Australasian Science: Australia's authority on science since 1938

Shark embryos ‘play dead’ to predator-simulated electric fields

Shark embryo research from the University of WA’s Oceans Institute has determined that embryos can remember previously encountered predator-like stimulus and reduce their response to it, creating implications for the effectiveness of electric shark repellents.

The team, including prominent shark experts Associate Professor Nathan Hart and Professor Shaun Collin, tested the response of Brownbanded Bamboo shark embryos (Chiloscyllium punctatum) to the simulated electrical fields of known predators.

Researchers found the shark embryo, which once hatched can grow up to a metre in length and is found along WA’s north coast, used electroreception to detect the presence of a supposed predator and react by ceasing breathing and movement.

UWA PhD student Ryan Kempster says this is the first study that shows a shark embryo’s ability to detect and ‘hide’ from a predator, whilst developing at the same time.

“I anticipated that shark embryos would show a response to predator-like stimuli but what I was surprised to find was that they also display an ability to remember previous stimuli and reduce their future responses when repeatedly exposed,” he says.

During the pre-hatching stage the bottom edge of the egg shell weakens and opens marginal seals which release sensory cues that could attract predators including Teleost fish and other sharks.

As the embryo increases in size it begins to undulate its tail to help circulate fresh seawater to assist in respiration.

But the team found the embryo, when exposed to predator-simulated electric fields, responded by ceasing gill movements and coiling its tail around its body resulting in almost no body movement during exposure.

However the embryo’s need to breathe eventually overcame the urge to remain undetected and after a certain period of time resumed gill movements, albeit much reduced, while still being exposed to the electric stimuli.

They also determined that the embryos became desensitised to the same electric field when repeatedly exposed to it in a 30-40 minute period resulting in a reduced response duration.

Mr Kempster says the study’s findings are a stepping stone to producing an effective shark repellent.

“This means that sharks may become conditioned to current repellent devices if the signals that these devices produce do not change substantially over time, thus reducing the effectiveness of a repellent the longer a shark encounters it,” Mr Kempster says.

“We hope to take what we have learnt from this study and test our findings on larger adult sharks to see what applications these results may have for the development of effective shark repellents.”

Nick Brant, Science Network WA