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

Diploneis salzburgeri Jovanovska sp. nov. (LM Figs 2–11, SEM Figs 12–23) Valves are weakly asymmetric, broadly lanceolate to weakly rhombic-elliptic becoming circular with smaller cell size (Figs 2–11). Valve length is 53.5–100.5 μm and valve width is 34.5–54.5 μm. The axial area is narrow and only slightly expanded close to the central area (Fig. 14); patterned with irregular round ornamentations opening into small depressions that do not penetrate the silica cell wall (white arrow Fig. 17). The central area is longitudinally elongate, 5.5–8.5 μm wide. Externally, the longitudinal canal is broad, lanceolate to linear, slightly expanded in the middle of the valve with four (rarely five to six) rows of cribrate areolae (>20 poroids) narrowing into two to one at the valve apices (Figs 12–14, 16). Internally, a thick non-porous slightly raised silica plate encloses the longitudinal canals (Figs 18, 21). Externally, the raphe is filiform, curved; the proximal ends are simple and weakly curved to one side, and positioned within an expanded teardrop depression (Figs 13, 14, 17). The distal raphe ends are unilaterally bent to the same side as the proximal ends and terminate at the junction of the valve face and mantle (Figs 12, 13, 16). 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 18–20, 23). The striae are parallel at mid-valve becoming radiate towards the valve apices, 8–10 in 10 μm. Striae are uniseriate throughout (Fig. 15). Externally, the striae are composed of small complex round to rectangular areolae covered with cribra (>20 poroids), 5–8 in 10 μm. Each stria and canal areola opens into a depression slightly lower than the rest of the non-porous valve surface, divided (typically in four) by narrow thickened bars that bear small fin-like silica ridges (white arrowed Fig. 15). The stria areolae are also divided by robust thickenings that form from the areolae walls (white arrowed Fig. 16). A few areolae are covered partially or entirely by thin flaps of silica, forming from the areolae walls (black arrowed Fig. 16). The inter-areolar thickenings bear transapical and longitudinal ridge-like shaped silica ornamentations serrated into ca. 4–7 notched edges (white arrowed Fig. 14). The areolae increase in size towards the valve margins (Figs 12, 15). Internally, the alveoli open via a single elongated opening covered with a thin silica layer (Figs 18, 22). Type:— REPUBLIC OF ZAMBIA, Lake Tanganyika, Ndole Bay, at 768 m elevation; mud, 12 m water depth, collected SCUBA diving, 8°28’34.7” S 30°27’06.7” E, W. Salzburger, 30 th September 2021 (holotype designated here, circled specimen BM-108978! = Fig. 7, isotypes ANSP-GC17207!, CANA-129323!). Type material CANA-129323. Registration: http://phycobank.org/103716 Pictures of the isolated specimen:— LM micrograph on 1000× magnification (Fig. S2b). Sequence data:— Plastid gene rbc L sequence (GenBank accession: OQ 660289) and nuclear encoded 18S (SSU rDNA) sequence (GenBank accession: OQ 629559). Etymology:— The specific epithet ‘ salzburgeri ’ was given in honor of Prof.Walter Salzburger, who has contributed significantly to the understanding of cichlid fish evolution in Lake Tanganyika and who collected the type material. Ecology and distribution:— This species has been observed in Lake Tanganyika along the coasts of Zambia and Tanzania (including the coast of Burundi; Cocquyt 1998, fig. 14: 2). It typically inhabits the sandy and muddy stretches between 6 and 33 m water depth in the southern, central, and northern parts of this alkaline lake with moderate mineral content and high-water transparency. It can also be found free-living (i.e. tychoplanktonic) or on submerged rocks, fishing nets, and other objects probably due to water currents and turbulence. Considering the robust and large size of Diploneis salzburgeri sp. nov., the species is not very abundant, but it is widespread and usually occurs together with D. cristata sp. nov., D. gigantea sp. nov., D. fossa sp. nov., D. major sp. nov., D. kilhamiana sp. nov., D. tessellata sp. nov., D. tanganyikae sp. nov., D. serrulata sp. nov., and D. cocquytiana sp. nov. in Isanga Bay, Chituta Bay, Mutondwe Island, Kalambo Falls Lodge, Ndole Bay, Cape Nangu at Kasaba Bay, Kalya Bay, Jakobsen Beach near Kigoma, Buhingu Island, and Mahale National Park (see Fig. 1c–f). Main differential characters:— Valve size and shape, external fin-like ornamentations across the valve, patterned axial area, areolae with fin-like extensions, and poroids>20 per areola. Similar species:— Diploneis tanganyikae sp. nov., Diploneis cristata sp. nov., Diploneis major sp. nov., and Diploneis erwin-reichardtii Lange-Bertalot, Fuhrmann & Werum (2020: 43). Taxonomic note:— Diploneis salzburgeri sp. nov. is very variable in the shape of the valves. Individuals ranging from narrowly elliptical to widely elliptical or even rhombic can be observed within and between populations. The nature of these different shapes and the causes of shape variation remain to be investigated, preferably by wholegenome sequencing. However, since our genetic data based on two markers and the morphometric data do not show a clear separation, we have considered these variations as conspecific. Morphological variations of this nature have also been observed in other robust Diploneis species, such as Diploneis hoevsgoelensis Jovanovska, Levkov & Edlund (2015: 206) (Jovanovska et al. 2015) and Diploneis alpina Meister (1912: 103) (Jovanovska et al. 2013).

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