Prey-capture behavior in gymnotid electric fish: Motion analysis and effects of water conductivity

Animals can actively influence the content and quality of sensory informati on they acquire from the environment through the positioning of peripheral sensory surfaces. This study investigated receptor surface positioning duri ng prey-capture behavior in weakly electric gymnotiform fish of the genus A pteronotus. Infrared video techniques and three-dimensional model-based tra cking methods were used to provide quantitative information on body positio n and conformation as black ghost (A, albifrons) and brown ghost (A. leptor hynchus) knifefish hunted for prey (Daphnia magna) in the dark. We found th at detection distance depends on the electrical conductivity of the surroun ding water. Best performance was observed at low water conductivity (2.8 cm mean detection distance and 2 % miss rate at 35 muS cm(-1), A. albions) an d poorest performance at high conductivity (1.5 cm mean detection distance and 11 % miss rate at 600 muS cm(-1), A. albifrons). The observed conductiv ity-dependence implies that nonvisual prey detection in Apteronotus is like ly to be dominated by the electrosense over the range of water conductiviti es experienced by the animal in its natural environment. This result provid es the first evidence for the involvement of electrosensory cues in the pre y-capture behavior of gymnotids, but it leaves open the possibility that bo th the high-frequency (tuberous) and low-frequency (ampullary) electrorecep tors may contribute. We describe an electrosensory orienting response to pr ey, whereby the fish rolls its body following detection to bring the prey a bove the dorsum, This orienting response and the spatial distribution of pr ey at the time of detection highlight the importance of the dorsal surface of the trunk for electrosensory signal acquisition. Finally, quantitative a nalysis of fish motion demonstrates that Apteronotus can adapt its trajecto ry to account for post-detection motion of the prey, suggesting that it use s a closed-loop adaptive tracking strategy, rather than an open-loop ballis tic strike strategy, to intercept the prey.