Planothidium paisiusii, Tseplik & Glushchenko & Maltsev & Genkal & Iurmanov & Kulikovskiy, 2025

Planothidium paisiusii sp. nov.

According to morphological and molecular data, the new species belongs to the genus Planothidium , characterized by a cavum on the rapheless valve, multiseriate striae and straight raphe branches with distal ends turned to one side and extended onto the mantle. The new species can be compared to such known species as P. straubianum Wetzel, Van de Vijver & Ector in Wetzel et al. (2019: 60), P. aueri (Krasske) Lange-Bertalot (1999: 275) , and P. curtistriatum Wetzel, Van de Vijver & Ector in Wetzel et al. (2019: 61). P. straubianum ( Wetzel et al. 2019) is the most similar to the new species; it can be differentiated by a more linear-elliptic valve outline and a small narrow cavum. The valve dimensions of the two species overlap, but the valves of P. straubianum are generally wider and have a higher maximum length as well (see Table 4). The axial area on the rapheless valve is linear in P. straubianum , more distinctly widened towards the centre in P. paisiusii sp. nov.; the striae on the rapheless valve in P. straubianum consist of 4–5 rows of areolae vs. 3–4 rows in P. paisiusii sp. nov. The outside of the rapheless valve in P. straubianum is ornamented with rounded depressions, while in P. paisiusii sp. nov. there is no ornamentation present.

P. curtistriatum ( Wetzel et al. 2019) is also a small-celled species, however, it has a distinct large axial area on the rapheless valve that distinguishes it from other Planothidium species including P. paisiusii sp. nov. Other differentiating features between P. curtistriatum and the new species are the valve outline which is more lanceolate in P. curtistriatum , coarser striae (11–14 in P. curtistriatum vs. 15–16 on the raphe valve and 14–18 on the rapheless valve in P. paisiusii sp. nov.), and the ornamentation on the rapheless valve (rounded depressions in P. curtistriatum , absent in P. paisiusii sp. nov.).

P. paisiusii sp. nov. can be compared to smaller valves of P. aueri ( Lange-Bertalot et al. 1996; Van de Vijver et al. 2002); there is resemblance in the cavum shape and the structure of the axial area on the rapheless valve. However, the valve outline of P. aueri is different from the new species and the quantitative features such as valve length and width and striae density also allow to differentiate these species (see Table 4): the valves of P. aueri are larger than P. paisiusii sp. nov. and the striae density is higher.

According to molecular data, P. paisiusii sp. nov. forms a separate branch in a subclade that includes several other species of Planothidium and is part of a larger clade containing Planothidium species. Mostly this subclade is formed by strains of P. victori ; the identity of other strains in this subclade was previously discussed in Tseplik et al. (2024b). Our data supports the idea of P. victori actually representing a complex of several cryptic species. Morphologically, P. victori can be differentiated from the new species by its protracted ends, large wide cavum, linear axial area on the rapheless valve, and the ornamentation on the rapheless valve in the form of small round depressions; the quantitative features overlap between the two species, however, the valves of P. paisiusii sp. nov. are generally smaller (maximum length 11.7 μm vs. 19.5 μm in P. victori ( Wetzel et al. 2019)) . Planothidium also contains cryptic diversity and several complicated species groups ( Jahn et al. 2017; Tseplik et al. 2024 a, b), and accumulation of DNA sequences of new strains is an important asset in resolving the taxonomy of this genus.

Our study is one of the first molecular studies of diatoms in Africa. Establishing a library of strains from different locations is a very important step in diatom research, and data like this is valuable for future studies in taxonomy, biodiversity, and biomonitoring.