Sharks are among the most intimidating creatures in the sea. If you've ever seen the movie Jaws, it’s no wonder why so many people are afraid of them. Like every living creature, however, sharks have weaknesses that can leave them vulnerable. In fact, flipping certain shark species upside down renders them immobile for up to 15 minutes, inducing a trance-like state known as "tonic immobility."
Tonic immobility can occur naturally, or it can be brought on purposely. Researchers induce tonic immobility in order to subdue sharks while handling them, often for tagging purposes. During tonic immobility, a shark’s muscles relax and its breathing slows, ultimately rendering the creature helpless. The precise reason for tonic immobility is unknown, but some researchers theorize that the reflex is a “playing dead” mechanism used for protection. The question remains, however, why such a weakness would exist in sharks, which have no natural predators. Another theory is that tonic immobility exists to aid the female shark’s fertilization during mating.
Many species of shark are susceptible to tonic immobility, including tiger sharks, silky sharks, sandbar sharks, blacktip reef sharks, whitetip reef sharks, and lemon sharks. Tonic immobility can happen in as little as one minute after the shark is flipped upside down, but suffocation may occur if the animal is forced into such a trance for a long period of time.
What's up with that shark?
- Evidence suggests that orcas sometimes use tonic immobility to prey on sharks. In 1997, an orca in the Farallon Islands near San Francisco was observed holding a white shark upside down for 15 minutes. Unable to defend itself due to its hypnotic state, the shark suffocated.
- In addition to flipping a shark upside down, tonic immobility can also be triggered in some sharks by massaging the front of the snout.
- Cristina Zenato, an expert scuba diver and conservationist based in the Bahamas, utilizes tonic immobility to remove hooks from sharks that are wounded in the field. Using this method, she has been able to help sharks up to 10 feet (3m) long.