Paralona pigra (Sars, 1862)

55. * Paralona pigra (Sars, 1862)

( Fig. 6A–C View FIGURE 6 )

Localities and specimens found: 92 – 1♀.

Distribution and ecology. Paralona pigra (formerly most commonly known as Chydorus piger Sars, 1862 ) is today the only member of the genus. It is recorded from the temperate and northern regions of Eurasia, North and South America, Africa and New Guinea ( Dumont & Smirnov 1996; Smirnov 1996; Korovchinsky et al. 2021b). There are no reports of findings of this chydorid within the Oriental biogeographic area, taking into account all available synonyms ( Kotov et al. 2013a; Błędzki & Rybak 2016; WoRMS Editorial Board 2024). It therefore appears to be the first record of the species not only in Vietnam, but in the entire region.

Paralona pigra is considered a benthic species. It primarily occurs in oligotrophic and mesotrophic lakes and ponds on sandy and silty sediments. The species is also found in groundwater ( Fryer 1968; Smirnov 1996; Kotov 2006; Korovchinsky et al. 2021b). The only female from our material was discovered in a low-flow section of a mountain river in December at a water temperature of approximately 16°C (see Appendix 1), which is quite consistent with habitat conditions for the species at more northern latitudes.

Remarks. Paralona pigra is not yet a fully studied taxon and needs revision. Because its specimens from different parts of the world have notable morphological differences, this crustacean may in fact represent a complex of related species ( Dumont & Smirnov 1996; Smirnov 1996; Korovchinsky et al. 2021b). For example, some populations of the species are characterized by the presence of a spine (or several spines) in the posterior-ventral corners of the valves, whereas others do not have such spines (Smirnov 1996). Our Vietnamese female has a distinct spine on each valve ( Fig. 6A, C View FIGURE 6 ).