In the murky waters of the Amazon River, electric eels (Electrophorus electricus) bombard their prey with quick-fire bursts of electricity. Just like a TASER, a rapid volley of electrical impulses causes repeated muscle contractions and immobilizes prey. Eels can then swallow their unlucky victims whole. Although this electrical weaponry is around two and a half times more powerful than the discharge from a UK electrical socket, this might not be enough to successfully stun many prey available in the Amazon. Kenneth Catania suspected that a unique behaviour might help eels intensify their attack to expand their menu.
Central to an eel's electrifying attack is an organ containing thousands of electrocytes, which runs down most of an eel's body. Individually, each electrocyte can only release a small charge, but when lined up in series like cells in a battery, electrocytes can discharge 600 V. In theory, to ‘amp’ up their attack, eels could increase the voltage that these electrocytes produce. However, it is possible that they may have evolved a more elegant alternative. In a typical attack, captured prey are locked between an eel's jaws, where they are exposed to electric impulses originating from behind the eel's head. However, Catania consistently observed eels looping their bodies around prey. This curling behaviour brings the negative pole of an eel's electrogenic organ (the tail) closer towards its positive pole (the head). Catania reasoned that prey sandwiched between the two poles of an eel's body would experience a more intense electrical field.
To test this idea, Catania inserted electrodes into dead pithead fish with viable muscles and presented these to eels. He found that when eels looped around fish, the voltage that the dead fish experienced more than doubled. By simply repositioning their bodies, these eels can exploit the physics of electrical fields to unlock their full electrogenic potential.
To see what effect this had on prey, Catania exposed pithead fish and crayfish tails to the high-voltage impulses that they would experience if held between the poles of an eel's body. He found that an amplified electric attack over-activates the victim's muscles, leading to a rapid drop in contractile force. The result is that by looping around their prey, eels quickly cause involuntary muscle fatigue.
Given that a standard monopole attack is already incredibly powerful, why do eels intensify their field? The answer probably lies in eel feeding behaviour. Eels must release their stunned victim to reposition it before swallowing it head first. This brief respite is the last opportunity for prey to recover and dart away. By causing fatigue in the muscle fibres central to anti-predation escape manoeuvres, the eels are able to incapacitate their prey so that they are less able to exploit this last chance of escape. This looping behaviour therefore helps eels subdue their victims, and might allow them to better tackle the well-armoured and electric-field-generating prey that may be available to them in the Amazon River.