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TONIC IMMOBILITY IN SHARKS
Tonic immobility is a reflex that causes a temporary state of inactivity in an animal – similar to hypnosis. It’s been observed in a variety of different species, including many sharks and rays. Although it can occur naturally, it can be induced in sharks and rays by stimulating the tiny sensory pores (ampullae of Lorenzini) located on the snout.©
Tonic immobility is often used by researchers when handling sharks (whether in the wild, captivity or laboratory conditions) to subdue them in order to minimise struggling and reduce the possibility of injury. The shark can be gently turned on its back, which may disorientate them and cause this trance-like response – the shark’s muscles relax and their breathing becomes deep and rhythmic – but when released, the shark snaps out of this state. This method has previously been used to help test the effectiveness of chemical shark repellents.
FUN FACT – Sharks usually enter tonic immobility in less than a minute and if undisturbed, can remain like this for up to 15 minutes before eventually righting themselves and swimming away.
One explanation for tonic immobility in some animals is that it could be used as a defence strategy, as playing dead could help deter potential predators. However as many types of shark are apex predators, they don’t have many natural predators and also don’t appear to enter tonic immobility in response to fear. Some scientists have suggested that this behaviour in sharks may in fact be related to mating but nobody knows for sure and there is still a great deal more to learn!
SHARKS AND ORCAS
There is evidence that suggests orcas may use tonic immobility to prey on sharks.
In 1997, in the waters around the Farallon Islands, a female Orca was observed holding a White Shark upside down for 15 minutes, causing it to suffocate. Whether intentional or not, the Orca likely caused the shark to enter tonic immobility.
In New Zealand, Orcas have also been observed turning themselves upside down before attacking stingrays, holding them in their mouths, and quickly righting themselves – flipping the prey over and inducing tonic immobility.
► Shark Senses
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