Tracking swimming Lefua echigonia to assess the impact of crayfish introduction

Lefua echigonia is an endangered freshwater species in Japan, whose habitats are decreasing due to urbanization and modernization of agriculture. In addition, invasion by non-native crayfish, Procambarus clarkii, is known to be a threat to L. echigonia. Whereas we observed P. clarkii at L. echigonia habitats such as large gravels, little is known about the impact of P. clarkii invasions on L. echigonia. In this study, we conducted a laboratory experiment using a 25-cm cylinder to observe swimming behaviour of L. echigonia with varying number of P. clarkii individuals and the presence/absence of a rectangular obstacle at the center. Swimming trajectories were obtained from a series of images taken 30 frames per second, based on which swimming speed, acceleration, and distance between individuals were calculated for the assessment of behavioural changes with P. clarkii introduction. No feed was given for both species during experiment. While both species moved along the wall during experiment under no obstacle condition, L. echigonia was more active than P. clarkia. Behavioural changes were observed when the obstacle was placed in the center. Foraging by P. clarkii was expected but no such a behaviour was observed even under the condition where more P. clarkii individuals were put together. Whereas no foraging behaviour was observed, some L. echigonia individuals stayed around the brick which is similar to P. clarkia. This suggests potential habitat overlaps between the two species. Further study can reveal species competition under more complex, dynamic instream conditions for which image analysis with high-speed/high-resolution cameras can be used as a tool for reliable assessment.