In the swamps and creeks of the Amazon and Orinoco river basins, a legendary predator lurks in the muddy depths: the electric eel. Said to be capable of paralysing animals the size of a horse with its electrical discharges, it has been the subject of countless experiments over hundreds of years: it inspired the design of the first battery by Volta and has been used to study the workings of the interface between nerve cells and muscles. Astonishingly, we still have little idea of how the eel actually uses its electric organ to capture prey. What we do know is that the electric eel is capable of producing three types of electrical discharges: a slow, low-intensity mode while exploring its environment, pairs or triplets of high-intensity pulses of unknown use, and an all-out, high-intensity mode, consisting of dozens of pulses over a few seconds, to capture prey or defend against attacks. A major hurdle to understanding electric eel predatory behaviour is that much of it simply occurs too quickly for the human eye to detect.
However, in one big sweep, Kenneth Catania of Vanderbilt University has filled in many of the gaps in our understanding in a highly insightful paper published recently in Science. He decided to overcome the limitations of the human eye by using a high-speed camera to track the behaviour of electric eels as they try to capture their prey (in this case small fish). Catania noticed that very quickly after the eel's first discharge of the high-intensity burst, prey were paralysed. How does this fast incapacitation occur? In order to address this question, Catania placed an experimentally incapacitated prey fish in the aquarium, separated from the eel by an agar wall, which, while acting as a physical barrier, still conducts electric fields.
As expected, when the eel generated the volleys of high-voltage pulses that it normally uses to capture prey, the prey fish tensed up. However, when Catania injected a drug into the prey that inactivates the connections between the nervous system and the musculature, this response no longer occurred. This shows that the eel incapacitates its prey by inducing electrical activity in the nervous system, which in turn makes the muscles contract uncontrollably.
When Catania looked more carefully at the eel's predatory behaviour, he saw that, in complex environments, pairs or triplets of discharges often precede high-voltage bursts. Moreover, the doublets or triplets always produce movement in the prey. Could this mode serve to detect prey under difficult conditions by forcing them to reveal themselves? In order to test this, Catania put incapacitated prey fish in a plastic bag in the aquarium, which electrically isolates them from the eel. Then, Catania connected the prey fish with electrodes, which allowed him to induce movement in the prey fish.
Catania noticed that, when an eel fired a doublet or triplet, the eel would only attack the prey when Catania induced movement in the fish. This shows that the eel uses the doublet or triplet to confirm that it is dealing with live prey, rather than one of the many inanimate objects in the complex environment that the murky bottom of a swamp provides.
This study elucidates how the electric eel uses its electric organ to draw out, detect and incapacitate its prey. Furthermore, it comprehensively shows that we should be grateful we're not small fish in the Amazonian basin.
