Brown-banded bamboo sharks can sense electrical fields even before they've hatched from their egg cases.

Despite being confined to the small space within the egg case, where they are vulnerable to predators, embryonic sharks are able to recognise dangerous stimuli and react with an innate avoidance response.

Normally, an embryo pulses its gills actively. But when researchers turned on electrodes to produce an electric field near the egg cases, the unhatched sharks froze and stilled their gills for several seconds.

The researchers suggest that freezing is a response to the presence of possible predators, such as other sharks. By freezing, the embryo may reduce its heart rate, which in turn may also reduce its own electric field, making it harder for predators to notice it.

The conditions under which this species habituates to electrical stimulation may also be useful in the development of electrical shark repellent devices.