A Simplified Method for Extracting the Movement Trajectories of Small Aquatic Animals

Understanding the motion behaviors of animals is crucial for unraveling the mechanisms underlying ethology across various domains, such as movement patterns, food detection, and defense strategies. In this study, we devised a simplified method enabling the movement of small animals to be tracked conveniently using high-resolution smartphone videos and freely available tracking software. Employing a laboratory video setup, we traced the swimming trajectory of the small copepod zooplankton Eodiaptomus japonicus, which has a body size of approximately 1 mm. From the tracked position data, we analyzed key motion parameters, including swimming distance, speed, and jump frequency. The results of our video analysis showed that adult female E. japonicus exhibited an average swimming speed of 9.8 mm s^-1, displaying a predominant cruising pattern with speeds of around 5.0 mm s^-1, punctuated by sporadic jumps, showcasing maximum instantaneous speeds reaching a remarkable 190.1 mm s^-1. Our successful tracking of the high-speed swimming copepod not only sheds light on its locomotion dynamics but also underscores the potential to refine this method to study the motion trajectories of diverse animal species.