Fractal Indices as Estimators of the Complexity of Ephemeroptera Nymph Movement in a Thermal Stress Experiment

Global warming increases water temperature of freshwater habitats, altering thermotaxis responses of hemimetabolous macroinvertebrates. Ephemeroptera nymphs are very vagile active in different types of microhabitats. Its movement complexity could shift during temperature augmentation, dynamic that has been received little attention. The present study examined the two-dimensional and movement behavior complexity of Ephemeroptera nymphs (Baetidae and Leptophlebiidae) under an experiment of induced thermal stress. Increasing temperatures could influence the movement patterns according to Fractal Dimension Index (FDI). FDI methods (radial, box-counting, and dilation) indicate the complexity of trajectories. In a acclimatized room, recording the movement of nymphs under three thermal conditions: control (temperature from sampling sites), control +5°C and control +10°C, from several microhabitats of rithronic streams, were conducted. An aquarium was continuously aerated with an air pump and covered at night to simulate the natural photoperiod, where nymphs were sheltered after capture. The results showed that thermal stress led to a decrease in FDI, suggesting a shift toward more directional and less complex movement patterns. Compared to families, microhabitats have more significative effect across FDI methods. Nymphs from structurally complex habitats (macrophytes and boulders) tended to maintain higher FDI values under stress, as opposed to those from simpler (gravel and sand), which showed a sharper reduction in movement complexity. Hence, the negative effects of heat stress on behavior may mitigate by structural heterogeneity of habitats. Positive correlations were found between habitat and locomotor traits such as Mean speed and Total path length, suggesting that complex habitats may enhance nymph mobility. Our findings support the use of fractal metrics as indicators of behavioral plasticity and complexity of animal movement, under environmental stress.