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We conducted a study of the peculiarities of the motor activity of juvenile and adult individuals of Daphnia magna in the process of thermal selection. On the example of Daphnia, we experimentally proved the assumption that the decrease in the motor activity of the ectotherms in the selected temperature range is a behaviour mechanism typical for thermal selection. The experiment was conducted on the offspring of one parthenogenetic female (pure line) of a laboratory culture maintained over several years at room temperature. The Daphnia individuals had been beforehand acclimated over several generations to 23.4 oС, and then were placed in a thermogradient apparatus. The control individuals were maintained at room temperature. The final temperature preference was determined using the so-called "chronic" method, when the tested organisms are maintained in a thermogradient apparatus over several days. The positions of the Daphnia individuals were recorded daily during 24 days. As a parameter which would characterize the motor activity of Daphnia, we used the parameter of average relative shifting, which was calculated as a difference (in cm) between the position they were found in two sequential records, divided by time (in min) between these records. Over the first 3 days, in the conditions of the temperature gradient, the Daphnia mostly selected heightened temperatures (24–28 oС). During the period from the 4th to 24th day, 74% of the Daphnia selected the range of 18–23 oС and 85% – 17–24 oС. Thus, the range of the ultimate selected temperatures decreased by 4 oС compared to the initially selected temperatures and enlarged by 3 oС towards the lower temperatures. In the thermogradient apparatus, the parthenogenetic females had the lowest value of the mean relative movement. In the control, at room temperature, this parameter of the adults was by 43% higher compared to the gradient. The pattern of the dynamic of the indicator in both variants of the experiment was similar for the periods and phases of fluctuations. With the juveniles the value of the parameter of mean relative shifting in the gradient was higher by 40% compared to the adult Daphnia, but close to the parameter of the adults in the control. We determined a relationship between the juveniles selecting the higher temperatures and higher mean relative shifting, which indicates a relationship between the behavioural selective reaction of juvenile Daphnia and general physiological condition of their organism. During the absence of the temperature gradient, the Daphnia were observed to show symmetry in their motor reaction. Occurrence of the asymmetry of the motor reaction of the Daphnia in the condition of the thermogradient, manifesting in the prevalence of shifting to lower or higher temperatures, can indicate more clearly manifested search reaction in the condition of a non-uniform environment.

Motor activity of Daphnia magna (Crustacea, Cladocera) during thermal selection: Peculiarities of search reactions in the non-uniform environment