Tonic immobility in sharks

If you have watched your fair share of shark week on the Discovery Channel, or you have scoured your way across YouTube, chances are that you have probably seen the video where a diver puts a shark into a trance called Tonic Immobility.

What is tonic immobility and when was this discovered?

Whilst operating shark cage diving trip in the 1990’s in Gansbaai, the crew would pat the sharks, stroke them, and get them to open their mouths as they approached the boat. During this rather ostentatious showmanship, the crew noticed that, sometimes, the shark would go limp when touched on the nose.

Tonic Immobility is a temporary state of inactivity or paralysis that is like hypnosis. This state can occur naturally, or it can be induced by another animal or human. Sharks are not the only animals that can experience tonic immobility, snakes, birds, rodents, and chickens are also able to enter this state. Some animals use tonic immobility to avoid an attack when being threatened. Animals such as rats, “play dead” when being hunted as a survival strategy. Animals also use this state to blend in with their surroundings to avoid being seen by their prey.

Tonic immobility in sharks can occur when a human gently touches or strokes the small sensory pores on their snout. Within seconds, the shark becomes calm and within one minute, it induces a catatonic state. This state causes the shark to be disoriented. This allows  a window of opportunity for a diver to slowly flip the shark on its back or vertically upside down. Once the shark has been flipped upside down, their muscles start to relax, and their breathing deepens. As soon as they are released from this manually induced state, they immediately return to normality. If the shark is left in this manual state, tonic immobility can last for around 15 minutes.  This technique is frequently used to tag sharks in a safe and calm manner for research purposes. Shark cage diving Cape Town is incredible.

Humans are not the only ones that can induce this trance manually with sharks. In 1997, the highly intelligent Orca, also known as the Killer Whale, was observed off the coast of the Farallon Islands, holding a Great White Shark upside down for 15 minutes causing the shark to suffocate and die. Once the shark was killed, the Orca proceeded to eat the liver. Unlike humans, the Orca bumps the shark on its back to induce the paralysis. Sharks are often threatened by Orcas.

Orcas have also been observed swimming upside down and grabbing stingrays. Once they have the stingray in their mouth, they immediately flip the ray over, leaving it paralysed.

Researchers are still unclear as to how tonic immobility benefits sharks. Some theories are that it is used by sharks as a survival strategy by playing dead. Other theories are that the shark uses this state for mating. It is believed that tonic immobility is an inherently stressful experience, and more research is being conducted on how inducing tonic immobility effects sharks and juvenile sharks and whether there are possible negative side effects to their psychology and behaviour.

Although tonic immobility is used mainly to tag sharks, prolonged or repeated inducement by humans should be avoided while more research is done on the effects of this state on sharks.

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