Shark embryos 'freeze' to evade predators

Embryonic brown-banded bamboo shark in egg case

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Bamboo shark embryos developing in egg cases stay still to evade predators, scientists say.

Australian researchers found that the embryos could identify electric fields simulating a nearby predator, despite being confined to a tiny egg case.

On sensing danger, they "froze" and temporarily stopped breathing to avoid being detected.

Sharks use jelly-filled pores on their heads called electroreceptors to recognise other animals.

These highly sensitive receptors enable sharks to locate prey, predators or potential mates from minute bioelectric fields.

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The study, by scientists from the University of Western Australia, Crawley, near Perth, suggests brownbanded bamboo shark (Chiloscyllium punctatum) embryos are able to employ similar methods to detect predators.

"Embryonic sharks are able to recognise dangerous stimuli and react with an innate avoidance response," explained Ryan Kempster, a shark biologist and member of the research team.

The team's findings are published in the journal PLoS ONE.

Brownbanded bamboo sharks grow up to 1.2m (3.9ft) in length and are found in the Indo-West Pacific region as well as coastal areas of northern Australia and southern New Guinea.

Embryos of some shark species are deposited in leathery egg cases to develop independently of the mother. This renders them vulnerable to predators such as other sharks and marine mammals.

As the embryo grows the seal of the egg case begins to open. This means predators are able to detect sensory cues given off by the embryo's movements inside the case.

"This is the first study that shows a shark embryo's ability to detect and 'hide' from a predator by staying completely still and stopping its breathing," said Mr Kempster.

Juvenile brown-banded bamboo shark Juveniles sport a banded pattern to deter predators

By simulating the electric fields given off by predators in a water-filled test tank, the team found the embryonic sharks responded by stopping movement of their gills and staying very still.

And if the embryos continued to be exposed to these electric fields when they needed to start breathing again, they did so at a reduced rate of movement of their gills, suggesting they were "hiding" from the perceived predator.

Shark-repellent potential

The scientists had predicted that the embryos would demonstrate a response to predator-like stimuli but were surprised by further findings.

Start Quote

There is more to sharks than just a sharp row of teeth”

End Quote Ryan Kempster Shark biologist, University of Western Australia

The embryos also displayed an ability to "recognise" previous stimuli when exposed to the electric fields, and accordingly "[reduced] their future responses when repeatedly exposed," explained Mr Kempster.

"This means that sharks may become conditioned to current repellent devices if the signals that these devices produce do not change substantially over time," he commented.

Mr Kempster, who also founded shark conservation group Support Our Sharks, said the results of the study may provide "a stepping stone" to producing more effective shark repellents.

The organisation aims to save sharks from being killed as by-catch in fishing nets and as a pre-emptive way of protecting human swimmers from potential attacks.

He said he hoped to test the study's findings on larger adult sharks to see how the results may be used for the development of such repellents.

He added: "It is my goal to show the world that there is more to sharks than just a sharp row of teeth, and that we have so much more to learn from these amazing animals."

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