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

Diploneis major sp. nov. (LM Figs 61–66, SEM Figs 67–72) Valves are weakly asymmetric, broadly elliptic to rhombic-elliptic becoming elliptic-circular with smaller cell size (Figs 61–67). Valve length is 30–62.5 μm and valve width is 21.5–37 μm. The axial area is narrow, linear to lanceolate, expanding into the longitudinally elongate and weakly asymmetric central area (Figs 61, 68), 3.7–6.5 μm wide. Externally, the longitudinal canal is broad, lanceolate to linear, slightly expanded in the middle of the valve with three to four rows of cribrate (8–12 poroids) areolae narrowing into two to one at the valve apices (Figs 67, 68). Internally, a thick non-porous slightly raised silica plate encloses the longitudinal canal (Fig. 70). Externally, the raphe is filiform, curved with proximal fissures positioned within shallow teardrop depressions; the fine terminal fissures are bent to the same side of the valve and terminate on the valve face at the mantle (Figs 67–69). 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 70–72). The striae are parallel at mid-valve becoming radiate towards the valve apices, 8–9 in 10 μm. Striae are uniseriate throughout (Fig. 67). The striae are composed of small complex round to rectangular areolae covered with cribra (>40 poroids), 9–10 in 10 μm (Fig. 68). The cribra has a volae-like shape that could be a real structure or the result of corrosion (white arrow in Fig. 68). Each areola opens into deep pits and it is divided by narrow thickened bars that bear small crested fin-like ridge silica ornamentations (Fig. 67). The crested fin-shaped silica ornamentations are serrated into ca. 3–5 notched edges (Fig. 68). The areolae increase in size towards the valve margins (Fig. 67). Internally, the alveoli open via a single elongated opening covered with a thin silica perforated layer (white arrow in Fig. 72). Type:— REPUBLIC OF ZAMBIA, Lake Tanganyika, Isanga Bay, at 768 m elevation; rock scrape, 20 m water depth, collected SCUBA diving, 8°39’10.7” S 31°11’34.8” E, A. Jordan, 8 th October 2021 (holotype designated here, circled specimen BM 108983! = Fig. 65, isotypes ANSP-GC17212!, CANA-129329!). Type material CANA-129329. Registration: http://phycobank.org/103714 Pictures of the isolated specimen:— LM micrograph on 1000× magnification (Fig. S2d). Sequence data:— Plastid gene rbc L sequence (GenBank accession: OQ 660283). Etymology:— The specific epithet ‘ major ’ is refers to the species great size. Ecology and distribution:— This species has only been observed on the Tanzanian and Zambian coasts of Lake Tanganyika. It is a rather rare and not very frequent species in the alkaline, moderately mineral waters, occurring mainly in the southern sub-basin on sand at 15 to 36 m water depth and on sand craters formed by cichlids and rarely on mud at more than 30 m depth. We found it particularly in Isanga Bay, Chituta Bay, Mutondwe Island, and at Cape Nangu in Kasaba Bay (Fig. 1c, f). Fewer individuals were also found in the central sub-basin in Kalya Bay and Mahale National Park (Fig. 1c, e). In both sub-basins, the species is mostly found together with D. fossa sp. nov., D. salzburgeri sp. nov., D. tumida sp. nov., D. gigantea sp. nov., and D. serrulata sp. nov. Main differential characters:— Valve shape, external fin-like ornamentations across the valve, fin-like formations within areolae, cribra with volae-like shape, and highly recessed areolae over the longitudinal canals. Similar species:— Diploneis salzburgeri sp. nov. and Diploneis baicaloparma Kulikovskiy & Lange-Bertalot (2015: 89).

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