Two-dimensional and anisotropic movement behavior of Ephemeroptera nymphs in an increasing thermal stress experiment

The global warming phenomenon leads to elevated water temperatures in freshwater habitats, which in turn alters the motility responses of hemimetabolous macroinvertebrates. Ephemeroptera nymphs are highly active in different types of micro-habitats. Its movement complexity may shift with increasing temperature, but how this dynamic occurs has received little attention. Our work studies the two-dimensional and anisotropic movement behavior of Ephemeroptera nymphs (Baetidae and Leptophlebiidae) under an experimental setting that provides controlled 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 and directionality (anisotropy). Experiments were conducted in a controlled arena, recording the movement of nymphs under three thermal conditions: control (temperature from sampling sites), control +5°C and control +10°C, from several micro-habitats of rithronic streams. The results showed that thermal stress led to a decrease in FDI, suggesting a shift towards more directional and less complex movement patterns, i.e., an increase in anisotropy. This response was influenced by taxonomic family, with Baetidae showing greater sensitivity and a steeper drop in FDI compared to Leptophlebiidae. Micro-habitat complexity also played a crucial role; nymphs from structurally complex habitats (macrophytes and boulders) tended to maintain higher FDI values under stress, as opposed to those from simpler substrates (gravel and sand), which showed a sharper reduction in movement complexity. This indicates that habitat structural heterogeneity may mitigate the negative effects of heat stress on behavior. Positive correlations were found between habitat fractal dimensions and locomotor traits such as Mean speed and Total path length, suggesting that complex habitats may enhance nymph mobility. These findings support the use of fractal metrics as sensitive indicators of behavioral plasticity and complexity of animal movement under environmental stress. Supplementary Material File (two-dimensional and anisotropic movement behavior of ephemeroptera nymphs in an increasing thermal stress experiment.docx) - Download - 1.11 MB Information & Authors Information Version history Copyright This work is licensed under a Non Exclusive No Reuse License. Collection

Authors Metrics & Citations Metrics Article Usage 231views 138downloads Citations Download citation Jorge Machuca-Sepúlveda, Mauricio Zamorano, Jorge Farías. Two-dimensional and anisotropic movement behavior of Ephemeroptera nymphs in an increasing thermal stress experiment. Authorea. 29 September 2025. DOI: https://doi.org/10.22541/au.175913190.04514180/v1 DOI: https://doi.org/10.22541/au.175913190.04514180/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.