Detection of Water Flow by Japanese Eel (Anguilla japonica): Behavioral and Pharmacological Analyses

Abstract

Eels migrate long distances and their spatial learning ability based on extra-maze visual cues has been examined in the laboratory. Here, I examine the discriminative properties of water flow in their spatial cognition. Individuals of the Japanese eel (Anguilla japonica) were trained in a circular pool containing four pipes. One pipe was open where the eels could enter, and the other pipes were closed. A small motor-driven water screw was placed beside each pipe. The water screw close to the open pipe was active to allow water flow, whereas the other screws were inactive. The position of the open pipe with the active screw was randomly changed, and the eels could learn the position of the open pipe after approximately 15 trials. Overall, five tests were conducted. Test 1: A generalization test showed a generalization gradient along the distance from the open pipe and the active screw. Test 2: A test using a screw with no blades (i.e., no water flow but possible vibration or sound by a motor is present) resulted in chance level performance, suggesting that the eels needed water flow to find the open pipe. Test 3: A test with a water pump that produced water flow by a mechanism different from that of the original motor-driven screw showed that the eels maintained the detection of water flow under this condition. Test 4: The eels also located the open pipe in a dark room test; therefore, visual cues were not used for the detection of the open pipe. To confirm that the water flow was detected by the lateral organ, streptomycin sulfate was dissolved in a home tank to impair the hair cells in the lateral line organ in Test 5, and the eels were not able to detect the open pipe under these conditions. This observation suggests that the detection of water flow depends on the lateral-line organ. In summary, the data show that eels can detect water flow to find hiding locations.