Neidiopsis baicalensis, Rodionova & Pomazkina & Sherbakova, 2025

Neidiopsis baicalensis sp. nov.

Light microscopy (LM): ( Figs 1–18 View FIGURES 1–18 ). Valves widely lanceolate in large specimens to elliptic-lanceolate in small cells, apices cuneate not protracted, 37–59 µm long, 15–18 µm wide. External axial area narrow at apices gradually expanding into the rhombic-rounded, transversely widened central area. Raphe filiform. External proximal raphe endings straight, distal curved to the same side. Striae slightly radiate, fine, 14–16 in 10 µm. A longitudinal canal along each margin.

Scanning electron microscopy (SEM): (19–35). Externally, striae composed of small open poroids, 20–23 in 10 µm, becoming denser towards apices, 30–35 in 10 µm. Areolae proximal in striae distinct, enlarged, slit-like in the center, rounded at poles, forming a row surrounding the sternum ( Figs 20–23 View FIGURES 19−26 ). Virgae wide. Striae uniseriate, areolae on the surface of canals biseriate return to being uniseriate on the deep mantle ( Figs 20, 22 View FIGURES 19−26 ). Apical valve areas smooth and non-porous ( Figs 22, 23 View FIGURES 19−26 ). Proximal raphe endings simple, drop-shaped, lay in grooves ( Figs 20, 21 View FIGURES 19−26 ). Distal fissures widely curved unilaterally, do not extend on the mantle ( Figs 22, 23 View FIGURES 19−26 ). Cingulum of at least two bands ( Figs 25, 26 View FIGURES 19−26 ). Internally, each stria consists of transversely elongated alveoli interrupted by a narrow hyaline area, continuing as offset rows of shorter alveoli ( Figs 27 View FIGURES 27 –33).Alveoli covered with porous hymens (Figs 28, 31). Paired slit-like foramina at alveolus margins form two rows along the valve (Fig. 32). External pores are visible in a partially damaged valve through alveoli lacking hymens ( Figs 34 View FIGURES 34 , 35). Proximal raphe endings terminate on an elevated, apically oblong central nodule, resembling a double helictoglossa, distal fissures end in small helictoglossae (Figs 28, 29, 31).

Type: RUSSIA. Lake Baikal: Southern basin, near the settlement Mangutie, sand sample collected by a diver at 2 m depth at station 41 (51.6069 °N, 103.91028 °E) on June 22 1998 (holotype: NS1000203 housed at the Central Siberian Botanical Garden, Novosibirsk, Russia, Herbarium of Vascular Plants. Holotype specimen is illustrated in Fig. 6 View FIGURES 1–18 . Isotypes: slides 0140–ВК, 0145–ВК Limnological Institute SB RAS, Irkutsk, Russia, station 40 near the city of Baikalsk).

Habitat and distribution: yet found in Lake Baikal in the type locality.

Etymology: The epithet baicalensis refers to the name of the type locality.

Ecology and associated diatom taxa: Neidiopsis baicalensis inhabits oligotrophic, slightly alkaline waters (pH 7.6–8.4) with low mineral content (less than 100 mg /l), typically found on silty–sand substrates at depths of 2– 20 m. Water temperatures range from 4 °C to 7 °C, with upper layers warming to 14 °C–16 °C during the summer season ( Votintsev 1961, Blinov et al. 2017, Podlesnaya et al. 2021).

At the type locality in the southern basin of Lake Baikal, Neidiopsis baicalensis was uncommon but formed a noticeable part of the benthic diatom assemblage, with less than 3% abundance. The dominant species (each with over 5% abundance) included widely distributed taxa such as Amphora pediculus (Kützing) Grunow in Schmidt (1875: fig. 26: 99), Navicula radiosa Kützing (1844: 91 , fig. 4: 23), Sellaphora bacillum (Ehrenberg) D.G. Mann in Reichardt (2018: 210), and Cocconeis placentula Ehrenberg (1838: 194) , as well as endemic species such as Aneumastus perodon Kulikovskiy, Metzeltin & Lange-Bertalot (2012: 48) , Aneumastus odon Kulikovskiy, Metzeltin & Lange-Bertalot (2012: 47) , Sellaphora baicalobacillum Metzeltin, Kulikovskiy & Lange-Bertalot in Kulikovskiy et al. (2012: 244), Skabitschewskia oestrupii (A. Cleve) Kulikovskiy & Lange-Bertalot in Kulikovskiy et al. (2015: 85), Amphora crispans Pomazkina & Rodionova (2014: 126) , and Amphora baicalopseudoeximia Pomazkina & Rodionova (2014: 124) . Other taxa, each with less than 1% abundance, were rare species with exclusively Baikalian distribution, including Lacustriella rombea Rodionova, Pomazkina & Sherbakova (2022: 174) , L. solita Rodionova, Pomazkina & Sherbakova (2022: 175–178) , Neidium spirale Pomazkina & Sherbakova in Pomazkina et al. (2021: 93–94), N. levii Sherbakova in Pomazkina et al. (2021: 65–66), Diploneis pumicosus Sherbakova, Pomazkina & Rodionova in Rodionova et al. (2019: 99), and D. implicatus Pomazkina, Rodionova & Sherbakova in Rodionova et al. (2019: 96).