Shark embryos keep still

Source:, photo by Zul M. Rosle
All living beings emit a weak electrical field. Sharks can 'feel' this with special sense organs, and some shark species use this sense to find their prey. Shark embryos, in their egg cases, also produce a weak electrical field.
And though young sharks can defend themselves just fine, and can themselves hunt for food, shark embryos are very vulnerable. While still in their egg cases, they are easy prey; but by remaining still, holding their breath (not moving their gills) and curling their tails around them, brownbanded bamboo shark embryos prevented themselves from being discovered.

This effect was discovered coincidentally by a scientist from the University of Western Australia while testing shark repellents. He used the shark embryos for repeated experimentation because, as he said, sharks swim away – but embryos in their egg cases stay in one place. A weak electric field was used as a repellent, and then the researcher observed the shark embryos’ reaction. Normally, the embryos move their tails to bring fresh water into the egg case, but in reaction to the electrical field, they ceased their movements and acted dead. The young sharks also seemed to remember the electrical field: reintroduction of the same voltage after 40 minutes triggered a less pronounced reaction. The scientist concluded that an electrical field used as a shark repellent would have to be varied constantly in order to keep the sharks from getting accustomed to it.

Shark repellent can be employed to protect humans (near beaches), but is primarily needed to protect sharks. Sharks are very frequently caught inadvertently. The use of such repellents in fishery is hoped to reduce shark bycatch.
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