Fractal analysis of Daphnia motion for acute toxicity bioassay

To quantify individual behavioral responses to toxic chemicals, the swimming motion of individual Daphnia magna was continuously monitored using a motion analysis system. The fractal dimension was introduced to compare the straightness or complexity of the swimming trajectory before and after exposure to toxic chemicals. Analysis indicated that the swimming trajectory of individual Daphnia has a fractal structure. The basal fractal dimension in the control medium was 1.35±0.01 (n = 50 Daphnia). Exposure to CuSO4 (10 μg/L), organophosphorus (Dichlorvos; 10 μg/L), and carbamate (Propoxur; 500 μg/L) pesticide caused a significant increase in the fractal dimension with a latency of 60 min, reaching a maximal level of 2.26±0.34, 2.43±0.19, and 2.51±0.21, respectively, after a 120‐min exposure. The magnitude of the change in the fractal dimension was related to the toxic chemical concentration and the exposure time. Threshold concentrations determined at 60 min of exposure were 10 μg/L for CuSO4, 5 μg/L for Dichlorvos, and 500 μg/L for Propoxur. The toxicity index (EC50) values after 120 min of exposure were 6.31 μg/L, 7.64 μg/L, and 466 μg/L for CuSO4, Dichlorvos, and Propoxur, respectively. Thus, the fractal dimension seems useful for analyzing and comparing complex trails, such as swimming trajectories, which could be used as the endpoint for an acute bioassay. © 2002 Wiley Periodicals, Inc. Environ Toxicol 17: 441–448, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/tox.10077