When a shark is turned upside down, it often enters a remarkable trance-like state. This response renders many shark species temporarily motionless, allowing minimal movement for breathing. This temporary incapacitation is natural, though more commonly observed when humans or other large marine animals intentionally invert a shark.
The Shark’s Trance
The trance-like state sharks enter when inverted is known as tonic immobility. This condition renders the shark incapacitated. In this state, the shark’s muscles relax, and its breathing often slows to a rhythmic pace. The dorsal fin may also straighten, contributing to its motionless appearance.
During tonic immobility, sharks exhibit a decrease in their sensory responsiveness, heart rate, and blood pressure. They also experience analgesia, meaning a reduction in their sensitivity to pain. This temporary paralysis can last for a few seconds up to fifteen minutes before the shark recovers. Great white sharks, however, seem less affected by this phenomenon compared to other species.
How It Happens
The mechanisms behind tonic immobility are still under investigation, but theories suggest inversion disrupts the shark’s equilibrium. This disorientation sends signals to the brain, triggering neurological responses. The ampullae of Lorenzini, electroreceptors located on the shark’s snout, are sensitive and can trigger this response when stimulated, such as by massage or flipping.
One theory suggests that tonic immobility is a passive defensive response, similar to “playing dead” seen in other animals. Some experts question this, noting that a motionless target might not deter all predators, especially orcas, which have been observed using this state against sharks. Another hypothesis proposes its role in mating rituals, as male sharks sometimes invert females before copulation, although this does not fully explain why males also experience it. Some research indicates that tonic immobility might simply be an evolutionary remnant rather than a functional adaptation, suggesting it has been lost independently in various shark and ray species over time.
Impact and Utility
Researchers and conservationists utilize tonic immobility as a tool for studying sharks. By inducing this state, scientists can safely handle and tag sharks, collecting data on movements, behavior, and population dynamics without causing stress. This method also minimizes the risk of injury for both the shark and the researchers during procedures.
The temporary nature of this state allows for efficient and ethical interventions. Once research tasks, such as taking blood samples or attaching tracking devices, are completed, the shark is righted. Upon being flipped back over, the shark recovers and swims away without harm. This controlled approach ensures the welfare of sharks while providing information for their conservation.