TONIC IMMOBILITY IN SHARKS

Nurse Shark in tonic immobility © Jillian Morris, Sharks in Focus.Tonic immobility is a reflex that causes a temporary state of inactivity in an animal. Similar to hypnosis.

It occurs in a variety of different species, including many sharks and rays. This trance-like state can occur naturally. It can also be induced in sharks and rays by stimulating the tiny sensory pores located on their snout.

Tonic immobility is often used by researchers when handling sharks to subdue them. Whether in the wild, captivity, or a laboratory. Subduing them minimises their struggling and reduces the possibility of injury. When the shark is gently turned on its back, it’s thought to disorientate them, causing them to enter the state. The shark’s muscles relax and their breathing becomes deep and rhythmic. When released the shark snaps out of this state.

But why would tonic immobility be useful for sharks? Perhaps it’s a defence strategy? Playing dead could deter potential predators. But some shark species that enter tonic immobility are apex predators. So, don’t have many natural predators. They also don’t appear to enter tonic immobility in response to fear. Some scientists suggest it may be related to mating but nobody knows for sure.

We’ve still got a great deal more to learn!


FUN FACTS:

  • Sharks usually enter tonic immobility in less than a minute. If undisturbed they can remain like this for up to 15 minutes.
     
  • Tonic immobility has been used to help test the effectiveness of chemical shark repellents.

SHARKS AND ORCAS

There's evidence to suggest that orcas use tonic immobility to prey on sharks.

In 1997, an Orca in the Farallon Islands was seen holding a White Shark upside down for 15 minutes. Whether intentional or not, the Orca likely caused the shark to enter tonic immobility. Defenceless, the shark, suffocated. This also happened again in 2000.

In New Zealand, Orcas also seem to use tonic immobility to hunt stingrays. Before attacking the orcas will turn themselves upside down. Then, holding the stingray in their mouth, they'll quickly right themselves. Flipping their prey upside down.


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