Efficient high-voltage protection in the electric catfish

To analyse the impact of natural and artificial high-voltage defensive discharges, two fish (either 2 electric catfish or 1 electric catfish and 1 goldfish) swam in an oval custom-made Plexiglas channel. The channel was separated into two halves by plastic mesh. In the centre of one half, a paddle wheel powered by an electric motor created a water current against which the two fish swam in the other half of the channel (12 cm diameter and 90 cm long). A porous filter pad in the channel separated the two fish. The fish were monitored from below at ≥1000 frames s−1 using a high-speed digital video camera (HotShot 2300cc, NAC Image Technology; 20 mm f/1.8 EX DG lens, Sigma). To optimise the contrast, the tank was illuminated from below with two LED floodlights (IP FL-50 COB 6400K, Eurolite). Four control goldfish (5–10 cm) and two electric catfish (16 cm) were monitored while exposed to either defensive discharges generated by the electric catfish or artificially generated discharges delivered by a pair of stimulation electrodes (silver wire) that were positioned 60 cm apart and perpendicular to the longitudinal axis of the Plexiglas channel. As the exact field strength and spatial properties of the defensive discharges within our setup were unknown, special care was taken to ensure that the positions of the test fish were similar in each experiment. More precisely, the discharging electric catfish was always positioned behind (at 5–10 cm distance) the receiving goldfish or catfish, respectively. Additionally, only experiments in which the discharging catfish and the receiving fish swam in line (with a tolerated maximal deviation of about 5 deg) were included in the analysis. All electrical discharges were recorded using two carbon electrodes in the water placed at opposite ends of the test aquaria. Each electrode was first fed into a voltage divider to reduce the output voltage to a quarter of the input voltage and connected to a differential amplifier (EXT 02F-2, npi electronic). Its output was used to trigger the high-speed video system so that it recorded the catfish 100 ms before and for another 100 ms after onset of its discharges.