Chydorus cf. sphaericus (Müller, 1776)

47. Chydorus cf. sphaericus (Müller, 1776) View in CoL

Localities and specimens found: 96 – 1♀; 98 – 1♀.

Distribution and ecology. Chydorus sphaericus s. lat. is one of the most known members of the chydorids, representing a cosmopolitan complex of closely related species with an intricate taxonomic history, many synonyms and, as previously thought, subspecies ( Smirnov 1971, 1996; Belyaeva & Taylor 2009; Kotov et al. 2013 a, 2016; Karabanov et al. 2022; WoRMS Editorial Board 2024). Chydorus sphaericus -like populations are well known in the Oriental and adjacent territories ( Chiang & Du 1979; Michael & Sharma 1988; Maiphae et al. 2008; Chatterjee et al. 2013; Korovchinsky 2013; Ji et al. 2015; Sinev et al. 2020), including Vietnam, where these crustaceans were often indicated as the subspecies Ch. sphaericus sphaericus (Müller, 1776) ( Shirota 1966; Dang et al. 1980, 2002; Dang & Ho 2001; Phan et al. 2015).

Chydorus sphaericus is considered to be a wide eurybiont, living in water bodies and habitats of different kinds ( Fryer 1968). In addition to the open and vegetated littoral areas, it is often found in the open pelagic zone (in plankton). In the latter case, the crustacean can attach to various floating substrates, such as algae and detritus ( Smirnov 1971, 1996; Korovchinsky et al. 2021b). In our material, two Ch. sphaericus -like females were found in a mountain river in winter, during the period of moderate current, low water and, in comparison with lowland areas, low temperature (see Appendix 1).

Remarks. Despite the long history of this taxon, the Ch. sphaericus group of species still remains incompletely investigated. Recent studies, including molecular-genetic, have revealed that the “typical” form, Ch. sphaericus s. str., is found mainly in the western and central Palearctic (together with other related forms), not counting cases of anthropogenic or other introductions outside this region. In turn, in other parts of the world the Ch. sphaericus -like populations are obviously represented by sibling species, some of which have already been examined and described whereas others remain to be studied in detail ( Belyaeva & Taylor 2009; Klimovsky & Kotov 2015; Kotov et al., 2016; Karabanov et al. 2022). It has not yet been precisely established which specific forms of the Ch. sphaericus group inhabit Southeast Asia.