The electric catfish can emit up to 300 volts to stun its unsuspecting prey. However, the fish isn’t just immune to its own jolts – it seems to be unable to be shocked at all.

Georg Welzel and Stefan Schuster at University of Bayreuth in Germany explored the degree to which electric catfish (Malapterurus beniensis) are insulated from electric shocks, both their own and those from outside sources.

In one test, in which a goldfish and one of the two electric catfish used in these trials shared a tank, Welzel and Schuster coaxed the catfish into discharging its electricity by gently brushing its tail. In another, they used a commercial electrofishing device to give the entire tank a jolt. In both trials, the goldfish spasmed and contorted its body briefly before recovering, but the catfish was unaffected.

“It was absolutely amazing to see how unexpressed and relaxed electric catfish swam through their tank when being confronted with electric shocks that usually narcotise other fish,” says Welzel.

The first two tests indicated that the catfish’s muscles were unaffected by electric shocks, but gave no clues as to whether its nervous system has the same insulation.

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To test for nervous system interference, the researchers added electrodes that maintained a current in the water. They then played a loud blast of sound to startle the fish into emitting a shock. If the animal’s nervous system was impaired by the ambient electric field, the fish probably wouldn’t react by producing a shock, says Welzel.

High-speed cameras were used to watch for even a slight delay between the fish’s reaction, if one occurred. When the sound was played, the catfish was unfazed, displaying what seems to be a nearly complete immunity to the effects of electricity.

The paradox of the electric catfish becomes more mysterious when you account for the fact that the fish often hunts by sensing weak electric fields emitted by its prey – so it isn’t as though they don’t have a way to detect electric fields.

“We hope to find out more about the nature of their protective mechanism in future experiments,” said Welzel. “This is a mystery that we now want to solve.”

Journal reference: Journal of Experimental Biology, DOI: 10.1242/jeb.239855