January 22, 2013 Featured Article Read More →

How do shark embryos avoid predators?

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.

Chiloscyllium_punctatum_egg
Egg case of a white-spotted bamboo shark (Chiloscyllium plagiosum) at the Shedd Aquarium (Stefanie Seskin).

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.

The study, by scientists from the University of Western Australia, Crawley, near Perth, suggests brown-banded 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.

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.

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.

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.

Read the full article at: Shark Embryos “Freeze” to Evade Predators

Posted in: Shark Bites
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