Mallomonas hippocrepica E.S. Gusev, Martynenko, Hoan Tran & Podunay sp. nov. (Figs 3–4, 7–17)

Cells are ellipsoidal, 9–12 × 5–7 μm, covered with overlapping scales and a few curved bristles associated with the apical scales. 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, 2.4–3.8 × 1.4–2.4 μm. Shield and anterior flanges are ornamented with numerous papillae and covered with the secondary siliceous layer, except for an area along the posterior submarginal rib. As a result, there is a depression in the shape of the letter “V” or “U” on the shield. There are no clearly defined anterior submarginal ribs on the scales. Anterior flanges are slightly raised above the shield and usually have 2–3 rows of papillae along the edge. The shield is patterned with densely spaced papillae. The posterior submarginal rib is well developed. At the distal end of each arm of the posterior submarginal rib, another short rib may develop on the outer side. There is one rimmed pore in the angle of the posterior submarginal rib. The posterior rim is smooth. The posterior flange is narrow and smooth. The collar scales are asymmetrical, elongate, 3.2–3.9 × 1.6–2.2 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 and connected with the dome. An additional short submarginal rib (anterior) departs from the dome on the “ventral” side. There are several rimmed pores in the angle of the posterior submarginal rib. The dome on the collar scales is ornamented with papillae. The apical part of the dome has serration. The rear scales are 1.2–2.2 × 0.7–1.5 μm, similar in structure to the body scales, but more oval and asymmetrical, and often bearing a spine. Bristles are 5–8 µm long, curved, smooth. Cysts are not observed.

Holotype (here designated): Portion of a single gathering of cells on SEM stub No. D 3/22 deposited at the Herbarium of the Steppe Institute of the Ural Branch of the Russian Academy of Sciences, Orenburg (ORIS). Material from pond in Cat Tien National Park, Dong Nai Province, Viet Nam, collected by E.S. Gusev on the 7 th of May, 2018. The strain was isolated by Yu.A. Podunay. Figure 14 is a representative scale from the type specimen.

Type Locality: Pond in Cat Tien National Park, Dong Nai Province, Vietnam. Latitude/Longitude: N11° 24.407’, E107° 24.398’.

Authentic strain: Representative living and fixed strain D3/22 and DNA sample deposited at the Collection of the A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences.

Genbank accession numbers for reference strain (D3/22): PQ677578 (nuclear SSU rDNA), PQ673582 (rbc L cpDNA) and PQ677569 (nuclear ITS rDNA).

Genbank accession numbers for other strain are listed in the Table 1.

Etymology: the species epithet means “horseshoe-shaped” and reflects the similarity between a depression along the V-rib and horseshoe.

Distribution: In addition to the type locality, this species has been observed in one more locality in Vietnam (Tables 1, 2). Mallomonas hippocrepica was found at quite narrow ranges of environmental parameters: pH from 6.4 to 7.4, specific conductance from 12 to 43 µS cm-1, and temperature 32–39 ºC (Tables 1, 2).