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

Diploneis tanganyikae sp. nov. (LM Figs 24–35, SEM Figs 36–46) Valves are weakly asymmetric, rhombic to rhombic-elliptic becoming elliptic-circular with smaller cell size (Figs 24– 35). Valve length is 43–75 μm and valve width is 26.5–40.5 μm. The axial area is narrow, lanceolate, widening at the center to form a longitudinally elongate and weakly asymmetric central area (Figs 36–38), 4.5–7 μm wide. Externally, the longitudinal canal is broad, lanceolate to linear, slightly expanded in the middle of the valve with three to five rows of cribrate (ca. 8–10 poroids) areolae narrowing into one at the valve apices (Fig. 36). Internally, a thick non-porous slightly raised silica plate encloses the longitudinal canal (Figs 41, 42). Externally, the raphe is filiform, curved; the proximal ends are weakly deflected and positioned within a teardrop depression (Figs 36, 38). The distal raphe ends are unilaterally bent to the same side and terminate at the junction of the valve face and mantle (Figs 36, 39). Internally, the raphe is curved with simple proximal and distal ends that are slightly elevated in a deep depression formed by the longitudinal canal (Figs 41, 43, 45). The striae are parallel at mid-valve becoming radiate towards the valve apices, 8–9 in 10 μm. Striae are uniseriate throughout (Fig. 40). The striae are composed of large round to rectangular areolae covered externally with cribra (ca. 15–30 poroids; white arrow in Fig. 40), 6–8 in 10 μm. Each areola opens into deep pits (Figs 36, 40). The stria areolae are divided by thick fin-like ridge thickenings that form from the areolae walls and bear silica ornamentations (white arrow in Fig. 39). The longitudinal inter-areolar crested fins have serrated ridge-like shaped silica ornamentations (ca. 3–9 notched edges – hardly visible on the illustrated specimens possibly because of corrosion; Fig. 40). The areolae increase in size towards the valve margins (Figs 37, 40). Internally, the alveoli open via a single elongated opening covered with a thin silica layer (Figs 44, 46). Type:— REPUBLIC OF ZAMBIA, Lake Tanganyika, Ndole Bay, at 768 m elevation; mud, 12 m water depth, 8°28’34.7” S 30°27’06.7” E, collected SCUBA diving, A. Indermaur, 30 th September 2021 (holotype designated here, circled specimen BM-108997! = Fig. 34, isotypes ANSP-GC17226!, CANA-129330!). Type material CANA-129330. Registration: http://phycobank.org/103718 Pictures of the isolated specimen:— LM micrograph on 1000× magnification (Fig. S2f). Sequence data:— Plastid gene rbc L sequence (GenBank accession: OQ660285). The sequence comes from an individual isolated from submerged wood of a palm tree in 0.1 m water depth (Acc. No. 532, Diatom Collection Elena Jovanovska; see coordinates in Table S1). As this is not the original habitat of this species and it probably occurred there due to water currents and turbulence, we selected the nearest suitable site as type material for this species. Etymology:— The specific epithet ‘ tanganyikae ’ is given in honor of Lake Tanganyika, where the species has been observed occurring and from which the type material was collected. Ecology and distribution:— Diploneis tanganyikae sp. nov. has been observed along the Tanzanian and Zambian coastlines of Lake Tanganyika, an alkaline system with moderate mineral content and high-water transparency. It is not very dominant in the muddy and sandy substrates between 9 and 33 m water depth, where it normally occurs. Higher species densities were found in the northern and central sub-basins (Fig. 1c–e), especially in Mahale National Park, Kiganza Bay near Gombe National Park, and Jakobsen Beach near Kigoma (also as tychoplankton at this site). In these bays, D. tanganyikae sp. nov. mostly coexists with D. salzburgeri sp. nov., D. kilhamiana sp. nov., D. cocquytiana sp. nov., D. tumida sp. nov., D. serrulata sp. nov., and D. decora sp. nov. Towards the south, the population density of the species gradually decreases, reaching only a few individuals in Ndole Bay, Cape Nangu at Kasaba Bay, Mutondwe Island, Chituta Bay, and Kalambo Falls Lodge (see Fig. 1c, f). Main differential characters:— Valve size and shape, canal width, striae density, external thick fin-like ornamentations across the valve, and fin-like ridges around areolae. Similar species:— Diploneis cristata sp. nov., D. salzburgeri sp. nov., and D. hoevsgoelensis. Taxonomic note:— Diploneis tanganyikae sp. nov. is morphologically variable in terms of the shape and width of the canal. It usually varies from broadly lanceolate to narrower linear. Since our current genetic and morphometric data do not reveal any differences between the two forms, we consider them conspecific, similar to what we found in D. salzburgeri sp. nov.

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