Abstract
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.
Highlights
The ability for search behaviour in the conditions of a thermally heterogeneous environment occurs due to the peculiarities of the physiology of ectotherms, which allow them to sense insignificant temperature differences
After the Daphnia were put in conditions of temperature gradient, over the first 3 days, they mostly chose high temperatures of 24.0– 27.9 oС (88.5% of records)
The level of motor activity of ectotherms is one of the most significant components of their food procuring efficiency. It seems that the reason for the decreased activity of the adult Daphnia in the conditions of the temperature gradient was most likely a well known phenomenon, when in the preferred temperature range, many animals demonstrate minimum motor activity
Summary
The ability for search behaviour in the conditions of a thermally heterogeneous environment occurs due to the peculiarities of the physiology of ectotherms, which allow them to sense insignificant temperature differences. For example, it was determined that fish can feel a difference in the temperature, that equals 0.03 oС (Bardach & Bjorklung, 1957; Steffel et al, 1976). For Daphnia, according to the results of the experiments in thermogradient devices, it was determined that they react to average changes in the temperature which equal 0.2– 0.5 oC (Gerritsen, 1982). It is considered that their thermal behaviour is based on non-directed behaviour mechanisms (orthokineses and klinokineses) which are caused by changes and differences in the temperatures (Lagerspetz & Vainio, 2006). There are no articles known to us, apart from the abovementioned, which include research on the behavioural mechanisms of the formation of the final temperature selection of Cladocera and, the peculiarities of their behaviour in the conditions of a temperature gradient
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
