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Baby sharks can fool passing predators even before they're born - by freezing on the spot or simply playing dead.

The embryonic sharklings can sense a hungry predator by detecting their electric fields and avoid being eaten by staying very still and slowing down their breathing.

Boffins already knew that adult sharks used highly sensitive receptors on their head to spot incoming predators and prey, but now scientists from the University of Western Australia have found that brownbanded bamboo shark embryos can do the same thing from inside their tiny egg case.

Brownbanded sharks, like a number of other shark species, are left in a leathery egg to develop on their own before hatching, making them vulnerable to passing hungry predators. But the study found that when the eggs in a tank were exposed to electric fields, they would hold their breath and stop moving. If they needed to breathe again before the danger passed, they would move their gills slowly to try to avoid detection.

"Embryonic sharks are able to recognise dangerous stimuli and react with an innate avoidance response," marine neuroecologist Ryan Kempster said.

Once they emerge after up to five months in their eggs, the baby sharks have the brown banding of their name, allowing them to mimic the colours of poisonous or less tasty prey. This colouring is lost as the shark grows up and takes on the more typical dark colouring of other shark species.

The boffins said that the study, published online in PLoS, was a step towards making more effective shark repellants. ®

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