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

Diploneis gigantea sp. nov. (LM Figs 73–78, SEM Figs 79–84) Valves are weakly asymmetric, linear-elliptic with slightly convex margins and round ends (Figs 73–79). Valve length is 67–106.5 μm and valve width is 37.5–46 μm. The axial area is narrow, expanding towards the narrow, lanceolate and weakly asymmetric central area (Figs 73, 80), 6–8 μm wide. Externally, the longitudinal canal is broad, lanceolate to linear, slightly expanded in the middle of the valve with four rows of cribrate areolae (10–15 poroids) narrowing into one at the valve apices (Figs 73–78, 80, 81). Internally, a thick non-porous slightly raised silica plate encloses the longitudinal canal (Fig. 83). Externally, the raphe is filiform, curved with proximal ends positioned within expanded teardrop depressions (Figs 79, 80). The distal raphe ends are unilaterally bent to the same side as the proximal ends and terminate at the valve face margin (Figs 79, 81). 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 82, 83, 84). The striae are parallel at mid-valve becoming radiate towards the valve apices, 7–8 in 10 μm. Striae are uniseriate throughout (Figs 79, 81). The striae are composed of small round to rectangular areolae covered with cribra (>30 poroids), 7–8 in 10 μm. Each areola opens in a depression slightly lower than the non-porous valve surface. The inter-areolar thickenings have crested fin-like ridge silica ornamentations, which are serrated into ca. 3–5 notched edges (in the illustrated specimens hardly visible but see white arrow in Fig. 81). The stria areolae are divided by thickenings, forming from the areolae wall (white arrow in Fig. 81). Areolae increase in size towards the valve margins (Fig. 81). Internally, the alveoli open via a single elongated opening covered with a thin silica layer (Fig. 82). Type:— REPUBLIC OF ZAMBIA, Lake Tanganyika, Kalambo Falls Lodge, at 771 m elevation; mud, 18 m water depth, collected SCUBA diving, 8°37’25.6” S 31°11’59.7” E, H. Buescher, 1 th September 2018 (holotype designated here, circled specimen BM-108994! = Fig. 75, isotypes ANSP-GC17223!, CANA-129338!). Type material CANA-129315. Registration: http://phycobank.org/103709 Pictures of the isolated specimen:— LM micrograph on 1000× magnification (Fig. S2a). Sequence data:— Plastid gene rbc L sequence (GenBank accession: OQ660290). The sequence comes from an isolated individual from the same area, but on sand in 5 m water depth (Acc. No. 91, Diatom Collection Elena Jovanovska; see coordinates in Table S1). However, as this species is extremely rare, we chose a nearby, deeper site as type material, where the species is more abundant but still very rare. Etymology:— The specific epithet ‘ gigantea ’ refers to the very large cell sizes of this species. Ecology and distribution:— This species has only been observed on the coasts of Tanzania and Zambia in the southern and central sub-basins of Lake Tanganyika. In the alkaline, moderately mineral-rich and highly transparent waters, the species usually inhabits sandy and muddy substrates (sometimes with mollusk shells) between 9 and 26 m water depth. It can also be found on submerged rocks below 20 m water depth and below 9 m water depth, where it is very rare. Diploneis gigantea sp. nov. was found mainly in the southern sub-basin at Kalambo Falls Lodge, Isanga Bay, Mutondwe Island, Cape Nangu at Kasaba Bay, Ndole Bay (Fig. 1c, f) and less frequently in the central sub-basin in Kalya Bay and Mahale National Park (Fig. 1c, e). In all these locations the species is extremely rare, typically one to two (very rarely a handful) specimens are found per slide together with D. salzburgeri sp. nov., D. cocquytiana sp. nov., D. elongata sp. nov., D. tessellata sp. nov., D. fossa sp. nov., and D. cristata sp. nov. Main differential characters:— Large valve size, linear-elliptic valve shape, striae density, and external fin-like ornamentations across the valve. Similar species:— Diploneis fossa sp. nov., Diploneis goohon Kulikovskiy & Lange-Bertalot (2015: 95), and Diploneis obtusa Kulikovskiy & Lange-Bertalot (2015: 99).

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