Stephanoarcus, Genkal & Glushchenko & Tseplik & Kulikovskiy, 2025

Genkal, Sergei, Glushchenko, Anton, Tseplik, Natalia & Kulikovskiy, Maxim, 2025, A reinvestigation of Stephanodiscus triporus (Stephanodiscaceae, Mediophyceae) with the description of Stephanoarcus gen. nov., Phytotaxa 712 (1), pp. 1-17 : 10

Stephanoarcus gen. nov.

In the study that described the genus Praestephanos ( Tuji et al. 2014) , the authors identified the position of rimoportulae and marginal fultoportulae, as well as the pattern of areolae change, as defining characteristics for this genus that separate it from Cyclostephanos and Stephanodiscus . According to Tuji et al. (2014), in species of Praestephanos the rimoportulae are positioned on the same level as the spines, and the marginal fultoportulae and the change in pattern of areolae are close to the valve edge, below the rimoportulae. The authors transferred 3 species to this genus, however, they also show that P. triporus actually differs from the other two species of Praestephanos ( Tuji et al. 2014, Fig. 3 ): in P. triporus the marginal fultoportulae are located at the very edge of the valve, and in P. suzukii and P. carconensis they are located approximately at 1/3 of the height of the valve mantle from the edge, with areolae present on the mantle lower than the marginal fultoportulae. This can also be seen in Praestephanos tokaiensis Kojima, K. Hattori, Ohtsuka & Saito-Kato in Kojima et al. (2024: 294) and Praestephanos miensis Kojima & Saito-Kato in Kojima et al. (2024: 299), in which a row of areolae can be seen below the level of marginal fultoportulae ( Kojima et al. 2024). Moreover, P. triporus differs from other Praestephanos species by other features as well, such as different valve face relief, fascicles consisting of only 2–3 rows of areolae vs. 4–12 in other species, and the external openings of marginal fultoportulae forming arch-like structures and not tubes. Due to these differences, we are suggesting separating P. triporus , together with several similar species, into a new genus Stephanoarcus gen. nov., defined by the presence of arch-like structures (vestibules) around the marginal fultoportulae, the fascicles continuing from the valve face to the valve mantle without changes in areolae structure, and the marginal fultoportulae being positioned at the very edge of the valve mantle. A schematic representation of morphological structures on the valve mantle in Stephanoarcus gen. nov. is given in Fig. 37 . Of the known Stephanodiscus species, S. vestibulis , S. lacustris and S. hashiensis also possess these characteristics, thus they are transferred into the new genus as well.

The arch-like structures around the marginal fultoportulae are quite diverse in their morphology (see schemes in Fig. 38 ). For example, in the type material of S. triporus arches with both wide and narrow openings were observed, sometimes with sector-shaped flaps visible on the sides of the arches ( Kiss et al. 2013, Figs 8–9 , 16 ); the same variability is shown in other studies (e.g., Tuji et al. 2014, Fig. 2C ; Houk et al. 2014, Pl. 155 Figs 7–10 ; Genkal et al. 2020, Table 63: 1–6 and others). In our material, we observed arches with varying width ( Figs 1, 3 ). S. vestibulis is also a widely known species; the type population possesses medium width arches with thickened edges ( Håkansson et al. 1986, Fig. 6 , 8 ), and also rounded ( Gotoh et al. 1998, Figs 2–9 ) or triangular (this study, Fig. 22 ) flaps were observed. The more recently described S. lacustris and S. hashiensis are characterised by arches with rounded ( Houk et al. 2014, Plate 150 Fig. 2 ) and sector-shaped ( Houk et al. 2014, Plate 151 Figs 11–13 ) flaps respectively, with very narrow arches being typical for S. hashiensis . Given the high variability of this feature, the defining characteristic of Stephanoarcus gen. nov. is the presence of the arch-like structure, not its shape. However, in future studies it would be very beneficial to research the patterns of this variability by observing populations from different geographic locations and cells in different stages of the life cycle, as well as using molecular genetic methods to determine the taxonomic significance of this feature. Molecular data obtained by Tuji et al. (2014) showed the close relationship between S. triporus and P. suzukii , however, this conclusion is not supported by morphological analysis, and the issue requires further research.

Figures

FIGURE 37. Schematic representation of the positioning of morphological features on the valve mantle in Stephanoarcus gen. nov. and close genera. FP—fultoportulae, RP—rimoportulae.

FIGURE 38. Variability of the arch-like structure in species of Stephanoarcus gen. nov. (schematic representation).

FIGURES 1–6. Stephanoarcus triporus. Scanning electron microscopy. 1–3. External views. 4–6. Internal views. Arch-like structures on valve mantle marked by white arrowheads. Scale bars 1 μm.

FIGURES 7–12. Stephanoarcus vestibulis. Scanning electron microscopy, external views. Scale bars 1 μm.

FIGURES 13–18. Stephanoarcus vestibulis. Scanning electron microscopy. 13, 14. External views. 15–18. Internal views. Arch-like structures on valve mantle marked by white arrowheads. Scale bars 1 μm (13–15, 17, 18), 0.5 μm (16).

FIGURES 19–24. Stephanoarcus vestibulis. Scanning electron microscopy, external views. Arch-like structures on valve mantle marked by white arrowheads. Scale bars 1 μm.