On-line determination of the velocity of simultaneously moving organisms by image analysis for the detection of sublethal toxicity

In this paper, an image analysis system for the measurement of the average velocity of many simultaneously moving similar objects is described. Using a low-cost PC framegrabber, trajectories of the moving objects are reconstructed in real time. Average object velocity is derived from a statistical analysis of all trajectories generated in short time intervals, and a simple geometrical model describing the displacement of the objects. This approach avoids the need to recognize individual objects in consecutive frames, and so the system can be used for the analysis of objects that cannot be tagged individually. Spatial distribution of the objects is determined by counting the objects in two defined areas of the active window. The calculations are continuously repeated, which results in an on-line measurement of the average velocity and spatial distribution of the population, and this allows for the monitoring of activity in time. Applications include the measurement of sublethal toxic effects in static systems or in flow-through on-line monitoring devices. The system is illustrated with an analysis of the response of the waterflea Daphnia magna to changes in its environment.