Cuttlefish can repel sharks

Scientists have discovered that cuttlefish use undersea electronic countermeasures to escape being detected and eaten by sharks.

Cuttlefish have a unique stealth ability that allows them to shield their electric fields from sharks.

The creatures were already known to be masters of camouflage. Now scientists have discovered they also employ undersea electronic countermeasures to escape being detected and eaten.

Sharks, one of the cuttlefish's main enemies, home in on faint bioelectric fields generated by the bodies of their prey which they pick up using sensitive detectors on their snouts.

A common cuttlefish at rest produces a bioelectric potential of 10-30 microvolts, which is about 75,000 times weaker than an AAA battery.

But the scientists found that when the animal feels threatened it freezes and drops the current down to about six microvolts.

It does this by covering body openings, the main source of the electric field, with its arms and breathing more slowly.

In the tests at Duke University in the US, captive cuttlefish in a tank were shown videos depicting the menacing silhouettes of a shark or predatory grouper fish.

They responded by reducing their electric fields - but ignored the shadow of a harmless crab.

The scientists, led by Dr Christine Bedore, wrote in the journal Proceedings of the Royal Society B: "The reduction of bioelectric fields created by the freeze-simulating stimulus resulted in a possible decrease in shark predation risk by reducing detectability.

"The freeze response may also facilitate other non-visual cryptic mechanisms to lower predation risk from a wide range of predator types."