The preferred and avoidance temperatures of Thermocyclops crassus (Fischer, 1853) and their relation to the temperature of optimal, pessimal and normal performance of the species

The ranges of the preferred and avoided temperatures in representatives of Thermocyclops crassus (Fischer, 1853) were determined by the results of experimental testing in a thermogradient aparatus and comparison of the obtained values with field observations of the optimal, pessimal and tolerated temperature conditions of development of these populations in nature. Copepods were sampled from pond located near Borok, Yaroslavl region, Russia (58o02'57'' N; 38o14'56'' E). The ambient water temperature was 15.0°С. Temperature preference was determined by the “chronic” method. The phenology of development of Th. crassus was observed in the field of water bodies in Central and Volga Federal Districts of Russia. The final thermal preferendum (FTP) achievement occurred with an increase in the preferred temperature: when animals were placed at a temperature in the thermal gradient (14.0–15.0°C) that approximated the temperature of the source pond, they moved to warmer water until they chose FTP (26.7°C). Obtained the values of FTP (25–30°C), temperature of normal performance (21–32°C) and pessimal temperatures of 9–20 and 33°С well coincide with numerous field observations for temperature conditions of development of species in northern waterbodies of Holarctic and with the temperatures at which populations of Th. crassus thrive in southern waterbodies of Holarctic and in tropical lakes. It is concluded that, despite the historically long existence of the species in the reservoirs of the temperate climatic zone, the northern populations of Th. crassus retained the temperature responses characteristic of their southern sister populations. And although the species has adapted to life in northern reservoirs at lower temperatures, when it becomes possible to choose, cyclops prefer temperatures above 25°C, which are optimal for southern populations living in tropical waters. These data once again confirm that the horizontal thermal gradient method can be used to infer temperature tolerance of freshwater cyclopoid copepods in nature.