalambo Falls Lodge, Mahale National Park, Kiganza Bay, and the Rukoma area (Fig. 1c–f). At all these sites it occurs on sandy (sometimes sandfish nests) and muddy substrates between 10 and 25 m water depth, typically together with D. serrulata sp. nov., D. cocquytiana sp. nov., D. gigantea sp. nov., D. salzburgeri sp. nov., D. kilhamiana sp. nov., D. disjuncta sp. nov., and D. clara sp. nov. Main differential characters:— Valve shape, striae pattern, narrow canals, recessed areolae over the canals, external fin-like ornamentations across the valve, and poroids>35 per areola. Similar species:— Diploneis clara sp. nov., Diploneis calcifuga Lange-Bertalot & Fuhrmann (2020: 28), and Diploneis altana Kulikovskiy & Lange-Bertalot (2015: 86).

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