Sharks can sense their prey’s minute electric fields, such as those produced when muscles twitch or nerve cells fire. This super-sense is part of what makes sharks such formidable hunters—you can’t hide from them if the very act of living can give you away. But the tables are turned when sharks are young. As they begin life, they’re as vulnerable as any other fish, and their electric sense helps them to hide instead of hunt.
The brown-banded bamboo shark lives in the western Pacific, between Japan and Australia. While some sharks give birth to live young, the bamboo shark is one of those that lays eggs. Each embryo is encased inside a pouch called a mermaid’s purse, which looks like a piece of square ravioli with a tendril at every corner. The purse’s tendrils fasten it to local vegetation and its colour makes it hard to see. Sealed inside, the growing embryo won’t stir the surrounding water with its movements, and its scent won’t diffuse away. It is well-hidden.
At some point, this sanctuary becomes more vulnerable. As the shark grows, the bottom edge of the mermaid’s purse weakens and opens, and the baby gets its first taste of the outside world. It starts wriggling its tail to waft fresh oxygen-rich seawater into the case. But it’s now vulnerable. A passing predator could pick up its smell drifting out from the case or feel the undulations of its tail.
But the baby shark isn’t entirely helpless. Ryan Kempster from the University of Western Australia has found that its electric sense develops shortly after its egg sac opens up. And that allows it to sense the electric fields of an approaching threat… and freeze.
Kempster collected egg cases that had been laid in an Australian aquarium and scraped off their outer layers. If he placed a light behind these denuded cases, he could film the silhouetted embryos within. He suspended the cases in a tank and exposed them to various electric fields.
When they sensed fields that match the breathing of a nearby predator, they quickly froze. They even kept their gills still, effectively holding their breath as long as they could. And they coiled their tail around their bodies in a little still ball. You can see some of this behaviour in the video below – there’s no tail-coiling but note how the gills freeze as soon as the electric field comes on.
Their freezing act in the final months of the embryo’s stay in its egg case, just before it emerges into the outside world. Their electric sense organs—jelly-filled pores called ampullae of Lorenzini—start to mature and they become more sensitive to ever-weaker electric fields. During this precarious window of time, their newly developed super-sense keeps them alive by hiding them from predators, before later turning them into predators that nothing can hide from.
Reference: Kempster, Hart & Collin (2013) Survival of the Stillest: Predator Avoidance in Shark Embryos. PLoS ONE 8(1): e52551. http://dx.doi.org/10.1371/journal.pone.0052551