Paralona pigra

55. * Paralona pigra (Sars, 1862) (Fig. 6A–C) 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).