Mallomonas sagittifera Podunay, Martynenko, Shkurina, Hoan Tran & E.S. Gusev (Figs 3–16)

Three types of scales have been observed: apical domed collar scales, domeless body scales and small rear scales with spines. The body scales are rhomboidal, slightly asymmetrical, 2.2–3.8 × 1.2–2.6 μm. Shield and anterior flange are ornamented with numerous papillae. Anterior flange is raised above the shield and usually have 2–3 rows of papillae along the edge at each side. Anterior submarginal rib is not developed. The shield is patterned with densely spaced papillae, with or without internal reticulation in form of triangular mesh. Posterior submarginal rib is well developed. There are several pits with rimmed pore inside are located along the posterior submarginal rib. The number of pits varied among cells from one strain from one to eight, usually three, rare such pits can be observed on the shield. Posterior rim is wide and smooth. Posterior flange is narrow, with an additional, thinner rib parallel to the posterior submarginal rib. The collar scales are asymmetrical, elongate, 2.8–3.8 × 1.7–2.6 μm. Ornamentation on the shield is the same as found on the body scales. The posterior submarginal rib on the “dorsal” side is well developed, connected with the dome. Posterior flange is narrow, with an additional, thinner rib parallel to the posterior submarginal rib. Anterior flange with one row of papillae or smooth. An additional submarginal rib (anterior) departs from the dome on the “ventral” side. There are several pits, up to six, with rimmed pores inside in the angle of the posterior submarginal rib. The dome on the collar scales is partially ornamented with papillae and has a sharply pointed peak at the apex. The rear scales are 1.3–2.0 × 0.9–1.5 μm, similar in structure and outline to the body scales, and each scale bearing a spine. Bristles are 5.8–7.6 µm long, curved, smooth.

Cysts were not observed.

Holotype (here designated): Portion of a single gathering of cells on SEM stub 63_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 pool near swamp Bau Thai in Cat Tien National Park, Dong Nai Province, Vietnam, collected by E.S. Gusev on the 6 th of May, 2018. The strain was isolated by Yu.A. Podunay. Figure 3 is a representative scale from the type specimen.

Type Locality: Pool near swamp Bau Thai in Cat Tien National Park, Dong Nai Province, Vietnam. Latitude/ Longitude: N11°30.377’ E107°21.658’.

Genbank accession numbers for the reference strain (B8/22): PV791366 (nuclear SSU rDNA), PV780783 (rbc L cpDNA) and PV791370 (nuclear ITS rDNA).

Genbank accession numbers for another strain (61_18): PV791367 (nuclear SSU rDNA) and PV780784 (rbc L cpDNA).

Etymology: the species epithet means “carrying arrows” and reflects the presence of an additional arrowheadshaped rib on the posterior flange.

Distribution: In addition to the type locality, this species has been observed in four more localities in Vietnam (Table 1). Mallomonas sagittifera was found under the following environmental parameters: pH from 5.2 to 7.4, specific conductance from 10 to 77 µS cm-1, and temperature 24–39 ºC (Table 1).