Diploneis serrulata sp. nov. (LM Figs 475–505, SEM Figs 506–517)

Valves are weakly asymmetric, rhombic to rhombic–elliptical with bluntly round apices (Figs 475–506). The length of the valve is 17–36 μm; and the width is 10–16 μm. The axial area is narrow and slightly expanding into the lanceolate central area (Figs 477, 506, 509), 1.5–4 μm wide. Externally, the longitudinal canal is lanceolate, slightly expanded in the middle of the valve with four rows of cribrate (<8 poroids) areolae narrowing into one at the valve apices (Figs 506, 509). Internally, a thick non-porous slightly raised silica plate encloses the longitudinal canal (Figs 512, 513). Externally, the raphe is filiform, curved with weakly deflected proximal ends positioned within expanded teardrop depressions (Figs 506, 509, 510). Internally, the raphe is curved with simple proximal and distal ends that are slightly elevated with a small helictoglossa and positioned in a depression formed by the longitudinal canal (Figs 512–515, 517). Striae are uniseriate becoming biseriate towards the valve margins (Figs 507, 508), 11–10(12) in 10 μm. The striae are composed of small round areolae covered with cribra (<10 poroids), 20–25 in 10 μm. The inter-areolar thickenings fin-like ridges serrated with ca. 8–12 notched edges (white arrow in Fig. 511). The fin-like ornamentations on the canal are slightly bent into semi-circular shapes (white arrow in Fig. 509), positioned towards the striae whereas those of the striae are only slightly bent towards the canal, changing opposite direction only at valve mantle (Fig. 506). Internally, the alveoli open via a single elongated opening covered with a thin silica layer (Fig. 516). A fine serrate edge is present aligning with the mantle margin (Figs 513, 514, 516). The valvocopula is serrated (white arrow in Fig. 507).

Type: — REPUBLIC OF ZAMBIA, Lake Tanganyika, Mutondwe Island, at 768 m elevation; sand and shells, 9 m water depth, collected SCUBA diving, 8°43’22.0” S 31°07’24.2” E, A. Indermaur, 9th October 2021 (holotype designated here, circled specimen BM-108979! = Fig. 478, isotypes ANSP-GC17208!, CANA-129328!). Type material CANA-129328. Registration: http://phycobank.org/103717

Pictures of the isolated specimen: —LM micrograph on 1000× magnification (Fig. S3p).

Sequence data: —Plastid gene rbcL sequence (GenBank accession: OQ 660286).

Etymology: —The specific epithet ‘serrulata’ refers to the small, sawtooth like structures on the fin-like ridge structures that ornament the valve.

Ecology and distribution: — Diploneis serrulata sp. nov. has been observed in Lake Tanganyika in the three sub-basins on the Zambian and Tanzanian sides. In the alkaline, moderately mineral-rich and highly transparent waters, the species occurs in relatively large numbers on muddy and sandy substrates (sometimes with mollusk shells) and on submerged rocks and sand fish craters between 7.5 and 33 m water depth. It has also been found on submerged rocks in the littoral areas at Jakobsen Beach and as free-living tychoplanktonic at 60 m water depth in Mahale National Park, probably due to water currents and turbulence. This small species lives almost everywhere in the lake, including Kalambo Falls Lodge, Isanga Bay, Chituta Bay, Mutondwe Island, Ndole Bay, Mahale National Park, Rukoma area, and Kiganza Bay (see Fig. 1c–f), together with different species in different locations, but mostly with D. kilhamiana sp. nov., D. salzburgeri sp. nov., D. cristata sp. nov., D. tessellata sp. nov., D. major sp. nov., and D. duplex sp. nov.

Main differential characters: —Valve shape, striae pattern, canals broadly expanded at mid-valve, external thick fin-like ornamentations with up to 14 notches distributed across the valve.

Similar species: — Diploneis decora sp. nov. and Diploneis lusatica Lange-Bertalot & Hofmann (2016: 163) .