First Record of Thermocyclops taihokuensis (Crustacea, Copepoda) in the Oka River Basin

A new finding of Thermocyclops taihokuensis (Harada, 1931) in European Russia is described. In October 2019, the East Asian copepod T. taihokuensis (Harada, 1931) was first recorded in the Shat Reservoir (54° N, 38° E) of the Oka River system. The Oka River is the right-bank tributary to the Volga River. The population of T. taihokuensis exceeded 100 000 ind./m 3 . The species actively reproduced at a water temperature of 13°C, sexually mature females with eggs in egg pouches accounted for 45% of their total number, and fecundity reached 17 ± 1 eggs per female. The Shat Reservoir is the westernmost habitat of the species in European Russia. It is assumed that the probability of further expansion of this species downstream the Oka River into the reservoirs of the midstream and downstream Volga is high.