Baby bamboo sharks can sense danger and freeze in response, before they've even left their eggs.
Scientists have found that still-developing bamboo sharks in their egg-cases respond to the electrical fields produced by predators by ceasing gill movements, wrapping their tails around their bodies and freezing in an effort to prevent themselves being detected.
In the Brownbanded bamboo shark (Chiloscyllium punctatum) used in the studies, the embryos develop independently from the mother in egg cases. During this time they are at risk of predators, which can detect sensory cues produced by movements inside the egg case, such as that caused by the embryo beating its tail to move fresh seawater in and out of the egg case, or even the moving of its gills as it breathes.
The scientists tested the reactions of 11 Brownbanded bamboo sharks to the simulated weak electrical field of predators.
"Despite being confined to a very small space within an egg case where they are vulnerable to predators, embryonic sharks are able to recognise dangerous stimuli and react with an innate avoidance response," said Ryan Kempster of The University of Western Australia's Oceans Institute. "This knowledge may help us to develop effective shark repellents."
However, the team found that if the same electric field was presented a number of times, the developing shark embryos could recognise it - along with the fact that there had been no consequences - and so didn't respond as strongly to subsequent exposures.
Kempster says this is important to remember in developing shark repellents that use electrical fields, suggesting that the current would need to be changed regularly to ensure that sharks didn't get so used to a particular field that the deterrent became ineffective.
Such deterrents would be useful in protecting not just humans from sharks, but also the sharks themselves by reducing shark bycatch and keeping them clear of fishing gear.
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