When a shark or ray is flipped, it enters a state called tonic immobility, where it becomes calmer and stops struggling.
We have all seen the videos. A diver calmly touches a shark’s snout and instantly, the shark becomes immobilized. Some scientists even describe the shark’s behavior as catatonic.
Why does the shark react in this strange way?
Researchers and recreational divers often find themselves in close proximity to sharks. (Photo Credit: -Greg Amptman/Shutterstock)
Sharks and rays can enter a trance-like state when their snout’s sensory pores are stimulated. These pores are activated by touch, resulting in a relaxed state known as tonic immobility.
The snout of a Tiger Shark has clusters of pores that can be seen up close. (Photo Credit : Wikimedia Commons)
Tonic immobility can be induced in various animals, not just sharks. This form of immobility can be induced in insects, lizards, and even humans.
There are different ways to achieve tonic immobility depending on the species. In fact, humans can also experience tonic immobility when triggered by a past traumatic event or when faced with a current traumatic situation.
When handling animals like chickens or trout, humans use tonic immobility to ensure the safety of both the handler and the animal. The beak of a chicken and the belly of a trout can be stroked to immobilize them, respectively.
Why Do Sharks Enter Tonic Immobility?
Divers and researchers typically stimulate specific pores on the snout of a shark called the Ampulla of Lorenzini. These pores are a highly sensitive network that can contain hundreds or even thousands of pores. Each pore opens into a canal lined with hair.
These hair-lined canals are similar to the small fuzzy hairs found in our own ears. Each canal leads to a chamber filled with gel. These chambers, known as ampullae, have excitatory nerve cells lining them.
The Ampullae of Lorenzini is a densely connected network of electroreceptive cells. (Photo Credit : Chris_huh/Wikimedia Commons)
This intricate sensory network provides sharks and rays with additional information about their prey and surrounding environment. The pores allow them to sense temperature and detect small stimuli in the water using electric fields. In fact, the class of animals known as Chondrichthyes are the most sensitive to electric fields compared to any other type of animal.
Sharks can detect weak electrical fields produced by muscle contractions. When animals near the shark move, whether they are prey or a threat like a pod of hunting orcas, sharks track the electrical activity to locate them.
However, excessive stimulation, such as stroking or tickling their snouts, can cause sharks to enter a paralytic state. Not all species of sharks and rays react the same way to the stimulation of these pores. Tiger sharks are particularly susceptible to overstimulation of their electrosensitive pores, while Great Whites do not show a similar reaction.
So how do researchers ensure their safety when working with Great Whites? They flip them.
Why Do Researchers Flip Sharks?
Handling or studying an animal is easier when we can anticipate or ensure their cooperation. However, certain sharks and rays do not have as effective of an “off” switch linked to the pores on their snouts.
A shark handler teaches someone how to use tonic immobility to “hypnotize” a Caribbean Reef Shark.
To study these species, researchers resort to flipping some sharks onto their backs.
Flipping a shark, whether it is a shark or a ray, has the effect of putting the animal into a state similar to hypnosis. In this state, the shark’s muscles relax and they breathe more deeply and consistently, resulting in a calm and less struggling behavior. This trance-like state can last for up to 15 minutes.
There are several theories as to why this phenomenon occurs. One theory suggests that it is a defense strategy, as playing dead or pretending to be unresponsive can deter predators. However, considering that sharks are apex predators and have few natural enemies besides orcas, this theory does not hold much weight. Another theory proposes that it is a mating mechanism, as studies have shown that female sharks are more susceptible to this state than males.
Interestingly, orcas seem to be aware of sharks’ susceptibility to tonic immobility and have incorporated it into their hunting strategies. In 1997, an orca was observed suffocating a great white shark by flipping it upside down for 15 minutes. A similar observation was made in 2000, where a hunting party of orcas chased down a school of stingrays and paralyzed them by flipping them upside down before devouring them.
The reason behind why sharks evolved with this mechanism to enter an immobile state is still unknown. Many theories have been proposed, but conclusive answers are lacking. Regardless of the evolutionary reason, this ability is beneficial for researchers studying sharks, as it allows them to collect data up close. However, it can also be a disadvantage for sharks when rare predators like orcas exploit this ability to prey on them.