You have for sure seen in some documentaries on video on the internet scuba divers touching sharks in a precise part of their head, stimulating the Ampullae of Lorenzini and make them fall into some sort of trance state but what exactly is this phenomenon and is it right to play with animals like that? In this article, I'll try to explain what causes that, but please note it is not in any way an encouragement to try that, remember while diving we are all guests in the ocean and this kind of game it is not ethically correct when done for entertainment and it could be very dangerous if you don't have a deep knowledge of shark behaviour and, as you will read it causes stress to the animal.
The tonic immobility is a response observed in many species from sharks to insects and that is caused by an external input, such as the perception of danger or touch input. it can be described as a state of reversible paralysis which mimics a dead state in order to avoid predators. In sharks, this state occurs when the animal is kept in a vertical position or upside down, where the muscular systems are proven to be more relaxed and the cyle of respiration becomes deeper and more regular (in terrestrial organisms the muscular tone is by opposite contracted).
As you might have already seen on the internet, some divers like to approach sharks and do this hypnotic game where the viewer might see a moment full of connection between the diver and the animal. But is that really the case? Probably not. A research of 2011 conducted on juvenile lemon sharks has shown that the tonic immobility causes physiological stress in the animals and chemicals of the blood changes such as an increase of the level of acidity in the blood., as well as a reduction of the efficiency of ventilatory rhythm.
But why many species of sharks do that if they are at the top of the trophic chain and eventually have no predator? Researchers are trying to understand that studying the behaviour and habits of certain species, even though there is still no certain answer yet.
One interesting fact is that killer whales have learned to induce tonic immobility in sharks as a hunting technique. The pod works together to trap the shark and put it in tonic immobility before eating it. Reports of this hunting technique are rare but it's not excluded that it might be a common behaviour in some pods of orcas.
The tonic immobility remains still a huge mystery to researches, but hopefully, more studies will be done in the future.
Despite tonic immobility might be useful to researchers or can be used by divers to remove the hooks from the mouth of the sharks, it should never be use as an entertainment game.