Thermal tolerance of Cyclops kolensis lilljeborg, 1901 (Copepoda: Cyclopoida) and its relationships with pessimal to optimal performance.

Cyclops kolensis Lilljeborg, 1901 belongs to the Arctic complex of Palaeoarctic species, yet in the past 20 years, its occurrence has extended to the summer months in waterbodies with high water temperatures. This species is considered one of the most active migrants from the northern waterbodies in the Volga reservoir cascade to the Volga delta. Here, we explored the ranges of the preferred and avoidance temperatures of C. kolensis from two geographically isolated populations. Thermal tolerance was measured in a thermogradient installation and compared to the temperatures at which members of these populations occurred in their source waterbodies. Temperature preference was determined using the "chronic" method. Individuals of C. kolensis possessed a bimodal final temperature preferendum of 2-6 °C and 13-21 °C, which corresponds to the optimal thermal conditions of the species in a pond. These ranges were the same for individuals of both populations irrespective of the geographical location and water temperature of their source waterbodies. The temperature range of normal performance was 2-4 to 21-25 °C, and the pessimal temperature ranges were from 1 - 2 to 3-4 °C and from 22-25 °C to 26-30°С. These temperature ranges coincide with field observations over a recent 20-year period of temperature conditions under which the species develops in nature. Our results allow us to characterize C. kolensis as an ecologically plastic species, which, despite its strong association with the cold-water Complex species, is adapting to a wider temperature range as global warming occurs.