Mallomonas cetosa Sterlyagova, Ignatenko, Nguyen Thi Lan & E.S. Gusev sp. nov. (Figs 1–18)

Three types of scales have been observed: apical domed collar scales, domeless body scales and small rear scales with spines. The body scales are wide-rhomboidal, 3.6–5.0 × 2.2–3.4 μm. The shield and anterior flange are ornamented with numerous papillae. The anterior flange is raised above the shield and typically has 2–4 rows of papillae along its edge on each side. The shield is patterned with densely spaced papillae and has inner reticulation of the secondary siliceous layer with polygonal or rounded meshes. There is a small area in the proximal part of the shield at the base of the posterior submarginal rib that lacks a secondary siliceous layer. This area contains several rimmed pores on the base plate, usually less than 10. The posterior submarginal rib is well developed, with inner struts. At the distal end of each arm of the posterior submarginal rib, another short rib may develop on the outer side. The posterior rim is narrow and smooth. The posterior flange has numerous transverse ribs between the posterior submarginal rib and the posterior rim. Rarely, these ribs can be poorly developed, but they are always visible. The collar scales are asymmetrical, elongate and have a broadly rounded proximal end and narrower distal end, 4.1–5.4 × 2.2–3.3 μm. Ornamentation on the shield and flanges is the same as that found on the body scales. The posterior submarginal rib on the “dorsal” side is well developed, connected with the dome. There are several rimmed base plate pores in the angle of the posterior submarginal rib. An additional short submarginal rib (anterior) departs from the dome on the “ventral” side. The posterior flange has numerous transverse ribs. The dome on the collar scales is ornamented with papillae, has a sharply pointed peak and serration at the apical part. Behind the distal part of the dome there is an expansion of the basal plate. The rear scales are 2.4–3.3 × 1.4–2.1 μm, similar in structure and outline to the body scales, but more asymmetrical, and each scale bears a spine. Bristles and cysts are not observed.

Holotype: — Portion of a single gathering of cells on SEM stub number 68_I_1 deposited at the Herbarium of the Steppe Institute of the Ural Branch of the Russian Academy of Sciences, Orenburg (ORIS). Material from a pond in Phu Yen Province, Vietnam. Figure 3 is a representative scale from the type specimen.

Type locality:— Vietnam: pond in Phu Yen Province, Dong Hoa District, Latitude/Longitude: N12°57.287’ E109°20.546’, 12 August 2024. Leg. E.S. Gusev.

Etymology: —The epithet cetosa (Latin for ‘whale’) was chosen due to the resemblance between the ribbed pattern on the posterior flange and baleen plates of whales.

Distribution: —To date, Mallomonas cetosa has been observed in 33 localities in Vietnam (Table 1). The scales of M. cetosa have been previously reported under different taxonomic epithets from China, Indonesia, Malaysia, Jamaica, Madagascar, Philippines and France (see below).

Ecology: — Mallomonas cetosa was found under the following environmental conditions: pH 4.6–7.9, conductivity 10–666 µS/cm, temperature 22–39 °C, and chlorophyll-“ a ” concentration 10–101 µg/L.